APOBEC3B and APOBEC3A as Key Drivers of Breast Cancer Hypermutation
The discovery of kataegis—localized clusters of hypermutation in breast cancer genomes—has reshaped our understanding of mutagenesis in cancer. This phenomenon is characterized by dense clusters of C:G to T:A transitions occurring on a single DNA strand, often adjacent to genomic rearrangements. Recent evidence strongly implicates the AID/APOBEC family of cytidine deaminases as the molecular engines behind this process. Through a combination of yeast models, genome-wide mutation profiling, and functional validation in human cells, this study identifies APOBEC3B and APOBEC3A as the primary enzymes responsible for generating the kataegis signature observed in breast tumors.
In budding yeast, expression of various APOBEC family members leads to increased mutation rates, with AID* and APOBEC3G* producing particularly high loads. However, only APOBEC3B and APOBEC3A generate mutation spectra that closely resemble those seen in breast cancer kataegic regions. These enzymes exhibit a pronounced preference for cytosines flanked by a thymine at the -1 position (5′-T-C), which aligns perfectly with the dominant motif found in tumor data. Importantly, while other APOBECs like AID or APOBEC3G prefer different flanking sequences (e.g., A/G at -1 or C at -1), APOBEC3B and APOBEC3A uniquely display high specificity for 5′-TC contexts, making them the most plausible candidates for driving kataegis.
Genome-wide analysis in yeast reveals that APOBEC3B and APOBEC3A induce mutation clusters with similar length distributions and inter-mutational spacing as those in human cancers.B3GAT1 Antibody manufacturer The average distance between mutations within these clusters is approximately 1.2 kb in yeast, comparable to values observed in PD4107a and PD4199a breast tumors. Moreover, both enzymes produce a strong bias toward strand-coordinated mutations, with over 88% of events confined to one strand—mirroring the polarity seen in clinical samples.
Functional studies in human cells confirm the oncogenic potential of these enzymes. When APOBEC3B is expressed in stably transfected KBM7 leukemia cells, it induces significant cell death and triggers robust DNA damage responses, including phosphorylation of histone H2AX and formation of 53BP1 foci. Similar effects are observed with APOBEC3A, indicating that both proteins can cause genotoxic stress even in the absence of viral infection. Notably, APOBEC3B is highly expressed across multiple breast cancer cell lines, and its expression level correlates with mutational burden. Treatment with phorbol ester or interferon-alpha further enhances APOBEC3A and APOBEC3B expression, suggesting that inflammatory signaling may amplify their activity in vivo.
Further support comes from patient-derived data: recent studies have shown that APOBEC3B expression correlates with a T-C mutator phenotype in primary breast tumors.4-Bromophenylacetonitrile manufacturer This provides direct evidence linking enzyme activity to the kataegis signature in human disease.PMID:35115103 The fact that APOBEC3B is not only active but also upregulated in tumor microenvironments strengthens the argument that it contributes significantly to ongoing genome instability.
While APOBEC3B appears to be the dominant player in many cases, APOBEC3A remains a strong candidate in others, particularly where distinct mutation patterns are observed. The presence of residual kataegis in UNG-deficient yeast suggests alternative pathways may exist, but the core mechanism—deamination of ssDNA during break repair—remains central.
Together, these findings establish APOBEC3B and APOBEC3A as major contributors to breast cancer hypermutation. Their ability to generate kataegis through targeted deamination of exposed ssDNA positions them at the intersection of innate immunity and carcinogenesis. Given their role in promoting mutational diversity and potentially enabling tumor evolution, inhibiting these enzymes could represent a promising strategy for preventing or reversing genomic instability in early-stage cancers. Future work will focus on identifying specific inhibitors and evaluating their impact on tumor progression and therapy resistance.MedChemExpress (MCE) offers a wide range of high-quality research chemicals and biochemicals (novel life-science reagents, reference compounds and natural compounds) for scientific use. We have professionally experienced and friendly staff to meet your needs. We are a competent and trustworthy partner for your research and scientific projects.Related websites: https://www.medchemexpress.com
Sexually transmitted infections (STIs) represent a major public health concern in Spain, characterized by high prevalence and significant complications when diagnosis and treatment are delayed. In 2016, it was estimated that approximately 146,500 people were living with HIV in Spain, corresponding to a prevalence rate of 0.37%. Alarmingly, about 13.7% of individuals with HIV remained unaware of their infection, underscoring gaps in testing and awareness. In 2020, 1,925 new HIV diagnoses were reported—a 41% decrease compared to 2019—largely attributed to pandemic-related disruptions in healthcare access and testing services. Of these cases, 84.3% involved men, with a median age of 36 years (interquartile range: 29–46). Despite free HIV testing available across all levels of the Spanish healthcare system, nearly half of new diagnoses were late (CD4 count <350 cells/μL), and this proportion increased with age: 35.4% among those under 25 and 58.9% among individuals aged 50 and older. Gonococcal infections also remain prevalent, with 12,359 cases reported in 2019—79.E-Cadherin Antibody site 7% among men and a male-to-female ratio of 3.9. The median age at diagnosis was 30 years (IQR: 24–38). Syphilis cases totaled 5,822 in 2019, with 88.7% affecting men and a male-to-female ratio of 7.4. The median age for syphilis diagnosis was 36 years (IQR: 29–46). Chlamydia trachomatis infections reached 17,718 notifications in 2019, with 54.4% occurring in women (9,788 cases), and a male-to-female ratio of 0.8. The median age at diagnosis was 26 years (IQR: 21–33). Lymphogranuloma venereum (LGV), caused by *Chlamydia trachomatis* serovars L1–L3, accounted for 453 cases, predominantly in men (98.9%), with only five cases reported in women. Median age at diagnosis was 37 years (IQR: 30–44).
Data on human papillomavirus (HPV) and genital herpes prevalence at the national level are not systematically collected. However, global estimates suggest an overall HPV infection prevalence of 11.7%, with higher rates observed in younger populations—peaking at 24.0% among women under 25. Most HPV infections are asymptomatic and resolve spontaneously within 1–2 years. Genital herpes remains the second most common STI worldwide after condyloma acuminata and is the leading cause of genital ulcers in sexually active adults in Spain.2,4-Dinitrobenzenesulfonic acid sodium salt supplier Traditionally linked to HSV-2, recent data show a growing role of HSV-1 as a causative agent, especially among young women, reflecting shifts in transmission patterns.PMID:35073512
Hepatitis C incidence has risen notably among gay, bisexual, and other men who have sex with men (GBHSM), particularly those co-infected with HIV. The EMIS survey in Spain revealed that 1.8% of GBHSM had ever been diagnosed with HCV, and 22.7% received a diagnosis within the past year. Additionally, 12.5% experienced recurrent HCV infections. Current guidelines recommend integrated HIV and HCV testing at least annually, with testing every three months for high-risk individuals based on sexual behavior, history of STIs, and use of PrEP or PEP. Key vulnerable groups include GBHSM, transgender individuals, people who inject drugs and their partners, and sex workers. Regular screening for gonorrhea, syphilis, and chlamydia is advised annually for GBHSM, with more frequent testing for those engaging in high-risk behaviors such as unprotected sex with new partners, prior STI diagnosis, or substance use. Given the high prevalence of chlamydia in young women, annual screening is strongly recommended for all sexually active women in this age group, with increased frequency based on individual risk factors.MedChemExpress (MCE) offers a wide range of high-quality research chemicals and biochemicals (novel life-science reagents, reference compounds and natural compounds) for scientific use. We have professionally experienced and friendly staff to meet your needs. We are a competent and trustworthy partner for your research and scientific projects.Related websites: https://www.medchemexpress.com
The accumulation of hyperphosphorylated tau protein into neurofibrillary tangles is a central pathological feature of Alzheimer’s disease and other tauopathies. Molecular chaperones, particularly Heat Shock Protein 70 (Hsp70), play a pivotal role in maintaining proteostasis by facilitating the proper folding, refolding, or degradation of misfolded proteins. Among its isoforms, Hsc70 (HSPA8) and Hsp72 (HSPA1A) are key regulators of tau turnover, with Hsp72 being more closely linked to the promotion of tau degradation via the ubiquitin-proteasome system. Pharmacological modulation of Hsp70 activity offers a promising strategy for reducing pathogenic tau levels. Allosteric inhibitors that stabilize a conformation of Hsp70 favoring substrate degradation have demonstrated efficacy in preclinical models.
