"@-Tides": the 1,2-dihydro-3(6H)-pyridinone unit as a beta-strand mimic.

The cyclic amino acid surrogate 1 was designed to mimic the extended conformation of a peptide unit and to provide hydrogen bond donor and acceptor functions conducive to beta-sheet formation. A convenient synthesis of this unit and solution and solid-phase methods for its incorporation into an oligomer alternating with peptide units have been devised. The resulting "@-tides", as these oligomers have been designated, show a high propensity for self-association in comparison to oligopeptides; insights into the structure and dynamical properties of their antiparallel dimers have been obtained by NMR.     

Structural and free‐volume analysis for alkyl‐substituted palladium‐catalyzed poly(norbornene): A combined experimental and Monte Carlo investigation

Increasing the length of an alkyl side chain on a rigid polynorbornene (PNB) backbone is shown to decrease the glass‐transition temperature of the resultant polymer, decrease the density of the bulk polymer, decrease the number and average free‐volume element size present, and decrease the permeability of gases through the polymer. Methyl‐, butyl‐, and hexyl‐substituted PNBs were investigated. Experimental results were compared with predictions based on molecular modeling. By using models that provided good agreement between the experimental and simulated wide‐angle diffraction patterns, the distributions of free‐volume elements were predicted. These predictions clearly indicate that the number of large free‐volume elements decreases as the length of the side chain increases, suggesting that the flexible aliphatic side chains can be largely accommodated within the free volume between the rigid PNB backbones. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 215–233, 2006     

Impact of average free‐volume element size on transport in stereoisomers of polynorbornene. II. Impact of temperature on solubility

This article evaluates the influence of temperature on the sorption of gases in two isomers of polynorbornene. The subject polymers were stereoisomers with nearly identical bulk density and total free volume. Because of differences in the mobility of the polymer backbone, the isomers packed differently resulting in differences in the average free‐volume element size within the matrix. The influence of these differences on free‐volume element size was characterized by the heat of sorption of gases in the matrix. The most pronounced differences were observed in the isosteric heats of sorption of condensable carbon dioxide and methane in the polymer isomers. This analysis suggests that the relative space available for sorption into free‐volume elements is higher in the methyl II isomer relative to methyl III. These conclusions support the physical characterizations reported in Part I of this series suggesting that the methyl II isomer has larger average free‐volume elements but fewer of them than the methyl III isomer. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1939–1946, 2003     

Effect of mild solvent post-treatments on the gas transport properties of glassy polymer membranes

This study examines the effect of treatment of defective glassy polymer membranes with a variety of vapors and liquids which have varying solvency power for the polymer. The pure-gas oxygen/nitrogen selectivities of defective, asymmetric membranes are shown to be permanently increased, in special cases, by treatment with certain solvents which have adequate solvency power to cause a critical level of swelling in the membrane skin layer. Three distinct types of membranes have been treated; asymmetric polysulfone membranes formed by dry/wet phase inversion, spin-coated poly(phenylene oxide)-ceramic composite membranes and solution-deposited polyimide-ceramic composite membranes. While the detailed fundamental processes controlling the elimination of surface defects are complex, our results suggest that plasticization of the selective skin layer, coupled with surface-tension driven cohesive forces are likely to be the key factors at play.     

Potential-energy and free-energy surfaces of glycyl-phenylalanyl-alanine (GFA) tripeptide: experiment and theory

The free-energy surface (FES) of glycyl-phenylalanyl-alanine (GFA) tripeptide was explored by molecular dynamics (MD) simulations in combination with high-level correlated ab initio quantum chemical calculations and metadynamics. Both the MD and metadynamics employed the tight-binding DFT-D method instead of the AMBER force field, which yielded inaccurate results. We classified the minima localised in the FESs as follows: a) the backbone-conformational arrangement; and b) the existence of a COOH...OC intramolecular H-bond (families CO(2)H(free) and CO(2)H(bonded)). Comparison with experimental results showed that the most stable minima in the FES correspond to the experimentally observed structures. Remarkably, however, we did not observe experimentally the CO(2)H(bonded) family (also predicted by metadynamics), although its stability is comparable to that of the CO(2)H(free) structures. This fact was explained by the former's short excited-state lifetime. We also carried out ab initio calculations using DFT-D and the M06-2X functional. The importance of the dispersion energy in stabilising peptide conformers is well reflected by our pioneer analysis using the DFT-SAPT method to explore the nature of the backbone/side-chain interactions.     

Optical method for measuring thermal accommodation coefficients using a whispering-gallery microresonator

A novel optical method has been developed for the measurement of thermal accommodation coefficients in the temperature-jump regime. The temperature dependence of the resonant frequency of a fused-silica microresonator's whispering-gallery mode is used to measure the rate at which the microresonator comes into thermal equilibrium with the ambient gas. The thermal relaxation time is related to the thermal conductivity of the gas under some simplifying assumptions and measuring this time as a function of gas pressure determines the thermal accommodation coefficient. Using a low-power tunable diode laser of wavelength around 1570 nm to probe a microsphere's whispering-gallery mode through tapered-fiber coupling, we have measured the accommodation coefficients of air, helium, and nitrogen on fused silica at room temperature. In addition, by applying thin-film coatings to the microsphere's surface, we have demonstrated that accommodation coefficients can be measured for various gases on a wide range of modified surfaces using this method.     

Study of structural and electronic transport properties of Ce-doped LaMnO3

The structural and electronic transport properties of La_1−x Ce _x MnO₃ (x=0.0–1.0) have been studied. All the samples exhibit orthorhombic crystal symmetry and the unit cell volume decreases with Ce doping. They also make a metal-insulator transition (MIT) and transition temperature increases with increase in Ce concentration up to 50% doping. The system La_0.5Ce_0.5MnO₃ also exhibits MIT instead of charge-ordered state as observed in the hole doped systems of the same composition.