Number Of H-bonds Research Articles

Effect of Adsorbed Alcohol Layers on the Behavior of Water Molecules Confined in a Graphene Nanoslit: A Molecular Dynamics Study.

With the rapid development of a two-dimensional (2D) nanomaterial, the confined liquid binary mixture has attracted increasing attention, which has significant potential in membrane separation. Alcohol/water is one of the most common systems in liquid-liquid separation. As one of the most focused systems, recent studies have found that ethanol molecules were preferentially adsorbed on the inner surface of the pore wall and formed an adsorbed ethanol layer under 2D nanoconfinement. To evaluate the effect of the alcohol adsorption layer on the mobility of water molecules, molecular simulations were performed to investigate four types of alcohol/water binary mixtures confined under a 20 Å graphene slit. Residence times of the water molecules covering the alcohol layer were in the order of methanol/water < ethanol/water < 1-propanol/water < 1-butanol/water. Detailed microstructural analysis of the hydrogen bonding (H-bond) network elucidated the underlying mechanism on the molecular scale in which a small average number of H-bonds between the preferentially adsorbed alcohol molecules and the surrounding water molecules could induce a small degree of damage to the H-bond network of the water molecules covering the alcohol layer, resulting in the long residence time of the water molecules.

In silico designing of peptide inhibitors against pregnane X receptor: the novel candidates to control drug metabolism

Drug-induced pregnane X receptor (PXR) activation may be unavoidable with some treatments. Besides, drug-induced PXR activation may lead to drug interactions with a number of treatments. Human PXR antagonists represent a means to counteract such interactions. Indeed, the ideal small molecule would have good drug-like properties, but be devoid of nonspecific cell-target effects and ability to induce cytotoxicity. Researchers assess on the possibility of deriving inhibitory peptides, which are derived from interfaces of “protein–protein interactions” (PPIs) could inhibit interactions of their origin domain by mimicking its mode of binding to cognate partners. Such peptides could serve as promising leads for rational design of inhibitory drugs. In this study, In silico designing of peptide inhibitors and peptidomimetics against PPIs was done to prevent unwanted drug metabolisms caused by PXR activation. In sum, no designed peptides had considerable higher energy of interactions and more H-bonds with the receptor compared to the SRC-1 peptides. Hence, none of the designed peptides are predicted to be more active against the target receptor. However, compared to the SRC-1 peptide, a designed peptidomimetic (mimetic3) had considerable higher energy of interactions with the receptor. It formed five H-bonds with PXR that was equal to the number of H-bonds that SRC-1 formed with the receptor. Therefore, by designing such mimetics, it would be probable to have a realistic chance of achieving peptides with improved binding affinity compared to SRC-1 and consequently prevent the unwanted drug metabolisms caused by PXR activation.

A Molecular Dynamics Simulation Study on the Relationship between Hydrogen Bond and Damping Properties of AO-70/NBR Composites

This work was try to study the number and types of hydrogen bonds (H-bonds) formed in hindered phenol AO-70/nitrile butadiene rubber (NBR) composites and their contributions to the damping properties by molecular dynamic (MD) simulation and experimental methods. MD simulation results showed that there were four types of H-bonds, namely, type A (AO-70) –OH...NC– (NBR) H-bonds in AO-70/NBR composites, type B (AO-70) –OH...O=C– (AO-70) H-bonds, type C (AO-70) –OH...OH–(AO-70) and D (AO-70) –OH...O–C– (AO-70) H-bonds, what's more, type A and type B H-Bonds formed more easily than others. Fourier transform infrared spectroscopy (FTIR) confirmed the existence of H-bonds. Meanwhile, the AO-70/NBR composites with AO-70 content of 109 phr had the largest number of H-bonds, smallest fractional free volume (FFV) and resulting in the optimistic damping performance of the composites.