In this study, we report the development of a new series of Hsp70 allosteric inhibitors derived from the rhodacyanine scaffold, focusing on enhancing metabolic stability while preserving biological activity. The lead compound, YM-08, exhibited favorable brain penetration and reduced tau levels but was rapidly metabolized in liver microsomes, with a half-life under three minutes. Metabolite identification studies revealed that the benzothiazole ring served as the primary site for CYP3A4-mediated oxidation. To address this issue, we introduced halogen substitutions—fluorine and chlorine—at various positions (3-, 4-, 5-, and 6-) on the benzothiazole ring. We also explored the impact of shifting the pyridine nitrogen from ortho to para position, hypothesizing that electronic and steric effects might influence metabolic vulnerability.
Seventeen analogs were synthesized using a multi-step protocol involving cyclization, methylation, condensation with 3-ethylrhodanine, and final deprotection.Benzanthrone In Vivo Compounds were evaluated for metabolic stability using mouse liver microsomes at 37°C over 30 minutes. Notably, JG-23, featuring a 4-chloro substitution and para-pyridine configuration, showed the most significant improvement, retaining 52% of its original concentration after 30 minutes—12-fold greater than YM-08.Diclofenac Epigenetics This enhancement was consistent across multiple time points and confirmed by kinetic analysis, which yielded a half-life of 36 minutes for JG-23 compared to less than 3 minutes for YM-08.PMID:34764602
Further validation confirmed that JG-23 effectively reduces total tau levels in both HeLaC3 and SH-SY5Y cell lines. In HeLaC3 cells, treatment with JG-23 at concentrations above 10 µM led to a dose-dependent decrease in tau protein, as measured by Western blot. Similarly, in SH-SY5Y cells, which express endogenous tau, JG-23 reduced tau levels by approximately 80% without altering expression of stress markers such as Hsp72 or Hsp90. These results indicate that JG-23 selectively enhances tau degradation without triggering cellular stress responses.
This work demonstrates that rational chemical modification—specifically halogenation and strategic heteroatom positioning—can dramatically improve the metabolic stability of Hsp70-targeting compounds without compromising their functional activity. JG-23 represents a robust and stable chemical probe suitable for investigating tau homeostasis in complex biological systems. Its favorable pharmacokinetic profile paves the way for future in vivo studies aimed at evaluating the therapeutic potential of Hsp70 modulation in tauopathy models.MedChemExpress (MCE) offers a wide range of high-quality research chemicals and biochemicals (novel life-science reagents, reference compounds and natural compounds) for scientific use. We have professionally experienced and friendly staff to meet your needs. We are a competent and trustworthy partner for your research and scientific projects.Related websites: https://www.medchemexpress.com
Vandetanib, a multi-targeted tyrosine kinase inhibitor, has emerged as an important therapeutic option for patients with advanced medullary thyroid cancer (MTC) and previously treated non-small-cell lung cancer (NSCLC). Its mechanism of action—targeting VEGFR-2, EGFR, and RET kinases—contributes to its antitumor efficacy. However, one of the most frequent and clinically significant adverse events associated with vandetanib is hypertension, which can impair quality of life, necessitate dose modifications, or lead to treatment discontinuation. Given the variability in reported incidence across individual trials and the lack of a consolidated estimate, a systematic evaluation of the overall risk and incidence of hypertension remains essential.
This meta-analysis aimed to determine the pooled incidence and relative risk of hypertension in cancer patients receiving vandetanib based on data from published clinical trials. A comprehensive literature search was performed using PubMed, Embase, and the Cochrane Library up to March 2012, supplemented by manual screening of conference abstracts from ASCO and ESMO (2001–2012), clinical trial registries (ClinicalTrials.gov), and reference lists of relevant articles. Eligible studies were prospective phase II and III randomized controlled trials that administered vandetanib at a daily dose of 300 mg, with available data on hypertension outcomes. Trials involving combination therapies, phase I studies, or those with inadequate reporting were excluded.
Eleven trials comprising 3,154 patients were included in the final analysis. Eight of these were randomized controlled trials with control arms (placebo or standard therapy), while three were single-arm studies. The Jadad score assessment indicated generally high methodological quality, with six trials scoring 5, two scoring 4, and three scoring 3 or below. Data extraction focused on the number of patients experiencing hypertension of any grade and high-grade (grade 3–4) severity, using the CTCAE v2 or v3 criteria.
The overall pooled incidence of all-grade hypertension among all cancer patients was 24.2% (95% CI: 18.1–30.2%). High-grade hypertension occurred in 6.4% (95% CI: 3.3–9.5%) of patients. Stratified by tumor type, the incidence varied significantly: MTC patients exhibited the highest rates (all-grade: 32.1%, high-grade: 8.8%), followed by NSCLC patients (all-grade: 21.8%, high-grade: 7.6%), and non-MTC/NSCLC tumors (all-grade: 15.4%, high-grade: 3.4%). These differences were statistically significant between MTC and NSCLC for all-grade hypertension (RR 1.48, 95% CI: 1.23–1.77, P < 0.001), but not for high-grade hypertension. Pooled relative risk analysis revealed a strong association between vandetanib use and increased hypertension risk. The summary RR for all-grade hypertension was 5.1 (95% CI: 3.76–6.92; P < 0.001), and for high-grade hypertension, it was 8.06 (95% CI: 3.41–19.04; P < 0.001). Subgroup analyses confirmed these findings across disease subtypes, with particularly high risks observed in NSCLC (RR 6.PIK3CG Protein Purity & Documentation 15) and MTC (RR 4.Laropiprant Immunology/Inflammation 99) patients.PMID:34543057 Heterogeneity was moderate (I² = 84.5% for all-grade, I² = 57.6% for high-grade), prompting the use of random-effects models for primary analyses.
No evidence of publication bias was found through Begg’s funnel plot and Egger’s regression test (P = 0.881 for all-grade RR; P = 0.602 for high-grade RR), supporting the robustness of the results. Potential limitations include reliance on reported adverse events rather than objective blood pressure measurements, possible underreporting due to CTCAE criteria thresholds (e.g., requiring >20 mmHg diastolic rise), and the absence of individual patient-level data for adjustment of confounders such as baseline cardiovascular risk.
In conclusion, this meta-analysis confirms that vandetanib is associated with a substantial and clinically meaningful risk of hypertension across multiple solid tumor types. The risk is highest in MTC patients, likely due to longer treatment duration and specific tumor biology. Early detection, regular monitoring, and prompt initiation of antihypertensive therapy are critical to prevent complications and maintain treatment continuity. Future studies should explore predictive biomarkers and optimal management strategies tailored to different tumor types and patient profiles.MedChemExpress (MCE) offers a wide range of high-quality research chemicals and biochemicals (novel life-science reagents, reference compounds and natural compounds) for scientific use. We have professionally experienced and friendly staff to meet your needs. We are a competent and trustworthy partner for your research and scientific projects.Related websites: https://www.medchemexpress.com
Fatty acid-binding protein 4 (FABP4), also known as adipocyte fatty acid-binding protein (A-FABP), has emerged as a key player in metabolic regulation and systemic inflammation. In this study, FABP4 levels were evaluated in a cohort of 296 apparently healthy Japanese individuals not receiving any medication, with the aim of assessing its role as a biomarker for metabolic phenotype. Serum FABP4 concentrations were significantly higher in females compared to males, consistent with greater adiposity in women. This gender difference was not observed in other FABP isoforms, suggesting a unique biological association of FABP4 with adipose tissue biology.
