Solid-state Carbon-13 Research Articles

Direct oxidation of alcohols to carboxylic acids using simple and economical Pd@Glu-HTC catalyst: Practical and scalable approach towards biomass based value added chemicals

Sustainable catalytic transformation of bio-based alcohols to high value-added fine chemicals is an important topic of research. This work described preparation of simple and economical Pd@Glu-HTC catalyst from biomass derived low cost d-glucose. Hydrothermal carbonization of glucose was carried out in first step to synthesize Glu-HTC support in a simpler, greener, economical and efficient manner followed by incorporation of palladium metal on surface of the catalyst in second step. The catalyst was characterized using techniques such as Fourier Transform Infrared Spectroscopy (FT-IR), Solid-state Cross-Polarization Magic Angle Spinning Carbon-13 (13C CPMAS), Energy-dispersive X-ray spectroscopy (EDAX), Powder X-ray diffraction (P-XRD), X-ray photoelectron spectroscopy (XPS), Thermogravimetric/Differential Thermal Analyzer (TG-DTA), Field emission scanning electron microscopy (FE-SEM) and High-resolution transmission electron microscopy (HR-TEM). The catalyst was evaluated for direct oxidation of alcohols to yield carboxylic acids and exhibited very good catalytic activity for wider substrate scope. Oxidation of alcohols was carried out using milder base, molecular oxygen and water as a solvent to achieve 92–99 % excellent yields. The practical utility of current strategy was also studied for gram scale synthesis of bio-based value added industrially important chemicals such as furoic acid (flavouring agent and preservative in industry), 2, 5-furan-dicarboxylic acid (monomer to 100 % fossil-free, recyclable polymer polyethylene furanoate (PEF), tetrahydro-2-furoic acid (production of many drugs) and vanillin (important product of flavor and fragrance industry). Pd@Glu-HTC catalyst was found to be reusable for four recycles and the catalytic performance was retained without any loss in its activity after four cycles.

Proteomic Characterization of Human Placenta: Insights into Potential Therapeutic Applications for Osteoarthritis.

Biologics have become increasingly prominent as therapeutics in recent years due to their innate immune-privileged nature, biocompatibility, and high levels of protein biofactors. The aim of the study is to characterise the biologic, lyophilized human placenta (LHP) and explore its therapeutic potential for osteoarthritis (OA). The presence of six bioactive constituents that regulate cell-extracellular matrix interaction was identified by liquid chromatography coupled to electrospray ionization and quadrupole time-of-flight mass spectrometry (LC-ESI-QTOF/MS). Metalloproteinase inhibitor 3 (TIMP3), alpha-1 anti-trypsin (a1AT), basic fibroblast growth factor (bFGF), and transforming growth factor β1 (TGFβ1) were detected and quantified using ELISA. The total protein content present in LHP by Bradford assay was found to be 409.35 ± 0.005μg/ml. The analytical techniques such as Attenuated Total Reflectance-Fourier Transform Infrared spectroscopy (ATR-FTIR), solid state carbon-13 Nuclear Magnetic Resonance (ssC13 NMR) spectroscopy, and Differential Scanning Calorimetry (DSC) revealed the secondary structure and conformational stability of LHP. X-Ray diffraction (XRD) studies showed its amorphous nature. Bioactivity assessment of LHP was performed in human keratinocytes (HaCaT) and human dermal fibroblasts (HDF) by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The LHP was highly proliferative against skin cells and non-toxic, based on the findings of the bioactivity assay. LHP has the potential to be used as a therapeutic agent for OA, as its characterisation unveiled its physical stability, significant concentration of bioactive components that are pertinent to cartilage repair and its conformational stability.

Dendrimer Coated Silica as a Sorbent for Dispersive Solid-Phase Extraction of Select Non-Steroidal Anti-Inflammatory Drugs from Water.