Molecular dynamics simulations of T-2410 and T-2429 HIV fusion inhibitors interacting with model membranes: Insight into peptide behavior, structure and dynamics

T-2410 and T-2429 are HIV fusion inhibitor peptides (FI) designed to present a higher efficiency even against HIV strains that developed resistance against other FIs. Similar peptides were shown to interact with model membranes both in the liquid disordered phase and in the liquid ordered state. Those results indicated that such interaction is important to function and could be correlated with their effectiveness. Extensive molecular dynamics simulations were carried out to investigate the interactions between both T-2410 and T-2429 with bilayers of pure 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) and a mixture of POPC/cholesterol (Chol) (1:1). It was observed that both peptides interact strongly with both membrane systems, especially with the POPC/Chol systems, where these peptides show the highest number of H-bonds observed so far. T-2410 and T-2429 showed higher extent of interaction with bilayers when compared to T-20 or T-1249 in previous studies. This is most notable in POPC/Chol membranes where, although able to form H-bonds with Chol, they do so to a lesser extent than T-1249 does, the latter being the only FI peptide so far that was observed to form H-bonds with Chol. This behavior suggests that interaction of FI peptides with rigid Chol rich membranes may not be as dependent from peptide/Chol H-bond formation as previous results of T-1249 behavior led to believe. As in other similar peptides, the higher ability to interact with membranes shown by T-2410 and T2429 is probably correlated with its higher inhibitory efficiency.

Universality of hydrogen bond distributions in liquid and supercritical water

Monte Carlo and molecular dynamics computer simulations using the rigid TIP4P and the flexible BJH intermolecular H2O potentials were carried out for 50 states of supercritical water characterizing a very wide range of thermodynamic conditions, 573≤T≤1273K; 0.02≤ρ≤1.67g/cm3; 10≤P≤10,000MPa. Good agreement with available experimental data of the simulated thermodynamic and structural properties give confidence to the quantitative statistical analysis of intermolecular hydrogen bonding under the conditions studied. Energetic, geometric, and angular characteristics of supercritical H-bonds and their distributions at a given temperature remain almost invariant over the entire density range studied from dilute gas-like (~0.03g·cm−3) to highly compressed liquid-like (~1.5g·cm−3) states. The increase of temperature from ambient to supercritical affects the characteristics of H-bonding in water much more dramatically than the changes in density along any supercritical isotherm. Compared to H-bonds in liquid water under ambient conditions, the H-bonds at 773K are, on average, 10% weaker, 5% longer, and less linear.Both above and below the H-bonding percolation threshold the fractions of H2O molecules involved in a certain number of H-bonds in liquid and supercritical water closely follow the universal binomial distribution as a function of the average number of H-bonds per one water molecule in the system, as predicted by the independent bond theory. This universal distribution remains intact even when dynamic criteria of H-bonding lifetimes are additionally applied.

STRUCTURAL AND FUNCTIONAL IMPACT OF G2032R MUTATION IN ROS1 – A THEORETICAL PERSPECTIVE

Objective: Drug resistance is an imperative issue in the treatment of patients with lung cancer. In this work, investigation of the drug resistance mechanism of G2032R mutation in ROS1 is carried out using computational simulation techniques.Methods: Molecular docking and molecular dynamics (MD) simulation approach have been utilized to uncover the mechanism behind crizotinib resistance in ROS1 at a molecular level. Normal mode analysis was carried out using ElNemo server which examines the movements and conformational changes in the protein structure. ArgusLab, PEARLS, and Autodock were employed for the docking analysis, whereas GROMACS package 4.5.3 was used for MD simulation approach.Results: The results from our analysis indicates that wild-type ROS1 (Protein Data Bank Code 3ZBF) could be more crucial for the crizotinib binding as it indicates largest binding affinity, minimum number of H-bonds, and higher flexibility than mutant-type ROS1. Moreover, the theoretical basis for the cause of drug insensitivity is the differences in the electrostatic properties of binding site residues between the wild and mutant ROS1 structures. Our analysis theoretically suggests that E-2027 is a key residue responsible for the ROS1 drug selectivity.Conclusion: Molecular docking and MD simulation results provide an explanation of the resistance caused by G2032R and may give a key clue for the drug design to encounter drug resistance.Keywords: ROS1, Crizotinib resistance, Molecular docking, Normal mode analysis, Molecular dynamic simulation.