Univariate analysis revealed that FABP4 levels were positively correlated with multiple markers of metabolic dysfunction: age, body mass index (BMI), waist circumference, systolic and diastolic blood pressure, total cholesterol, LDL cholesterol, triglycerides, fasting glucose, insulin, HOMA-R, brain natriuretic peptide (BNP), and high-sensitivity C-reactive protein (hsCRP).S2116 Epigenetic Reader Domain Notably, FABP4 showed one of the strongest correlations with insulin resistance, as reflected by HOMA-R (r = 0.319, p < 0.001). The correlation remained significant when stratified by gender—male r = 0.40, female r = 0.38—indicating a robust relationship across sexes. Multiple regression analysis confirmed FABP4 as an independent predictor of HOMA-R after adjusting for confounding factors such as age, gender, waist circumference, blood pressure, HDL cholesterol, and hsCRP. The model explained 40.6% of the variance in insulin resistance (R² = 0.406), underscoring the substantial contribution of circulating FABP4 to metabolic dysregulation. These findings suggest that elevated FABP4 levels may precede overt clinical manifestations of metabolic syndrome, serving as a sensitive indicator of early insulin resistance.Polyoxin D Protocol
Further analysis indicated that FABP4 is not merely a passive marker but likely actively contributes to metabolic pathology through its roles in lipid trafficking, adipokine signaling, and inflammatory pathways.PMID:34219288 Its secretion from adipocytes into circulation, even in the absence of cell damage, supports its function as a hormone-like mediator linking adipose tissue activity to systemic metabolism. Moreover, the strong correlation with BNP and hsCRP hints at potential involvement in cardiovascular stress and low-grade inflammation.
In conclusion, FABP4 emerges as a powerful, non-invasive biomarker capable of identifying subclinical metabolic abnormalities in the general population. Its ability to reflect insulin resistance, obesity-related risk, and early vascular stress positions it as a valuable tool for early detection and risk stratification in preventive medicine. Future longitudinal studies are warranted to determine whether FABP4 levels can predict the onset of type 2 diabetes, cardiovascular disease, or metabolic syndrome progression.MedChemExpress (MCE) offers a wide range of high-quality research chemicals and biochemicals (novel life-science reagents, reference compounds and natural compounds) for scientific use. We have professionally experienced and friendly staff to meet your needs. We are a competent and trustworthy partner for your research and scientific projects.Related websites: https://www.medchemexpress.com
Under conditions of prolonged nutrient deprivation, cardiac cells face a critical challenge in maintaining energy homeostasis and cellular integrity. This study demonstrates that epoxyeicosatrienoic acids (EETs) play a pivotal role in promoting survival by orchestrating an adaptive autophagic response in HL-1 cardiomyocytes and neonatal cardiomyocytes (NCMs). The protective effects of EETs were evaluated using the dual-function analog UA-8, which mimics EET activity while inhibiting soluble epoxide hydrolase (sEH), thereby increasing endogenous EET levels. Upon starvation, untreated control cells exhibited progressive loss of viability, marked by increased lactate dehydrogenase (LDH) release, reduced beating rate, and elevated caspase-3 and proteasome activities—hallmarks of apoptotic and degenerative stress. In contrast, UA-8 treatment significantly attenuated these detrimental changes, preserving cell membrane integrity, contractile function, and metabolic capacity.
A central finding was that EETs enhanced autophagic flux during starvation.Alogliptin Epigenetic Reader Domain Immunoblot analysis revealed a robust increase in microtubule-associated protein light chain 3-II (LC3-II) levels in UA-8-treated cells, indicating active autophagosome formation.Lactamide Biological Activity Fluorescence microscopy confirmed a higher number of LC3-positive puncta, particularly in cells treated with UA-8, suggesting accumulation of autophagosomes.PMID:35122502 Electron microscopy further visualized autophagic vacuoles containing mitochondria, confirming mitophagy—a selective form of autophagy targeting damaged organelles. These morphological features were absent in controls and diminished upon inhibition of autophagy with either Atg7 shRNA or 3-MA, underscoring the dependence of EET protection on functional autophagy.
Mitochondrial preservation was a key outcome of EET-mediated autophagy. Despite prolonged starvation, cells treated with UA-8 maintained higher enzymatic activities of citrate synthase, succinate dehydrogenase (SDH), and cytochrome c oxidase (COX IV), all essential components of the mitochondrial respiratory chain. Western blot analysis showed sustained expression of mitochondrial proteins including VDAC, SDH, and COX IV, indicating no net loss of mitochondrial mass. This suggests that EETs do not prevent mitochondrial degradation but instead promote selective removal of dysfunctional mitochondria through autophagy, thereby maintaining a healthier pool of organelles capable of supporting ATP production.
The signaling cascade underlying this process involves pmKATP channels and AMPK activation. Inhibition of pmKATP with HMR-1098 abolished UA-8-induced phosphorylation of AMPK and blocked the upregulation of LC3-II. AMPK, a master regulator of cellular energy balance, is activated under low ATP conditions and promotes autophagy by inhibiting mTORC1 and activating ULK1. In starved cells, UA-8 triggered AMPK phosphorylation, linking EET signaling to metabolic sensing and autophagy initiation. This pathway ensures that only severely damaged components are removed, preserving functional mitochondria and enhancing cell resilience.
These findings reveal that EETs act as metabolic sentinels, fine-tuning autophagy to protect cardiac cells during deep metabolic stress. By selectively clearing damaged mitochondria and sustaining functional ones, EETs shift the cellular fate from death to survival. This mechanism provides a new perspective on how lipid mediators regulate cellular quality control and offers potential for therapeutic intervention in ischemic heart disease, heart failure, and other conditions involving energetic crisis and mitochondrial dysfunction.MedChemExpress (MCE) offers a wide range of high-quality research chemicals and biochemicals (novel life-science reagents, reference compounds and natural compounds) for scientific use. We have professionally experienced and friendly staff to meet your needs. We are a competent and trustworthy partner for your research and scientific projects.Related websites: https://www.medchemexpress.com
Metabolic stress, such as nutrient deprivation or serum withdrawal, challenges cellular homeostasis and triggers adaptive responses to maintain essential functions. Among the most critical adaptations is the preservation of phosphatidic acid (PA) levels, a lipid molecule indispensable for mTOR signaling and membrane integrity. When one pathway for PA synthesis is compromised, cells activate compensatory mechanisms involving alternative enzymatic sources—LPAAT, DGK, and even ER-associated kinases—to sustain PA production. This redundancy ensures that mTOR remains active and cell cycle progression can proceed under adverse conditions.
In Ras-driven cancer cells, serum withdrawal leads to a significant upregulation of phospholipase D (PLD) activity, particularly PLD1 and PLD2. These cells exhibit a heightened dependence on exogenous lipids due to impaired desaturation of fatty acids, stemming from reduced stearoyl-CoA desaturase-1 (SCD1) expression. As a result, endogenously synthesized fatty acids remain saturated, limiting their utility in membrane biosynthesis.Edonerpic Biological Activity To overcome this deficit, these cells increase macropinocytosis—a process that internalizes extracellular proteins and lipids, including albumin-bound fatty acids. This scavenging mechanism provides both amino acids and lipid precursors necessary for PA generation via LPAAT and DGK pathways.
The compensatory activation of PA synthesis extends beyond PLD. Inhibition of PLD does not lead to decreased PA levels; instead, it often results in elevated PA from unknown sources, suggesting the existence of robust backup systems. One such mechanism involves the endoplasmic reticulum kinase PERK, which phosphorylates diacylglycerol (DG) to produce PA during ER stress induced by low glucose or hypoxia. This pathway provides a direct link between metabolic stress and PA production, ensuring mTOR signaling persists even when canonical PLD activity is suppressed.
Another key player is diacylglycerol kinase (DGK), which converts DG into PA. DG can originate from stored triglycerides via lipolysis or from phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis by phospholipase C (PLC). However, PIP2 is present in limited quantities, making the PLC-DGK route energetically costly and likely restricted to specific contexts. In contrast, DG derived from lipid droplets offers a more sustainable source. The ability to utilize multiple DG sources enhances flexibility in PA production under varying metabolic demands.
Notably, the functional significance of PA may depend on its acyl composition. While PLD generates PA with mixed acyl chains—typically one saturated and one unsaturated—LPAAT and DGK pathways may produce PA with distinct profiles. Recent studies demonstrate that dipalmitoyl-PA (two saturated chains) destabilizes mTORC2, whereas PA containing unsaturated fatty acids stabilizes both mTOR complexes. This suggests that not all PA is functionally equivalent. Cells may thus regulate the type of PA produced based on metabolic context, potentially using saturated PA as a negative signal to inhibit mTOR under unfavorable conditions.Endosulfan MedChemExpress
These compensatory mechanisms are not merely survival strategies—they also influence cancer progression.PMID:34796907 Elevated PLD activity under stress promotes migration and invasion in a PLD-dependent manner, indicating that PA signaling contributes to metastatic behavior. This dual role—supporting both survival and motility—highlights the importance of PA in shaping cancer cell fate under environmental stress.