Non-steroidal anti-inflammatory drugs (NSAIDs) have been recognized as a potentially serious threat to the natural environment. NSAIDs are popular painkillers, and the main pathway for them to reach natural water is via discharge from wastewater and sewage treatment plants. In order to monitor contamination caused by these drugs, as well as their impact on the environment, a new material based on Silica Gel 60, functionalized with a dendrimeric copolymer of methylamine and 1,4-butanediol diglycidyl ether (named MA-BDDE), was prepared. Initial physicochemical characterization of the MA-BDDE material was carried out using ATR FT-IR spectroscopy as well as solid-state carbon-13 NMR spectroscopy. Its effectiveness at NSAID extraction was evaluated by the application of five select drugs in dispersive solid-phase extraction (dSPE): aspirin, ketoprofen, naproxen, diclofenac and ibuprofen. This was followed by their simultaneous determination using the HPLC-UV/Vis system demonstrating good sensitivity, with limits of detection values within the 63-265 ng mL-1 range. A comparison of the sorption capacity of each pharmaceutical with unmodified base silica showed an at least tenfold increase in capacity after modification. Initial MA-BDDE application in a quick, low-waste extraction procedure of those select NSAIDs from spiked surface water samples yielded promising results for its use as a sorbent, as recovery values of analytes adsorbed from various samples were found to exceed 72%.

Organic matter from water treatment residuals stabilizes amorphous iron to maintain phosphorus adsorption by iron hydroxides

Functional materials enriched with amorphous iron are commonly used for environmental remediation. However, their performance is often susceptible to aging and reductive dissolution effects. Amorphous iron in drinking water treatment residuals (DWTRs) primarily exists in organic matter-bound forms, possessing a stable phosphorus adsorption capability. In this study, in-depth investigations were conducted to evaluate the effect of organic matter extracted from DWTRs on amorphous iron stability in iron hydroxides by using incubation tests for 180 days. The results showed that organic matter from different DWTRs exhibited similar FTIR and XPS patterns; whereas, the solid-state carbon-13 NMR analysis and the carbon/nitrogen atom ratios indicated variations in structures and origins. Notably, organic matter addition increased the 0.01 and 0.005 M-oxalic acid-ammonium oxalate extractable iron, especially the later one which was greater than the control by approximately 24 times, inhibiting iron aging in iron hydroxides. The addition also promoted the transformation of iron from acid-soluble and reducible to oxidizable and non-European Community Bureau of Reference method-extractable forms, reducing the iron release potential. This effect was intensified when organic matter contained relatively low levels of carboxylic carbon and nitrogen/oxygen-alkyl carbon while high levels of carbon=oxygen/ketone carbon. The inhibited iron aging further led to stronger phosphorus adsorption capabilities of iron hydroxides, and the reduced iron release potential caused the adsorbed phosphorus to exist in more stable forms. Therefore, organic matter from DWTRs stabilized amorphous iron to maintain phosphorus adsorption capabilities of iron hydroxides. These mechanisms show potential for application in the development of iron-based materials.

Preparation of chitosan-graft N-hydroxyethyl acrylamide copolymers as an in vitro-engineered skin

Amino-polysaccharides derivatives such as grafted chitosan (CS) have attracted increasing interest in biotechnology. The work aims to assess the novel CS-g-PHEAA as a model for in vitro-engineered skin. Gamma radiation-induced copolymerization has achieved N-hydroxyethyl acrylamide (HEAA) grafting onto CS. The functionalization of CS by the amino group has been confirmed through solid-state carbon-13 nuclear magnetic resonance. As evidenced by cell metabolic activity, viability, and proliferation results of human dermal fibroblast (HDF) co-cultivated with the biomaterials, the biological assays are independent of the choice of solvent. Also, the grafting degree, the HDF growth, and monolayer migration favored using CS-g-PHEAA prepared in bulk. Hence, it has potential as a future bioengineered skin substitute material.

Mechanism of carbon structure transformation in plastic layer and semi-coke during coking of Australian metallurgical coals