Structure-related protein tyrosine phosphatase 1B inhibition by naringenin derivatives

Naturally occurring flavonoids co-exist as glycoside conjugates, which dominate aglycones in their content. To unveil the structure-activity relationship of a naturally occurring flavonoid, we investigated the effects of the glycosylation of naringenin on the inhibition of enzyme systems related to diabetes (protein tyrosine phosphatase 1B (PTP1B) and α-glycosidase) and on glucose uptake in the insulin-resistant state. Among the tested naringenin derivatives, prunin, a single-glucose-containing flavanone glycoside, potently inhibited PTP1B with an IC50 value of 17.5±2.6µM. Naringenin, which lacks a sugar molecule, was the weakest inhibitor compared to the reference compound, ursolic acid (IC50: 5.4±0.30µM). In addition, prunin significantly enhanced glucose uptake in a dose-dependent manner in insulin-resistant HepG2 cells. Regarding the inhibition of α-glucosidase, naringenin exhibited more potent inhibitory activity (IC50: 10.6±0.49µM) than its glycosylated forms and the reference inhibitor, acarbose (IC50: 178.0±0.27µM). Among the glycosides, only prunin (IC50: 106.5±4.1µM) was more potent than the positive control. A molecular docking study revealed that prunin had lower binding energy and higher binding affinity than glycosides with higher numbers of H-bonds, suggesting that prunin is the best fit to the PTP1B active site cavity. Therefore, in addition to the number of H-bonds present, possible factors affecting the protein binding and PTP1B inhibition of flavanones include their fit to the active site, hydrogen-bonding affinity, Van der Waals interactions, H-bond distance, and H-bond stability. Furthermore, this study clearly depicted the association of the intensity of bioactivity with the arrangement and characterization of the sugar moiety on the flavonoid skeleton.

The functional and structural stabilization of trypsin by sucrose

Docking and spectroscopic techniques were performed to probe the stabilizing effect of sucrose on the dynamics, structure and activity of trypsin. The thermodynamic folding properties, melting temperature (Tm), enthalpy change (ΔH°) and entropy change (ΔS°) were measured by thermal stability studies to understand the picture of trypsin folding. Sucrose acted as an enhancer for trypsin stability. Fluorescence spectroscopy revealed the static model of the quenching. The number of binding sites was 1. The Absorption, Fluorescence and circular dichroism spectral analysis illustrated that sucrose could protect the native structural conformation of enzyme and prevent the enzyme unfolding. Fluorescence spectroscopy and the molecular docking technique simulation displayed that the hydrogen bonding and Vander Waals forces played a main role in stabilizing the trypsin-sucrose complex, and the number of direct H-bonds between sucrose and trypsin was low; thus, the direct interactions had little contribution in the stabilizing effect and the indirect interactions caused by the preferential hydration were resulting from a molecular mechanism principally causing the stabilizing effects of sucrose.Upon sucrose conjugation, the kcat/Km value of the enzyme was increased. Tm of the trypsin-sucrose complex was increased due to the higher H-bond formation and the lower surface hydrophobicity after sucrose modification. Sucrose acted as enhancers for trypsin stability and activity. The result shows the ability of sucrose to protect the native structural conformation of trypsin. These results explicitly describe that stabilizing sucrose is preferentially excluded from the surface of trypsin, since water has a higher tendency toward favorable interactions with functional groups of trypsin than with sucrose.