In summary, the maintenance of PA levels through redundant and adaptable pathways underscores its central role in cellular adaptation. Cancer cells exploit these mechanisms to thrive in harsh microenvironments, making PA metabolism a compelling target for therapeutic intervention. By disrupting compensatory PA production, it may be possible to selectively sensitize tumor cells to metabolic stress, thereby inhibiting growth and preventing metastasis.MedChemExpress (MCE) offers a wide range of high-quality research chemicals and biochemicals (novel life-science reagents, reference compounds and natural compounds) for scientific use. We have professionally experienced and friendly staff to meet your needs. We are a competent and trustworthy partner for your research and scientific projects.Related websites: https://www.medchemexpress.com
Electromyographic (EMG) signals provide a direct window into neuromuscular activity, making them ideal for non-invasive human-machine interaction systems. In facial gesture recognition, surface EMG signals capture the electrical activity of facial muscles during various expressions and movements. However, raw EMG data is inherently noisy, non-stationary, and complex, necessitating effective feature extraction and selection to ensure accurate classification. This study focuses on identifying the most informative time-domain features from facial EMG signals and evaluating their performance in distinguishing ten distinct facial gestures.
A total of ten healthy participants performed ten predefined facial gestures—smiling bilaterally, smiling unilaterally, opening the mouth, clenching molars, raising eyebrows, frowning, closing both eyes, and closing each eye individually—five times per gesture. Bipolar surface electrodes were placed over three primary facial muscle groups: frontalis (forehead), left temporalis, and right temporalis. The recorded signals were sampled at 1000 Hz and filtered using a band-pass filter (30–450 Hz) to remove high-frequency noise and power-line interference. Signal segments were then divided into 256 ms non-overlapping windows, resulting in 39 frames per gesture per channel. From these segments, ten time-domain features were computed: Integrated EMG (IEMG), Mean Absolute Value (MAV), Mean Absolute Value Slope (MAVS), Maximum Peak Value (MPV), Root Mean Square (RMS), Simple Square Integral (SSI), Variance (VAR), Mean Value (MV), Wave Length (WL), and Sign Slope Changes (SSC).
Each feature was evaluated for its ability to distinguish between different facial gestures. Statistical analysis revealed significant differences in the distribution of feature values across gesture classes. For instance, MPV consistently exhibited higher peak amplitudes during expressive movements such as smiling or frowning, while WL showed increased variability during prolonged muscle contractions. Features like MAV and RMS demonstrated strong correlation with overall signal energy, yet struggled to differentiate subtle differences in gesture dynamics. Conversely, SSC and MAVS captured rapid changes in muscle activation, proving useful for detecting transient movements like eyebrow raises.
To identify the most discriminative features, mutual information (MI) was used to measure statistical dependencies among features. High MI values between MAV and MAVS indicated redundancy, suggesting that they carry similar information. In contrast, low MI values between WL and other features highlighted its unique, often less reliable, characteristics. Feature ranking was conducted using two criteria: Minimum Redundancy–Maximum Relevance (MRMR) and Recognition Accuracy (RA). MRMR prioritized features with high relevance to the target class and low redundancy among themselves, while RA ranked features based on their individual classification performance.
The results showed that MPV ranked highest under RA, achieving an average recognition accuracy of 87.1% when used alone. It also demonstrated the lowest standard deviation across subjects (1.Glypican-4/GPC4 Proteinmedchemexpress 1%), indicating consistent performance.Octylamine site Features such as IEMG and SSI also performed well but required more computational resources.PMID:35093367 WL, despite being included in several combinations, consistently yielded poor results (average accuracy 24.5%), likely due to its sensitivity to baseline drift and noise. Confusion matrix analysis confirmed that MPV enabled clear separation between gesture classes, particularly for G1 (opening mouth), G2 (left smile), G3 (right smile), and G5 (clenching molars), where misclassification rates were below 5%.
Combining features did not improve performance beyond using MPV alone. In fact, feature sets constructed via MRMR or RA generally resulted in lower accuracy and longer training times. This suggests that adding redundant or weakly relevant features introduces noise and increases data overlap between classes. The inclusion of MV, which ranked second under MRMR, significantly degraded performance due to its low individual accuracy, even though it contributed complementary information in theory.
These findings underscore the importance of selecting features based on empirical performance rather than theoretical relevance. MPV emerged as the most robust and discriminative single feature, capable of capturing both amplitude and timing characteristics of facial muscle activation. Its stability, high accuracy, and minimal computational cost make it ideal for real-time applications in assistive devices. Future research will explore adaptive feature selection strategies and hybrid feature sets incorporating frequency- and time-frequency domain features to further enhance recognition accuracy without sacrificing speed.MedChemExpress (MCE) offers a wide range of high-quality research chemicals and biochemicals (novel life-science reagents, reference compounds and natural compounds) for scientific use. We have professionally experienced and friendly staff to meet your needs. We are a competent and trustworthy partner for your research and scientific projects.Related websites: https://www.medchemexpress.com
The transition from active growth to dormancy in temperate and boreal woody perennials involves a complex reprogramming of cellular metabolism, particularly during the onset of winter. In Sitka spruce (Picea sitchensis), this process—known as winter hardening—is essential for survival under freezing temperatures while maintaining photosynthetic tissues. Despite the absence of significant photosynthetic activity during cold periods, metabolic remodeling remains extensive, requiring substantial energy and carbon substrates. This study investigates how such metabolic demands are met through heterotrophic pathways.
Transcriptomic analysis across five time points—from late summer to early winter—revealed that genes associated with chloroplast function were predominantly down-regulated after TP2, indicating a reduction in photosynthetic capacity. However, key components of antioxidant systems, especially those localized to thylakoid membranes, were up-regulated. Notably, thylakoid-bound ascorbate peroxidase (tAPX) showed one of the strongest increases in expression, suggesting an enhanced role in redox homeostasis. Additionally, plastid terminal oxidase (PTOX), involved in dissipating excess excitation energy, was also significantly induced.Tolvaptan Autophagy These findings support the idea that even as photosynthesis slows, protective mechanisms remain highly active.2,6-Dichloro-4-nitroaniline web
In contrast, mitochondrial-related genes exhibited widespread up-regulation.PMID:35216364 Components of both the conventional and alternative electron transport chains (ETC) were activated, including external and internal NAD(P)H dehydrogenases, alternative oxidase A1, and uncoupling protein 2. The activation of these alternative pathways allows continued electron flow without forming reactive oxygen species (ROS), which is critical under low-temperature stress where standard ETC components may become inhibited. Furthermore, genes encoding enzymes involved in the tricarboxylic acid (TCA) cycle and glycolysis showed moderate but consistent changes, indicating sustained respiratory activity.
Metabolite profiling confirmed these transcriptional patterns. While sucrose levels remained stable, starch synthesis genes were up-regulated while degradation genes were suppressed—suggesting a shift toward starch accumulation rather than mobilization. However, despite increased transcript levels for invertases and sucrose synthases, no corresponding rise in hexose availability was observed. This implies that stored carbohydrates may not be readily accessible, or that other sources are being utilized.
Indeed, evidence points to glycolytic bypasses enabling efficient ATP production. The cytosolic malate dehydrogenase and phosphoenolpyruvate carboxylase were up-regulated, facilitating the conversion of phosphoenolpyruvate to malate, which is then transported into mitochondria via dicarboxylate/tricarboxylate transporter (DTC). Within mitochondria, malate can enter the TCA cycle or be decarboxylated by NAD-dependent malic enzyme to produce pyruvate and NADH. The latter was reflected in a 9- to 11-fold increase in transcript levels of the relevant malic enzyme gene, indicating robust flux through this pathway.
This metabolic strategy allows for the transfer of reducing equivalents from the cytosol to mitochondria, supporting both conventional and alternative ETCs. It also provides additional carbon skeletons for the TCA cycle, sustaining respiration and biosynthetic processes during winter. Moreover, substrate-level phosphorylation steps in glycolysis—such as those catalyzed by pyruvate kinase—were consistently up-regulated, preserving ATP under conditions where oxidative phosphorylation may be compromised.