The transformation of the carbon structure of coal during the coking process influences the coke microstructure and microtexture, and ultimately, the coke quality. This study investigates the impacts of parent coal properties on the evolution of carbon structures of selected Australian coals from the plastic layer to semi-coke stage during coking using electron spin resonance (ESR), Synchrotron attenuated total reflection Fourier transform infrared microspectroscopy (Synchrotron ATR-FTIR) and solid-state carbon-13 nuclear magnetic resonance (13C NMR) analyses. The stable radical concentration, the g-value, and the linewidth measured by ESR were combined with IR and Solid-state 13C NMR results to improve the understanding of carbon structural transformation at the plastic and post-resolidification stages of coke formation. In addition, micro gas chromatography (micro-GC) was used to study the evolution of gaseous species. The results suggested that the coal undergoes crosslinking reaction, condensation, and re-polymerization within the thermoplastic range, resulting in loss of oxygen to form condensed carbon-bearing crosslinking structures. Due to differences in their chemical structure, macerals significantly influenced crosslinking structures during plastic layer formation. Higher rank coals generated more stable radicals in the plastic phase due to their lower H/C and O/C ratios than low-rank coals with higher vitrinite contents. Lower fluidity and lower rank coals formed oxygen-bearing cross-links at the early plastic stages, hindering fluidity development and carbon ordering at high temperatures. Above the resolidification point, the continuous transformation of C-O and C–H bonds to C–C bonds was accompanied by the release of H2 and CO2, leading to increased ordering and anisotropy of coke carbon structures.

Phenoloxidase activity and organic carbon dynamics in historic Anthrosols in Scotland, UK.

Phenolic compounds are chemical precursor building blocks of soil organic matter. Their occurrence can be inhibitory to certain enzymes present in soil, thereby influencing the rate of decomposition of soil organic matter. Microbe-derived phenoloxidases (laccases) are extracellular enzymes capable of degrading recalcitrant polyphenolic compounds. In this study, our aim was to investigate the relationships between phenoloxidase enzyme activity, organic carbon content and microbial abundance in the context of long-term anthropogenically amended soils. To achieve this, we used a series of complementary biochemical analytical methods including gas chromatography, enzyme assays and solid-state Carbon-13 Cross Polarisation Magic-Angle Spinning Nuclear Magnetic Resonance Spectroscopy (13C CPMAS NMR). Using several anthrosols found in St Andrews (Scotland, UK) that had been subjected to intense anthropogenic modification since the medieval period (11th century AD) to present-day, we were able to scope the impact of past waste disposal on soils. The long-term anthropogenic impact led to organic matter-rich soils. Overall, phenoloxidase activity increased by up to 2-fold with soil depth (up to 100 cm) and was inversely correlated with microbial biomass. Solid-state 13C NMR characterisation of carbon species revealed that the observed decline in soil organic matter with depth corresponded to decreases in the labile organic carbon fractions as evidenced by changes in the O/N-alkyl C region of the spectra. The increase in phenoloxidase activity with depth would appear to be a compensatory mechanism for the reduced quantities of organic carbon and lower overall nutrient environment in subsoils. By enzymatically targeting phenolic compounds, microbes can better utilise recalcitrant carbon when other labile soil carbon sources become limited, thereby maintaining metabolic processes.

Characterisation of compound middle lamella isolated by a combination of wet-beating, sedimentation, and methanol dialysis

Abstract A simple method was developed to isolate the compound middle lamella (CML) in its natural state using wet-beating, sedimentation, and methanol dialysis. The isolated CML fraction was characterised. The CML was isolated in 2.0% yield from the mature wood of Ginkgo biloba. Scanning electron microscopy (SEM) images of the isolated CML fraction showed the presence of flake-like structures. However, some small amounts of secondary wall substance was observed. The lignin content of the CML fraction was approximately 45%. Thioacidolysis reactions with the isolated CML fraction revealed that the lignin β-O-4 structure was scarce in the isolated fraction and that numerous structures with two or more consecutive condensed-type bonds were present. Results from the solid-state carbon-13 nuclear magnetic resonance (13C NMR) experiments indicated that the CML fraction had low crystallinity, indicating low cellulose content. Liquid-state NMR analysis of the lignin from the CML fraction revealed the absence of p-hydroxyphenyl (H) units.

Decomposition of woody debris in Mediterranean ecosystems: the role of wood chemical and anatomical traits