Functional Characterization of the Urea Transporter UreI from Helicobactor Pylori

More than 50% of the world's population is infected with Helicobactor pylori, a pathogenic bacterium responsible for numerous gastroduodenal disorders such as chronic gastritis, peptic ulcer disease, and gastric cancer. A small proton-gated inner membrane channel HpUreI represents H.pylori's life insurance. It ensures survival in the acidic gastric juice, by means of urea transport from the periplasm to the cytoplasm, where urea is hydrolyzed by urease. In turn, the hydrolysis products ammonia and carbon dioxide buffer the periplasm to pH 6.1. MD simulations suggested that water is transported along with urea through the channel. We tested that hypothesis by estimating the unitary transport rates for both molecules. To this end we overexpressed HpUreI in E.coli, purified the transporter and reconstituted the protein into large unilamellar vesicles (LUVs). We observed water flux by osmotically deflating the vesicles or inducing urea transport under isosmotic conditions at neutral and acidic conditions. We extracted the unitary transport rates from the intensity of scattered light by (i) extending our recent treatment of the Rayleigh-Gans-Debye equation for solvent transport (1) across vesicular membranes to solute transport and (ii) determining the number of reconstituted transporters (2) and their oligomeric state (1) with fluorescence correlation spectroscopy before and after detergent-mediated vesicle solubilization. In addition we confirmed the hexameric arrangement by acquiring high-speed AFM images of the proteoliposomes spread on mica.(1) Horner A, et al. (2015) The mobility of single-file water molecules is governed by the number of H-bonds they may form with channel-lining residues. Science Advances 1(2):e1400083. (2) Hoomann T, Jahnke N, Horner A, Keller S, & Pohl P (2013) Filter gate closure inhibits ion but not water transport through potassium channels. Proc. Natl. Acad. Sci. U. S. A. 110(26):10842-10847.

What is the effect of carbon nanotube shape on desalination process? A simulation approach

Effect of carbon nanotube (CNT) shape on desalination efficiency was investigated by computer simulation. Three shapes of CNT including tubular, cone, and hetero-junction (a combination of two different diameter CNTs which connected by a diverged CNT) shaped were simulated at high pressure (>50MPa). For the first time, we placed neutral charge CNT between two inert walls to illustrate support of CNT that is an inert polymer. Cone shaped CNT conducted water ~4 times faster than (6,6) CNT and rejected the ions similar to (6,6) CNT. Actually, cone-shaped CNT possesses the shorter residence time and higher number of water in the tube. This is because of the higher capacity and lower energy barrier against water to pass through it. However, hetero-junction CNT showed a slower water conductance because of the formation of water cluster at the large end of CNT that implies a high-energy barrier against water. In addition, H-bond number is also responsible for energy barrier. H-bond number in bulk of water is 4 and should be decreased to 2 to penetrate in the tubular and hetero-junction shaped CNT. Nevertheless, in a cone shaped CNT H-bond number falls down to 3 only at the entrance of the tube.

Micro-Environmental Signature of The Interactions between Druggable Target Protein, Dipeptidyl Peptidase-IV, and Anti-Diabetic Drugs.

Druggability of a target protein depends on the interacting micro-environment between the target protein and drugs. Therefore, a precise knowledge of the interacting micro-environment between the target protein and drugs is requisite for drug discovery process. To understand such micro-environment, we performed in silico interaction analysis between a human target protein, Dipeptidyl Peptidase-IV (DPP-4), and three anti-diabetic drugs (saxagliptin, linagliptin and vildagliptin). During the theoretical and bioinformatics analysis of micro-environmental properties, we performed drug-likeness study, protein active site predictions, docking analysis and residual interactions with the protein-drug interface. Micro-environmental landscape properties were evaluated through various parameters such as binding energy, intermolecular energy, electrostatic energy, van der Waals'+H-bond+desolvo energy (EVHD) and ligand efficiency (LE) using different in silico methods. For this study, we have used several servers and software, such as Molsoft prediction server, CASTp server, AutoDock software and LIGPLOT server. Through micro-environmental study, highest log P value was observed for linagliptin (1.07). Lowest binding energy was also observed for linagliptin with DPP-4 in the binding plot. We also identified the number of H-bonds and residues involved in the hydrophobic interactions between the DPP-4 and the anti-diabetic drugs. During interaction, two H-bonds and nine residues, two H-bonds and eleven residues as well as four H-bonds and nine residues were found between the saxagliptin, linagliptin as well as vildagliptin cases and DPP-4, respectively. Our in silico data obtained for drug-target interactions and micro-environmental signature demonstrates linagliptin as the most stable interacting drug among the tested anti-diabetic medicines.