Collectively, these results demonstrate that Sitka spruce relies heavily on heterotrophic metabolism during winter hardening. Energy and carbon are derived not from photosynthesis, but from stored reserves and efficient metabolic rerouting. The integration of alternative respiratory pathways, redox regulation, and strategic use of bypass reactions enables the plant to maintain essential functions while minimizing oxidative damage. This adaptive mechanism ensures survival under prolonged freezing conditions and highlights the importance of metabolic flexibility in evergreen conifers.MedChemExpress (MCE) offers a wide range of high-quality research chemicals and biochemicals (novel life-science reagents, reference compounds and natural compounds) for scientific use. We have professionally experienced and friendly staff to meet your needs. We are a competent and trustworthy partner for your research and scientific projects.Related websites: https://www.medchemexpress.com
The adsorption and structure of sodium dodecyl sulfate (SDS) layers onto positively charged films have been monitored in situ with vibrational sum-frequency-generation (SFG) spectroscopy and surface plasmon resonance (SPR) sensing. Substrates with different charge densities and polarities used in these studies include CaF2 at different pH values as well as allylamine and heptylamine films deposited onto CaF2 and Au substrates by radio frequency glow discharge deposition. The SDS films were adsorbed from aqueous solutions ranging in concentration from 0.067 to 20 mM. In general the SFG spectra exhibited well resolved CH and OH peaks. However, at SDS concentrations between 1–8 mM the SFG CH and OH intensities decreased close to background levels. Combined data sets from molecular conformation, orientation, and order sensitive SFG with mass sensitive SPR suggest that the observed changes in SFG intensities above 0.2 mM are related to structural arrangements in the SDS layer. A model is proposed where the SFG intensity minimum between 1–8 mM is associated with a monolayer containing two head group orientations, one pointing towards the substrate and one pointing towards the solution phase. The SFG peaks observed at concentrations below 0.2 mM are dominated by the presence of adsorbed contaminants such as fatty alcohols (e.g., dodecanol), which are more surface active than SDS. As SDS solution concentration is increased above 1 mM SDS molecules are incorporated in the surface layer, with dodecanol continuing to be present in the surface layer for solution concentrations up to at least critical micelle concentration.
Molecular level information about the interaction of surfactants with surfaces is of great interest for a range of industrial and scientific areas. Particularly sodium dodecyl sulfate (SDS) is used in such varied fields as cleaning applications, lubrication, stabilization of emulsions, preparation of nano- and microparticles, and even as model systems for biological membranes in protein research. The formation of SDS layers also raises important questions about the dynamics of molecular self-assembly on surfaces. Surfactant adsorption is complex and involves a delicate, concentration dependent balance of forces between charge interactions of the headgroups with the surface, intermolecular van der Waals interactions, electrostatic repulsion between the headgroups, and interactions with the surrounding liquid phase. On charged surfaces, SDS head groups with negative charge bind at the interface and anchor the molecules to the surface. Upon completion of monolayer coverage, the surface charges are increasingly screened by surfactant headgroups. At higher concentrations typically around the critical micelle concentration (cmc) it has been suggested that a second inverted surfactant layer is formed by hydrophobic interactions of the tail groups in the first and second layer. Studies of surfactants at interfaces have been done with a number of techniques including neutron reflectivity, spectroscopic ellipsometry, optical reflectometry, quartz crystal microbalance, filter viscometry, atomic force microscopy, and vibrational sum-frequency-generation (SFG) spectroscopy. While most of these techniques can accurately determine the amount of surfactant adsorbed at the interface, SFG spectroscopy is the only technique capable of providing insight into the molecular conformation, orientation, and order of surfactant layers in situ. The adsorption kinetics and energetics of a variety of surfactants including SDS have been studied in great detail, but the number of spectroscopic studies is still very limited. For a detailed understanding of SDS at surfaces it is essential to combine molecular-level structural information directly with adsorbed mass information to obtain a full understanding of the surfactant film structure. The investigation of SDS at interfaces is further complicated by contaminants with high surface activity making it exceedingly difficult to prepare sufficiently pure SDS for surface studies. In fact, concentrations of dodecanol at 0.1% can significantly reduce the surface tension at the air/water interface at solution concentrations up to the cmc. Commercially available `as-received’ SDS typically contains 0.1–1% dodecanol, so unless extensive purification is done, impurities will be present in the SDS films and have a significant effect on their surface structure and composition. This is especially true at low SDS solution concentrations, but dodecanol will continue to be present and have an effect for solution concentrations up to at least the cmc. To address effects of dodecanol on the structure of SDS, Bain and coworkers performed SFG studies of mixed dodecanol/SDS monolayers at hydrophobic solid surfaces. These studies showed that dodecanol has an effect on SDS conformation and packing density at concentrations below the SDS cmc. Importantly, the above-cited studies clearly show that any SDS layer prepared under technologically and commercially relevant conditions, where it is not feasible or practical to invest the time and money required to produce ultra-high purity SDS, will be affected by contaminants in the SDS to varying but significant degrees. In this work, we therefore chose to use `as-received’ SDS without further purification as a realistic system that can be directly related to the use of SDS in practical applications. We recognize that impurities will change the recorded spectra and the structure of the materials at the interface, thus we will discuss the obtained data in this context. We here combine SFG measurements with mass sensitive SPR data for SDS solutions ranging from micro- to millimolar concentrations to study the binding of SDS to positively charged surfaces. Using radio frequency glow discharge deposited allylamine (AAm) and heptylamine (HApp) surfaces as positively charged model surfaces allowed us to prepare identical substrate chemistries for both SFG and SPR. The latest SFG-based model of SDS adsorption onto positively charged surfaces, put forward by Richmond and co-workers, describes neutralization of net surface charge near 0.2 mM SDS solution concentration related to monolayer coverage and onset of bilayer formation at higher concentrations. The data reported here indicate that the monolayer-bilayer transition observed by Richmond et al. is most likely caused by contaminants and that the related adsorption model needs to be extended to account for additional film structure transitions.
SDS solution preparation: Surfactant Dodecyl Sulfate (SDS) was purchased from Bio-Rad (99% purity). Experiments with both as-received and recrystallized SDS were done. SDS consists of a 12-carbon tail and anionic sulfate headgroup. The as received powder was dispersed in degassed, deionized, and distilled water at room temperature. A 20 mM SDS solution was prepared by heating at 40°C overnight with moderate stirring. After dissolution, the solution was cooled to room temperature and then aliquots from the 20 mM SDS solution were diluted to obtain lower SDS solution concentrations. The SDS solutions were adjusted to pH 3.5 and pH 5.4 with acetic acid or 1M NH4OH. For recrystallization the as-received SDS was dissolved into ethanol and then filtered with a 20 m syringe-filter. The ethanol was removed from the SDS solution via evaporation in a vacuum dessicator. Radio Frequency (RF) Glow Discharge Coating: The radio frequency glow discharge (RFGD) deposition was performed using published procedures. AAm (98% purity) and HApp (99% purity) were purchased from SigmaAldrich. AAm and HApp films were deposited in an RFGD system. The vacuum system consisted of a rotary pumped glass vacuum chamber with an external RF electrode. The coupled external electrode was connected to the 13.56-MHz RF power source. After loading the samples into the reactor and evacuating it to the base pressure of 10 mTorr with a rotary vacuum pump, oxygen was introduced inside the chamber and the pressure was maintained at 350 mTorr. The samples, holder and interior walls of the chamber were then cleaned by an 80 W oxygen discharge for 30 min. After oxygen etching, the chamber was evacuated to base pressure. The substrates were further cleaned and activated using a 30 W Ar discharge for 30 sec at 350 mTorr. AAm and HApp films were then coated onto Au pieces and CaF2 prisms. First an adhesion-promoting layer of AAm was deposited at 80 W and 350 mTorr for 30 sec. Then the final AAm coating process was done at 10 W and 350 mTorr for 5 min. The deposition process for HApp was 80 W for 1 min (adhesion layer) followed by 10 W for 5 min (final layer), both at a pressure of 250 mTorr. The effective thickness of the deposited coatings was determined by spectroscopic ellipsometry to be 130 nm (AAm) and 220 nm (HApp). The refractive index of the AAm was determined to be 1.581. The rms roughness of the HApp film was determined by atomic force microscopy to be 0.5 nm.