Data about woody debris (WD) decomposition are very scarce for the Mediterranean basin. The specific aim of this work is to explore the relationships between WD traits with the decay rate. We carried out a three-year litterbag decomposition experiment using ten WD types incubated in two plant communities (i.e. shrubland and woodland) and in laboratory conditions. WD was characterized for 31 chemical and anatomical traits, including macro- and micronutrients, lignin, and cellulose as well as organic chemistry by Solid-state Cross-Polarization Magic Angle Spinning Carbon-13 Nuclear Magnetic Resonance (13C CPMAS NMR) and Fourier transform infrared spectroscopy/ Attenuated Total Reflection (FT-IR/ATR spectroscopy). WD decay rate was negatively associated with di-O-alkyl, lignin/N and C/N ratios, but positively with N concentration. Less consistent but positive correlations were recored for K, Mn, and Na concentration. The alkyl C and carboxylic C regions, associated with aliphatic and amide compounds, was positively correlated with WD decomposition. Conversely, di-O-alkyl C and O-alkyl C fractions, largely associated with cellulose and hemicellulose, were negatively correlated with WD decay rate. Finally, the positive correlation between Na concentration and WD mass loss in field conditions suggest a role of this neglected micronutrient for wood decomposition. WD specific density and anatomical features, have a minor capability to explain decomposition rate. Our findings demonstrate a major role of WD chemical traits in explaining the variability of decomposition in Mediterranean ecosystems.

13C CP/MAS NMR Can Discriminate Genetic Backgrounds of Rice Starch

Solid-state cross-polarization magic-angle spinning carbon-13 nuclear magnetic resonance (13C CP/MAS NMR) spectroscopy is used to analyze starch derived from plants including wheat, maize, and potato, but few reports have described its application to rice starch. Here, we combined 13C CP/MAS NMR with deconvolution and subtraction methods to analyze rice lines including mutants that are deficient in at least one enzyme involved in amylose and/or amylopectin biosynthesis. We found that differences in the content of ordered structures between rice lines could be evaluated using C1 signal deconvolution and subtraction. The content of the V-type ordered structure increased with increasing amylose content. Furthermore, starch derived from a starch synthase (SS) IIIa/starch branching enzyme (BE) IIb-deficient mutant formed B- and V-type ordered structures and significantly more nonordered structures than the other rice lines. These data indicate that 13C CP/MAS NMR analysis is useful for discriminating the genetic backgrounds of starch derived from different rice cultivars.

Development and Evaluation of Matrices Composed of β-cyclodextrin and Biodegradable Polyesters in the Controlled Delivery of Pindolol.

Polymer-drug conjugates are currently being more widely investigated for the treatment of hypertension. In view of the above, in the first stage of our work, we used nontoxic β-cyclodextrin (β-CD) as effective, simple, inexpensive, and safe for the human body initiator for the synthesis of biocompatible and biodegradable functionalized polymers suitable for the medical and pharmaceutical applications. The obtained polymeric products were synthesized through a ring-opening polymerization (ROP) of ε-caprolactone (CL), d,l-, and l,l-lactide (LA and LLA). The chemical structures of synthesized materials were elucidated based on 1H NMR and solid-state carbon-13 cross-polarization/magic angle spinning nuclear magnetic resonance (13C CP/MAS NMR) analysis, while the incorporation of β-CD molecule into the polymer chain was confirmed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). Furthermore, molecular modeling has been applied to investigate the intrachain rigidities and chain architectures for several representative structures. The obtained and thoroughly characterized branched matrices were then used to generate the first β-cyclodextrin/biodegradable polymer/β-blocker conjugate through the successful conjugation of pindolol. The conjugates were fabricated by carbodiimide-mediated coupling reaction. The branched biodegradable materials released the drug in vitro in a sustained manner and without “burst release” and thus have the ability to treat different heart diseases.

Biodegradable polyurethanes foam and foam fullerenes nanocomposite strips by one-shot moulding: Physicochemical and mechanical properties

Biodegradable polyurethanes foam as well as foam strips have been formulated by the renowned one-shot process. Commercially available biodegradable trifunctional polyol, polycaprolactone triol (major, ~77 wt%) and isophorone diisocyanate were chosen to react in the presence of other essential components to formulate the PU foams from the paper cup as well as foam strips using mould at ambient temperature. The formulation of foams has been efficiently attempted by successive steps by alteration of the components such as chain extender, quantity of water and isocyanate index. In situ addition of fullerenes (0.1, 0.4 and 1.0 wt%) into the composition of PU foam strips formulation in mould found to afford the three samples of PU-fullerenes nanocomposite. In case of polyurethanes-fullerenes nanocomposite foam strips, nanomaterial of fullerenes was identified to disperse uniformly in the bulk of sample to incorporate the enhancement in physicochemical and mechanical properties value as compared to the control PU foam strip. The battery of analyses such as physicochemical, morphological and mechanical properties was studied for each foam samples, including control and PU-fullerenes nanocomposite strips. Solid state carbon-13 nuclear magnetic resonance spectral data for the foam revealing desired properties was collected to corroborate the chemical functional groups in the molecular structure network.