Different Ways of Hydrogen Bonding in Water - Why Does Warm Water Freeze Faster than Cold Water?

The properties of liquid water are intimately related to the H-bond network among the individual water molecules. Utilizing vibrational spectroscopy and modeling water with DFT-optimized water clusters (6-mers and 50-mers), 16 out of a possible 36 different types of H-bonds are identified and ordered according to their intrinsic strength. The strongest H-bonds are obtained as a result of a concerted push-pull effect of four peripheral water molecules, which polarize the electron density in a way that supports charge transfer and partial covalent character of the targeted H-bond. For water molecules with tetra- and pentacoordinated O atoms, H-bonding is often associated with a geometrically unfavorable positioning of the acceptor lone pair and donor σ*(OH) orbitals so that electrostatic rather than covalent interactions increasingly dominate H-bonding. There is a striking linear dependence between the intrinsic strength of H-bonding as measured by the local H-bond stretching force constant and the delocalization energy associated with charge transfer. Molecular dynamics simulations for 1000-mers reveal that with increasing temperature weak, preferentially electrostatic H-bonds are broken, whereas the number of strong H-bonds increases. An explanation for the question why warm water freezes faster than cold water is given on a molecular basis.

Coarse-Grained Simulations Complemented by Atomistic Molecular Dynamics Provide New Insights into Folding and Unfolding of Human Telomeric G-Quadruplexes.

G-quadruplexes are the most important noncanonical DNA architectures. Many quadruplex-forming sequences, including the human telomeric sequence d(GGGTTA)n, have been investigated due to their implications in cancer and other diseases, and because of their potential in DNA-based nanotechnology. Despite the availability of atomistic structural studies of folded G-quadruplexes, their folding pathways remain mysterious, and mutually contradictory models of folding coexist in the literature. Recent experiments convincingly demonstrated that G-quadruplex folding often takes days to reach thermodynamic equilibrium. Based on atomistic simulations of diverse classes of intermediates in G-quadruplex folding, we have suggested that the folding is an extremely multipathway process combining a kinetic partitioning mechanism with conformational diffusion. However, complete G-quadruplex folding is far beyond the time scale of atomistic simulations. Here we use high-resolution coarse-grained simulations to investigate potential unfolding intermediates, whose structural dynamics are then further explored with all-atom simulations. This multiscale approach indicates how various pathways are interconnected in a complex network. Spontaneous conversions between different folds are observed. We demonstrate the inability of simple order parameters, such as radius of gyration or the number of native H-bonds, to describe the folding landscape of the G-quadruplexes. Our study also provides information relevant to further development of the coarse-grained force field.

Effect of salt valency and concentration on structure and thermodynamic behavior of anionic polyelectrolyte Na+-polyethacrylate aqueous solution.