X-Ray Photoelectron Spectroscopy (XPS): X-ray photoelectron spectroscopy (XPS) experiments were carried out using an S-Probe XPS instrument. The base pressure was below 10-9 Torr. XPS studies were performed using a monochromatized AlK1,2 X-ray source and an analyzer pass energy of 150 eV. The Au substrates and the CaF2 prism were mounted on standard sample stubs by means of double-sided adhesive tape and core-level spectra were recorded at a 55° photoelectron take-off angle. The photoelectron take-off angle is defined as the angle between the surface normal and the axis of the analyzer lens. The x-ray beam spot size was about 800 m and the x-ray power was 200 W. All binding energies (BEs) were referenced to the hydrocarbon C 1s peak at 284.6 eV. Atomic % compositions were calculated using the Hawk Data Analysis v7 software, which incorporates the appropriate sensitivity factors for the S-Probe XPS instrument.
Vibrational Sum-Frequency-Generation (SFG) Spectroscopy: SFG spectra were acquired using a picosecond Nd:YAG laser with a pulse duration of 35 ps at a repetition rate of 50 Hz. Visible (532 nm) light and tunable IR pulses are overlapped at the sample interface. The substrate films were deposited onto one side of an equilateral CaF2 prism, which was brought into contact with the sample solution in a Teflon liquid cell. The laser beams were brought in through the backside of the prism to probe the substrate/solution interface in situ in near-total internal reflection geometry. The visible and IR beams were overlapped at the sample spatially and temporally with incidence angles of 67° and 55° relative to the surface normal, respectively. The energy for both beams was 190–240 J per pulse in the CH and OH spectral regions and approximately 50 J per pulse for the IR beam in the SO spectral region. A spectral resolution of 2 cm-1 was used for the ppp polarization combination between 2800 and 3000 cm-1 with 200 shots accumulated at each wavenumber. For the ssp polarization combination between 2800 and 3850 cm-1 and 1000 and 1100 cm-1, the spectral resolution was 4 cm-1 with 100 shots accumulated at each wavenumber. All spectra were divided by the visible and IR intensities and plotted without further smoothing. The recorded SFG intensities ISFG in the SO region were fitted in accordance to the following equation:
where χ(2) is the second-order susceptibility, Ak the amplitude of the k-th resonance, ωk its frequency and δk the phase difference between substrate and resonant response. Γk represents the linewidth of the k-th vibration and Ivis, IIR the intensities of the two incident beams.
Surface Plasmon Resonance (SPR): The SPR sensorgrams were measured using a commercial T100 spectrometer with a 760 nm LED light source and a sample interface assembly (SIA Au kit). The SPR detection was based on the p-polarized reflected light from the AAm or HApp coated gold substrate, which was mounted in a micro-fluidic flow system. The reflection spectra are represented as a function of time. The flow rate was maintained at a slow speed (10 μL/min) to prevent the delamination of the plasma coated films. The data were obtained by first introducing pure water into the flow cell containing a fresh RFGD film, then switching from pure water to an SDS solution of known concentration while recording the SPR signal in real time. The SDS solutions were flowed over the samples for 30 s (SDS concentrations <2 mM) or 10 s (SDS concentrations >2 mM). The spectra were collected with a computer using the standard Biacore software. The SPR measurements were repeated three times for each SDS solution concentration. After each injection of an SDS solution, the flow system was completely rinsed with flowing pure water for 1 hour. Each SPR spectrum was collected using a fresh AAm or HApp film. Since the SPR signal did not fully saturate after 30 s of SDS adsorption for solution concentrations below 3 mM, the final signal was estimated by extrapolating the kinetic data using the following equation: y = y0 + Ae-x/t. The SPR signal values were then converted into adsorbed layer thickness, d (in nm) by using the following equation:
where ld is a characteristic decay length, S is the SPR sensitivity value, R is the SPR response, a and s are the refractive indices of the adsorbed film and SDS solution. The sensitivity value (1.041 × 106 SPR units/RIU) was determined from calibration using ethylene glycol/water solutions with known refractive indexes. The decay length is typically defined as 37% of the laser wavelength (760 nm). The SDS refractive index of 1.461 was used for a. Measured refractive indices of the SDS solutions showed them to be the same, within experimental error, of pure water. So the refractive index of pure water was used for s. Using the refractive index of solid SDS in equation (2) results in the calculation of an effective thickness, where the closer the structure of the adsorbed SDS film is to the structure of solid SDS the closer this effective thickness is to the actual film thickness. Further discussion regarding thickness calculations with equation (2) and the errors associated with those calculations are provided in reference 44.
Stability of the AAm and HApp films: The compositions of the RFGD coatings were examined by XPS before and after exposure to water and an 11 mM SDS solution to characterize their stability and adhesion to the Au and CaF2 substrates during the SFG and SPR experiments. The 11 mM SDS concentration were chosen to be above the cmc where the SFG signals are also maximized. The XPS experiments were done at 3 different spots on the same sample. The XPS data shows the composition of the pristine HApp films were very similar to those of films soaked in the SDS solution. However, the AAm/Au film composition changed slightly after the longest exposure time (30 min) to the SDS solution. Sulfur was detected (3.2 atomic %) with a S2p BE of 168.6 eV. Also, this SDS-soaked AAm films had an approximately 5 atomic % higher oxygen concentration, 5 atomic % higher carbon concentration and 10 atomic % lower nitrogen concentration than the unsoaked AAm film. The detection of a S2p peak at 168.6 eV and an increase in the oxygen signal are consistent with SDS being retained on the AAm surface. The significant decrease in nitrogen concentration along with a small increase in the carbon concentration also suggest partial removal of the outer AAm film could be occurring at long SDS exposure times. To minimize the effects of residual SDS and film erosion from previous exposures to SDS solutions, fresh RFGD films were used for each SDS solution concentration in the SPR experiments. Thus, the total exposure time of the RFGD films to the SDS solutions was < 1 min, well below the time where XPS detected changes in the AAm films. The C, N, and O atomic % values in Table 1 were renormalized to 100 atomic % to show the composition of the RFGD coating on the Au and CaF2 substrates. The only sample where substrate peaks were detected was the AAm/Au film after 30 min water exposure (0.4 atomic % Au). The only sample where sulfur was detected was also the AAm/Au film after 30 min SDS solution exposure (3.2±0.6 atomic % S). ppp polarization SFG spectra in the CH stretching region: After verifying with XPS that the RFGD films were stable on the CaF2 surfaces, SFG spectra were collected using the CaF2 prism with RFGD deposited films and the bare CaF2 prism. SFG spectra of AAm and HApp films, as well as bare CaF2 in increasing SDS concentrations are shown in Figure 3 (with as received SDS used to prepare the SDS solutions). The spectra were collected using ppp polarization combination (sum-frequency, visible and IR beams all p-polarized). Since AAm films are hydrophilic and the AAm film becomes charged at lower pH values, the SDS solution was maintained at a pH of 3.5. The spectra of the AAm coated substrate below 37 μM SDS concentration are featureless, while at 47 μM and higher SDS concentrations the spectra exhibit resonances near 2870 cm-1, 2934 cm-1, and 2958 cm-1 (data below 67 μM not shown). These modes can be assigned to the symmetric CH3 stretching vibration, a CH3 Fermi resonance and the asymmetric CH3 stretching vibration. Given that `as-received' SDS was used to form these films, the observed CH peaks for SDS solutions below 0.2 mM are likely due the presence of the highly surface active ordered fatty alcohol contaminants (e.g., dodecanol) and not ordered SDS. The spectra at both 0.2 mM and 1 mM solution concentrations show extremely weak signals. The presence of methyl bands and the almost absence of methylene resonances is indicative of well-aligned, all-trans hydrocarbon chains with few gauche defects (both dodecanol and SDS contain C12 alkyl chains). This is due to symmetry considerations of an alkane chain in an all-trans configuration. In this case a center of inversion is present between the methylene groups, making the methylene CH stretches SFG inactive. This is a consequence of the SFG selection rules, which precludes signals from molecules possessing inversion symmetry or isotropic arrangements. To verify that the transitional features observed on AAm are general phenomena and valid for other positively charged interfaces, we have also collected SFG spectra for SDS adsorption onto positively charged HApp surfaces. CH stretching SFG data for SDS adsorbed onto HApp surfaces are shown in Figure 3(B). The HApp films are more hydrophobic compared to the AAm films, but also remain uncharged at neutral pHs. Therefore, the SDS adsorption was done from aqueous acidic solution (pH of 3.5) as well, so the HApp films will have a positive charge. The spectra contain the expected CH3 resonances near 2871 cm-1, 2938 cm-1 and 2957 cm-1 similar to those recorded on AAm films. However, the signal to noise ratio is somewhat lower on the HApp films compared to the AAm films, which might be explained by a rougher surface. The main difference is that for HApp films the lack of SFG signal was observed at solution concentrations of 0.2 mM and between 1 and 5 mM, compared to solution concentrations of 0.2 and 1 mM for AAm films. To further confirm that the adsorption mechanism depends on SDS concentration, SFG spectra from SDS adsorption onto a CaF2 substrate were acquired at pH 3.Mucicarmine supplier 5 and pH 5.4. Streaming potential measurements indicate that the point of zero charge of CaF2 is at pH 6.2, so CaF2 should be positively charged at both pH 3.5 and 5.4. The SFG spectra at pH 5.4 and pH 3.5 are very similar. Only the 0.067 mM spectral intensity was observed to vary significantly, which is not unexpected since at this solution concentration the surface film should be predominately comprised of the fatty alcohol contaminants. SDS adsorption onto the CaF2 substrate exhibits no SFG signal for solution concentrations of 0.2 and 3.5–8 mM. In the other concentration ranges the peaks around 2870 and 2934 cm-1, and 2957 cm-1 appear with comparable intensities to those observed on the HApp and AAm films. For all samples studied the CH region exhibits two minima in spectral intensity. The first minimum is at 0.2 mM SDS solution concentration and is independent of the substrate used. The second minimum ranges from 1 to 5 mM SDS solution concentration, depending on the surface. The lack of an SFG signal can, in principle, be explained by a disordered layer, low surface coverage, or centrosymmetric molecular arrangements such as bilayers or monolayers with opposing head groups. It is also interesting to note that the shape of the three resonances (i.e., the relative signal phases) changes dramatically across the transitional concentrations above 0.2 mM and additionally, in the case of AAm, at concentrations above 11 mM. The phase changes of the SFG signals are indicative of structural changes in the film and will be further discussed in the section analyzing the SO3 stretching vibrations of SDS.