Hydrothermal Synthesis and Structure of a Dinuclear Molybdenum(III) Hydroxy Squarate with a Mo-Mo Bond.

The reaction of molybdenum(II) and chromium(II) acetates with squaric acid in degassed and deionized water under hydrothermal conditions at 150 °C is described. The products have been formulated as M2(μ-OH)2(μ-C4O4)2(H2O)4·2H2O, where M = Cr (1) and Mo (2), based on combustion elemental analysis, infrared spectroscopy, magic angle spinning (MAS) solid-state carbon-13 nuclear magnetic resonance (NMR), and single-crystal X-ray diffraction. The edge-shared bioctahedral structures involve doubly bridging hydroxide ligands and μ-squarate ligands. The chromium compound lacks a direct metal–metal-bonding interaction, while in contrast the molybdenum compound contains a Mo–Mo bond [2.491(2) Å]. The nature of the Mo–Mo-bonding interaction is compared with that of other similar d3–d3 dimers.

Amido-amine derivative of alginic acid (AmAA) for enhanced adsorption of Pb(II) from aqueous solution

The present work reports the alternate synthesis of amido-amine derivative of alginic acid (AmAA) with high degree of functionalization. The AmAA have been characterized for percentage functionalization, functional group change, surface morphology and thermal decomposition behavior. The results indicate that the amido-amine derivatisation of alginic acid (AA) with >95% functionalization, significantly improves its Pb(II) adsorption efficiency (395.72 mg/g to 535.87 mg/g) over the AA. The equilibrium and kinetic studies showed that Langmuir and Freundlich adsorption isotherm models fitted well to the experimental data, and these followed pseudo-second order kinetic model. The FTIR (Fourier transform infrared spectroscopy) and 13C CP-MAS NMR (Cross-polarization magic angle spinning carbon-13 solid state nuclear magnetic resonance spectroscopy) analysis revealed that Pb(II) binds to the carboxyl group in case of AA and to the carbonyl & amine group in case of AmAA, which leads to increase in its adsorption efficiency. The study concludes that the functionalization of amido-amine on AA improves its adsorptive efficiency for Pb(II) from aqueous medium.

Chelating modified cellulose bearing pendant heterocyclic moiety for effective removal of heavy metals.

Cellulose bearing pendant Schiff base with heterocyclic chelating groups (CMC-Bz) was synthesized, which were fully characterized using various instrumental techniques such as solid state carbon-13 nuclear magnetic resonance (13C-NMR), Fourier transform infrared (FTIR), scanning electron microscope (SEM) and energy dispersive X-ray analysis (EDX) spectra. The adsorption of toxic metals onto cellulosic material was tested in a batch mode operation. The adsorption functional factors such as pH, adsorbent dose, metal ion concentration, equilibrium time and temperature were experimentally optimized for the maximum removal of Cu(II) and Pb(II) ions. Adsorption isotherms were evaluated with Langmuir, Freundlich, Temkin and Redlich-Peterson isotherms. Kinetic parameters and equilibrium adsorption capacities were investigated for pseudo-first-order, pseudo-second-order and intra-particle diffusion models. Thermodynamic parameters and reusability were also evaluated.

Differences in swelling of chemical pulp fibers and cotton fibers – Effect of the supramolecular structure

The swelling and dissolution of cellulose are key parameters in the production of regenerated cellulose fibers. Since cotton is almost pure cellulose, it has been proposed that the recycling of cotton textiles may be accomplished through incorporating the cotton textiles into the production of regenerated cellulosic fibers. In this study, the supramolecular structure before and after pretreatment was characterized using solid-state carbon-13 nuclear magnetic resonance (13C NMR), and the findings related to the swelling of dissolving pulp and cotton were quantified with a fiber analyzer. The cotton and dissolving pulp samples were subjected to three different pretreatments: mild acid hydrolysis, acid hydrolysis in ethanol, and a hydrothermal treatment. The results showed that cotton was harder to swell than the dissolving pulp. This indicated that either waste cotton requires another type of activating pretreatment than those included in this study or very good solvents if it is to be included in the production of regenerated fibers.