The intermolecular structure and solvation enthalpy of anionic polyelectrolyte atactic Na+-polyethacrylate (PEA) in aqueous solution, as a function of added salt concentration C s (dilute to concentrated) and valency (NaCl versus CaCl2), were investigated via molecular dynamics simulations with explicit-ion-solvent and atomistic polymer description. An increase in C s leads to a decrease in α, which stabilizes to a constant value beyond critical C s. A significant reduction in R g in the presence of CaCl2 salt was observed, due to ion bridging of PEA by Ca2+ ions, in agreement with results available in literature on other similar polycarboxylates. An increase in salt valency reduces the value of critical C s for the onset of stabilization of the overall size and shape of the polymer chain. The critical C s ratio for the divalent to monovalent salt case is in excellent agreement with results of Langevin dynamics studies on model systems available in the literature. PEA-water H-bond half-life increases with C s for CaCl2, but no appreciable effect is seen for NaCl. The hydration of PEA becomes stronger in the presence of divalent salt. The strength of H-bond interaction energy is greater for cations as compared to anions of the salt. The salt cation effect in displacing water molecules from the vicinity of PEA, with increase in C s, is greater for NaCl solution. The decrease in water coordination to PEA carboxylate groups, due to increased C s, is more pronounced in NaCl solution. The nature of the behavior of the solvation enthalpy of PEA and the type of intermolecular interactions contributing to it, is in agreement with experimental observations from the literature. The hydration enthalpy of PEA in divalent CaCl2 aqueous salt solution is more exothermic compared to monovalent NaCl salt solution, in agreement with experimental data. The solvation of PEA is thermodynamically more favorable in the case of CaCl2 solution. The exothermic solvation enthalpy, H-bond lifetime, number of H-bonds and H-bond interaction energy are greater in magnitude in CaCl2 aqueous solution.

Synthesis, characterization and antibacterial properties of two POM-based supramolecular compounds with the natural active ingredient: (THB)3(H5PMo12O40) and (THB)3(H3PW12O40)

Two supramolecular compounds of Keggin POMs supporting the natural active ingredient, (THB)3(H5PMo12O40) (1) and (THB)3(H3PW12O40) (2) (THB=tetrahydroberberine), have been synthesized and characterized by X-ray single-crystal diffraction, elemental analyses, IR, XRPD and TGA. A crystallization screening of a series of pH value and stoichiometric ratio combinations was performed in order to investigate the synthetic conditions of two compounds. Compound 1 was synthesized at pH=1.5 and has a 3:7 stoichiometric ratio, in which THB molecules exist two configurations and link to polyanion via 14 H-bonds. Compound 2 was obtained at a relatively high pH (pH=2.0) and has the same stoichiometric ratio as 1, in which THB molecules exist only one configuration and link to polyanion via 8 H-bonds. The discrepancies of pH values, and volumes and oxygen basicities of polyanions could significantly affect the number of H-bonds between POMs and THBs, and then the number of H-bonds may affect configurations of THB molecules. This result implicitly indicates that it is providing a new thought and method for chiral resolution of (±)-tetrahydroberberine. Additionally, two title compounds show advantages in terms of antibacterial properties against Staphylococcus aureus and Escherichia coli in comparison to their corresponding parent compounds.

Ab initio molecular dynamics study of H-bonding and proton transfer in the phosphoric acid-N,N-Dimethylformamide system.

Car-Parrinello molecular dynamics simulations of phosphoric acid (H3PO4)-N,N-dimethylformamide (DMF) mixtures over the whole composition range have been carried out. It has been found that the neutral molecules are the dominant species in this system. The concentration dependences of the average number of H-bonds per proton acceptor atom in P=O and C=O groups as well as per proton donor atom in DMFH+ ions towards phosphate species have been discussed. The H-bonding between components in all investigated mixtures of H3PO4 and DMF is possible. A significant fraction of the protonated DMF forms appears at phosphoric acid mole fraction higher than 0.37, indicating a high probability of proton transfer from phosphate species to oxygen atoms in C=O groups. The intermolecular proton transfer between phosphate species themselves is mainly observed when xH3PO4 > 0.19. Satisfactory agreement with available experimental data for structural characteristics of the investigated system was obtained.