ssp polarization SFG spectra in the CH and OH stretching regions: The ordering of SDS and water at the AAm, HApp, and CaF2 surfaces was studied in the CH and OH regions using the ssp polarization combination. The SFG response in the OH stretch is generally stronger in ssp polarization compared to ppp polarization. For SDS solution concentrations above 1 mM the OH stretching regions exhibit broad resonances between 3000 and 3650 cm-1. The peaks observed at 2862 and 2932 cm-1 can be assigned to the CH3 symmetric stretching mode and the CH3 symmetric stretching Fermi resonance. Similar to the ppp polarization spectra, the presence of primarily methyl stretches in the C-H stretching regions indicates a considerable degree of alignment of all-trans fatty alcohol and SDS alkyl chains. The OH stretching signals only become significantly stronger at concentrations above 0.2 mM. This increase in intensity is most probably related to electrostatic interactions between the charged SDS molecules starting to assemble at the interface and polar water molecules in the vicinity of the interface. Increasing the SDS solution concentration leads to a minimum in intensity that is observed at 3 mM for films of AAm, 5 mM for HApp, 8 mM for CaF2 at pH 3.5, and 8 mM CaF2 at pH 5.4. This decrease in OH signal intensity is accompanied by a decrease in CH signal intensity, with the exception of CaF2 at pH 5.4. In this case the OH stretch minimum occurs at an SDS solution concentration of 8 mM, whereas the CH stretch has its minimum at an SDS solution concentration of 5 mM. Assuming that electrostatic interactions have the largest influence on the OH signal strengths, this suggests that the surface becomes neutralized near the SDS solution concentration range where the second minimum in the CH stretch is observed.
Contributions of contaminants to the SFG spectra: In both polarization contributions, a strong SFG CH signal was obtained from 67 μM SDS solutions and no SFG signal was observed from 0.2 mM SDS solutions. The appearance of signals below 0.2 mM are likely from impurities such as unsulfated alcohols, which have been observed in various commercial SDS products. These fatty alcohols can successfully compete with SDS for surface adsorption sites at SDS solution concentrations below 0.3 mM and remain in the surface film at least until the solution concentrations reaches the SDS cmc. The as-received SDS purity was 99%. To remove some of the fatty alcohol impurities from the `as-received’ SDS, solutions of recrystallized SDS were examined. The corresponding SFG CH spectra shown in Figure 5 are featureless for recrystallized SDS solutions below 0.2 mM, but exhibit CH vibrational peaks above 0.4 mM. No SFG signal was observed from 5 mM recrystallized SDS solutions, similar to `as-received’ SDS solutions (see Figure 3(C)). It is interesting to note that the CD region of the SFG spectrum recorded from a 67 μM fully deuterated SDS solution is featureless (see insert in Figure 5), as is the spectra from a 0.2 mM fully deuterated SDS solution (data not shown), suggesting the amount of deuterated fatty alcohol contamination in the deuterated SDS solutions is lower than the undeuterated fatty alcohol contamination in the as-received undeuterated SDS solutions. Since the fatty alcohol is a contaminant, it is not surprising its concentration varies among different types and sources of SDS. Thus, results from the recrystallized SDS and deuterated SDS solutions are consistent with SDS present on the surface only above SDS solution concentrations of 0.2 mM. Contaminants such as dodecanol from the as received SDS are likely the dominant adsorbed species contributing to the SFG spectra observed at solution concentrations below 0.2 mM. Dodecanol will continue to be present in the SDS films as the solution concentration is increased to the SDS cmc.
ssp polarization SFG spectra in the O–SO3 stretching region: While the previous paragraph discusses spectral contributions at higher frequencies, this section discusses the SFG O–SO3 SDS headgroup stretching vibrations between 1000 and 1150 cm-1. Figure 7 shows the ssp polarization SFG spectra recorded at the SDS/CaF2 interface at a fixed pH 3.5 value and different SDS concentrations. The first observation is the absence of SFG intensity at SDS solution concentrations 2 mM. This further confirms that SDS is not adsorbing onto the CaF2 surface in an ordered fashion at sufficient surface density from the low concentration SDS solutions. However, the presence of strong CH bands for the as received SDS solutions at concentrations below 0.2 mM indicates the existence of an ordered layer at these concentrations, but the absence of SO3 vibrations makes it unlikely that SDS is the source for these CH spectral features. Bain et al. previously showed that low concentrations of dodecanol form well-ordered films, suggesting dodecanol or similar fatty alcohol is responsible for the CH signals observed at solution concentrations below 0.2 mM made with as-received SDS. At solution concentrations above 0.2 mM the alkyl chains in both dodecanol and SDS will contribute to the CH signals. Furthermore, starting at 3.5 mM a double peak with frequencies of 1074 and 1087 cm-1 appears and continues to grow in intensity at concentrations where the CH signals are vanishing (between 5 and 8 mM, see Figs. 3 and 4). The presence of SO3 vibrations and the absence of the CH signals in this concentration range can, therefore, be related to a molecular SDS arrangement where the methyl groups are in a symmetric environment while the sulfate headgroups are in an ordered, non-symmetric environment. A symmetric methyl group arrangement can be accomplished by an equal number of opposing SDS chains. This can be realized in a bilayer or a monolayer with opposing orientations of SDS molecules. The most likely bilayer configuration would have the SDS headgroups in the inner layer facing the substrate and the SDS headgroups in the outer layer facing the water phase. The most likely monolayer configuration would require interdigitating SDS chains. If an SDS bilayer does form at solution concentrations between 3 and 8 mM, the reappearance of methyl vibrations at higher solution concentrations would suggest the formation of an additional layer on top of the bilayer as shown in Figure 6(A). This seems energetically unfavorable due to the adjacent location of the charged headgroups from the second and third layers as well as the hydrophobic chains extending into the water phase. A more plausible explanation is a single layer with opposing headgroups and interdigitating alkyl chains as shown in Figure 6(B). Other SDS adsorption models such as hemi-micelles have been considered (e.g., see references 11 and 26). However, the arrangement of the SDS molecules in these models is not consistent with the results of these study. So only the planar, layered models are discussed in detail here. As the effective surface charge becomes neutralized with increasing SDS adsorption, it becomes more favorable for SDS molecules to expose their headgroups towards the water phase. At the highest SDS solution concentrations (i.e., above 8 mM) this would lead to an unequal population of sulfate headgroups pointing towards the water phase and methyl groups pointing towards the substrate, thus, breaking the methyl group symmetry and resulting in the reappearance of the CH bands. At the SDS solutions above the cmc the presence of fatty alcohol molecules in the film should be minimized since Bain et al have shown that above the cmc dodecanol mostly resides in the SDS micelles. Following this, SDS continues to adsorb with both headgroups pointing towards the substrate as well as headgroups pointing towards the water phase. As the sulfate headgroup is experiencing a different environment in both orientations, e.g. one is exposed to a charged surface while the other is adjacent to water, their vibrational stretching frequencies are different. It has been shown that for SDS at the air/water interface with the headgroups pointing towards the water phase, only a single peak close to 1070 cm-1 is observed. Therefore, we assign the 1074 cm-1 peak to sulfate groups pointing towards the water phase and the 1087 cm-1 peak to sulfate groups pointing towards