Spectroscopic-Chemical Fingerprint and Biostimulant Activity of a Protein-Based Product in Solid Form.

A solid biostimulant (AA309) obtained through thermobaric hydrolysis applied on trimmings and shavings of bovine hides tanned with wet-blue technology was chemically characterized, and its effects in maize (Zea mays L.) were evaluated. AA309 contained 13.60% total nitrogen (N), mainly in organic forms (13.40%), and several amino acids, especially lysine, phenylalanine, glycine, aspartate, and isoleucine. AA309 was further analyzed using Fourier Transform Infrared (FT-IR) spectroscopy, which revealed the presence of amide I and amide II bands, indicative of peptide structures. When supplied to maize plants for 15 days at two N dosages (2.1 or 4.2 mg/kg), AA309 induced positive physiological responses, likely because of its content in amino acids functioning as signaling molecules. The low dosage was the most effective in improving leaf (+24%) and root (+98%) dry weight, photosynthetic activity (+70%), and accumulation of N (+80%), proteins (+65–75%) and antioxidants (+60%). Spectroscopic analyses (Solid-state Cross-Polarization Magic Angle Spinning Carbon-13 Nuclear Magnetic Resonance, CP/MAS 13C–NMR, and High resolution-magic angle spinning nuclear magnetic resonance, HR-MAS NMR) on plant tissues revealed the increase in proteins, lignin structures and cutin in AA309-treated plants compared to untreated plants. Our results indicate that AA309 could be used as a valuable biostimulant in agriculture.

Effect of composition and microstructure of humic acid on 3,3',4,4'-tetrachlorobiphenyl sorption.

The fate and transport of polychlorinated biphenyls (PCBs), a class of persistent organic compounds, in soils was markedly affected by their sorption/desorption on soil organic matters (SOM) due to high hydrophobicity of PCBs. Humic acid (HA), an important fraction of SOM, has no steady composition and microstructure from different origins, resulting in their diverse sorption capacity. Therefore, the effect of composition and microstructure of HA on sorption of 3,3',4,4'-tetrachlorobiphenyl (PCB77) was investigated in this study. The primary sorption mechanism of PCB77 on HAs was also demonstrated by using Fourier transform infrared (FTIR) and solid-state carbon-13 nuclear magnetic resonance (13C-NMR) spectroscopy analysis. Elemental content analysis of three HAs showed that the content of unsaturated C (the total of aromatic, carboxylic, and carbonyl carbon) followed an order of YHA>SHA>AHA. Furthermore, YHA and SHA had remarkably rough and complicated particle surfaces but AHA had relatively smooth surface and the well-proportioned and interspersed particle sizes. The results of the sorption experiment showed that the sorption capacity of PCB77 on HAs followed a similar order of YHA>SHA>AHA, indicating the content of unsaturated C of HAs controlled the sorption of PCB77 on HAs. Sorption of PCB77 on either AHA or SHA did not change with increasing ionic strength of background solution, implying that there was no H-bond or electrostatic interaction between PCB77 and HAs. The result of FTIR and 13C-NMR spectra showed the primarily possible mechanism was π-π conjugative interaction and hydrophobic binding between PCB77 and HAs.

Electrooxidative Copolymerization of 3,4-Ethylenedioxithiophene and Benzene from a Mixture of Each Monomer

Abstract Electrooxidative copolymerization of 3,4-ethylenedioxithiophene (EDOT) and benzene from a mixture of the monomers with concentrations of 5 mM EDOT and 5 M benzene produced copolymer films on an anode surface. The formation of copolymer films and characterization of their electrochemical properties was confirmed using Fourier transform infrared (FTIR) spectroscopy, ultraviolet–visible absorption (UV–vis) spectroscopy, energy dispersive X-ray (EDX) spectroscopy, cyclic voltammetry (CV), and solid-state carbon-13 nuclear magnetic resonance (13C NMR) spectroscopy measurements. Furthermore, the copolymerization ratio could be controlled by selection of the oxidation potential applied for copolymerization.

Sex-related chemical differences in keratin from fingernail plates: a solid-state carbon-13 NMR study

The carbon-13 solid-state NMR reveals chemical differences in fingernail keratin between young, healthy males and females.