Molecular-dynamic study of liquid ethylenediamine

Models of liquid ethylenediamine (ED) are built using the molecular dynamics approach at temperatures of 293–363 K and a size of 1000 molecules in a basic cell as a cuboid. The structural and dynamic characteristics of liquid ED versus temperature are derived. The gauche conformation of the ED molecule that is characteristic of the gas phase is shown to transition easily into the trans conformation of the molecules in the liquid. NH···N hydrogen bonds are analyzed in liquid ED. The number of H-bonds per ED molecule is found to vary from 5.02 at 293 K to 3.86 at 363 K. The lifetimes in the range of the temperatures and dissociation activation energy for several H-bonds in liquid ED are found to range from 0.574 to 4.524 ps at 293 K; the activation energies are 8.8 kJ/mol for 50% of the H-bonds and 16.3 kJ/mol for 6.25% of them. A weaker and more mobile spatial grid of H-bonds in liquid ED is observed, compared to data calculated earlier for monoethanolamine.

Estimating the energy of intramolecular hydrogen bonds in chitosan oligomers

The effect the number of chitosan monomer units CTSn (n = 1–5), the protonation of chitosan dimers, and the interaction between CTSn (n = 1–3) and acetate ions have on the energy of intramolecular hydrogen bonds is investigated by means of QTAIM analysis and solving the vibrational problem within the cluster-continuum model. It is established that the number of H-bonds in CTSn is 2n − 1 and the total energy of H-bonds grows by ~20 kJ/mol. It is concluded that the hydrogen bonds between CTS and acetate ions play a major role in the stabilization of polyelectrolyte complexes in dilute acetic acid solutions of CTS.

Radical Formation Initiates Solvent-Dependent Unfolding and β-sheet Formation in a Model Helical Peptide

We examined the effects of Cα-centered radical formation on the stability of a model helical peptide, N-Ac-KK(AL)10KK-NH2. Three, 100 ns molecular dynamics simulations using the OPLS-AA force field were carried out on each α-helical peptide in six distinct binary TIP4P water/2,2,2-trifluoroethanol (TFE) mixtures. The α-helicity was at a maximum in 20% TFE, which was inversely proportional to the number of H-bonds between water molecules and the peptide backbone. The radial distribution of TFE around the peptide backbone was highest in 20% TFE, which enhanced helix stability. The Cα-centered radical initiated the formation of a turn within 5 ns, which was a smaller kink at high TFE concentrations, and a loop at lower TFE concentrations. The highest helicity of the peptide radical was measured in 100% TFE. The formation of hydrogen bonds between the peptide backbone and water destabilized the helix, whereas the clustering of TFE molecules around the radical center stabilized the helix. Following radical termination, the once helical structure converted to a β-sheet rich state in 100% water only, and this transition did not occur in the nonradical control peptide. This study gives evidence on how the formation of peptide radicals can initiate α-helical to β-sheet transitions under oxidative stress conditions.

Microstructure Changes in Polyester Polyurethane upon Thermal and Humid Aging.

The microstructure of compression molded Estane 5703 films exposed to 11%, 45%, and 80% relative humidity and 70 °C for 1 and 2 months has been studied by small-angle neutron scattering (SANS), Fourier transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC), and differential scanning calorimetry (DSC). Scattering data indicated increase of the interdomain distance and domain size with a higher humidity and longer aging time. GPC data showed a progressive shortening of polyurethane chains with increasing humidity and aging time. The shortening of the polyurethane chains caused a drop of the glass transition temperature of soft segments, and promoted crystallization of the soft segments during long-time storage of the aged samples at room temperature. FTIR showed a substantial increase in the number of inter-urethane H-bonds in the aged samples. This correlates with the increase of the hard domain size and the degree of phase separation as measured by SANS. The data collected reveals that the reduced steric hindrance caused by hydrolysis of ester links in polybutylene adipate residues promotes the organization of hard segments into domains, leading to the increase of domain size and distance, as well as phase segregation in aged Estane. These findings provide insight into the effects of humidity and thermal aging on the microstructure of aged polyester urethane from molecular to nanoscale level.