the substrate. The double peak also explains the absence of an intensity minimum in the SO3 vibrations (see Figure 7), in contrast to the intensity minima observed in the CH vibrations. Fig. 8 shows also a curve fit of these spectra using Equation (1). The two peaks can be separated and their phase can be determined. Before discussing the phase, it is important to realize that nonpolar methyl groups are not affected by the surface charge or the presence of the water phase. Therefore, an opposing orientation would result in opposing orientation of IR transition dipole moments (TDMs) and, therefore, a phase shift around π that leads to a cancellation of the signals (destructive interference). The same holds true for the headgroups, except that the energy is split. This is further illustrated in Figure 8 showing the phases from the 2 peaks as well as their intensities as a function of solution SDS concentration. Fitting 2 peaks with similar phases was not possible with physically meaningful peak widths. The difference in phases is close to 120° indicating a change in sign of χ(2) as a result of an opposing orientation of TDMs, e.Ethyl 6-bromohexanoate supplier g.PMID:35059429 upward and downward. Most importantly, the energetic splitting does not lead to a vanishing signal, even if the phases are closer to 180° than to 0°. The 2 peaks located at 1074 and 1087 cm-1 become easily visible at 3.5 mM SDS solution concentration. Both contributions are increasing with SDS solution concentration until the cmc is reached. (Note that, in consideration of the error bars, a thorough quantitative analysis of relative headgroup orientations, i.e. if more SDS molecules are pointing up than down, cannot be performed unambiguously.) Finally, the signal becomes generally lower at 20 mM SDS solution concentrations, possibly due to the presence of micelles in the vicinity of the interface disrupting the organization of the SDS monolayer or due to the release of the remaining dodecanol from the SDS monolayer. As a summary of the SFG spectroscopy data, SDS only starts to adsorb at the CaF2 interface for solution concentrations of 0.2 mM and above. Below these solution concentrations fatty alcohol impurities (i.e., dodecanol) are absorbing at the interface that are responsible for the CH signals. Above 0.2 mM contributions of SDS with the headgroup orientated towards the substrate lead to neutralization of the surface charge. In addition SDS with headgroups orientated towards the water interface are present. The doublet SO3 peaks are increasing with SDS solution concentration while the CH peak intensities reach a minimum, the latter due to destructive interference of opposing methyl group TDMs at the same vibrational frequency. Above this concentration micelles are formed in the solution phase. This will be discussed further in a following section. While the SFG data, which provides information about the molecular structure of the SDS layer suggests different headgroup orientations, it is not sensitive to the thickness of the corresponding layer (e.g., whether the arrangement corresponds to a bilayer or a monolayer with opposing headgroup orientations). The amount of material in the SDS layer needs to be determined with a technique sensitive to the surface coverage and thickness of the SDS layer.
SPR data of SDS adsorbed to AAm and HApp films: The SPR results in Figure 9 show that a small amount of material is adsorbed on the films at SDS solution concentrations below 0.2 mM. As-received SDS was used for these experiments, so the SPR signal detected from these solutions is likely due to adsorption of the fatty alcohol contaminant in the as-received SDS. The SPR signal shows a slight decrease near a solution concentration of 0.2 mM where the SFG CH intensity disappears, likely due to SDS molecules beginning to co-adsorb with the fatty alcohol, causing the adsorbed fatty alcohol to disorder and suggesting that this change in the structure and morphology of the adsorbed film could be responsible for the slight decrease in the SPR signal. Most importantly, the surface concentration of the adsorbed species is sub-monolayer at SDS solution concentrations below 1 mM (see next paragraph), ruling out the possibility that the minimum in the SFG CH intensity observed for solution concentrations of 0.2 mM is due to formation of an SDS monolayer, as proposed in previous studies. Above solution concentrations of 0.2 mM the SPR signal continues to increase as the solution concentration increases. From SDS solution concentrations of 0.2 mM up to the cmc, this increase is primarily due to an increase in adsorbed SDS, consistent with the observed increase in the SFG sulfate peaks. Above the cmc the further increase in the SPR signal is likely due to the presence of micelles in the region sampled by the evanescent wave, since its sampling depth extends beyond the adsorbed SDS film and into the liquid phase. The thickness of the adsorbed film was determined by converting the SPR response to a thickness using equation (2). Based on the length of the SDS molecule an SDS monolayer should have a thickness between 1.5 and 2 nm, depending on the exact tilt of the SDS molecules from the surface normal. The calculated thicknesses for the absorbed film only reach the lower limit of this range (1.5 nm) at an SDS solution concentration of 11 mM. Thus an SDS monolayer is not formed until at or slightly above the SDS cmc. At the lower SDS solution concentrations the thicknesses are below those expected for a monolayer (e.g., ~0.6 nm for 1 mM SDS and ~1 nm for 3 mM SDS). Thus, at SDS solution concentrations where the second minimum in the CH intensity occurs (1–5 mM) the SDS surface coverage is still in the submonolayer to monolayer regime and well below the thickness expected for an SDS bilayer (~3.5 nm).
SDS adsorption model: Monolayer with opposing headgroups: In Figure 10 a model is proposed for how the SDS monolayer with opposing headgroup orientations is formed as the SDS solution concentration increases. With increasing number of adsorbed SDS molecules, the charged substrate becomes neutralized. Since the tail groups of SDS are hydrophobic, further adsorption leads to an SDS monolayer with opposing headgroup orientation. This is supported by (i) a minimum intensity for the methyl vibrations because of an equal number of methyl groups in downward and upward orientations in the monolayer (between solution concentrations of 3 and 8 mM), (ii) SFG spectral analysis for the SO3 band that exhibited two contributions with positive and negative phases (see Figure 8). Since SDS is in an upright orientation in the 3 to 8 mM concentration range, this indicates one sulfate group points toward liquid phase and the other sulfate group binds to the substrate. Finally, the surface coverage determined by SPR corresponds to a monolayer rather than a bilayer, all of which is consistent with a model where an SDS monolayer is formed at the interface with opposing headgroup orientation. The observed decrease in both SO3 and CH band above 14 mM can be interpreted as a disordering of the sulfate and methyl groups. These solutions are well above the cmc, so it is likely the micelles present in solution may pass close enough the SDS monolayer to influence its order and orientation. For example, the sulfate headgroups from the SDS layer that point towards the water phase could experience an electrostatic repulsive interaction with the charged sulfate groups on the outer surface of the micelles, resulting in a disordering of those SDS molecules. Another possible explanation for the decrease in SO3 and CH band intensities above 14 mM is that removal of dodecanol from the adsorbed monolayer at these SDS solution concentrations causes some disordering of the adsorbed SDS molecules.
Conclusion: In summary, based on the SFG and SPR results, the formation of films from solutions made with as-received SDS (i.e., SDS containing fatty alcohol contaminants) on positively charged surfaces exhibits five structural phases. (i) c<57 μM: At micro-molar solution concentrations the film coverage and order is very low, if any. The ordered species observed at these low concentrations are likely due to contaminants such as fatty alcohols adsorbing onto the substrate. (ii) 57 μM