Crystallization of Arthrobacter sp. strain 1C N-(1-D-carboxyethyl)-L-norvaline dehydrogenase and its complex with NAD+

The novel NAD+-linked opine dehydrogenase from a soil isolate Arthrobacter sp. strain 1C belongs to an enzyme superfamily whose members exhibit quite diverse substrate specificites. Crystals of this opine dehydrogenase, obtained in the presence or absence of co-factor and substrates, have been shown to diffract to beyond 1.8 ? resolution. X-ray precession photographs have established that the crystals belong to space group P21212, with cell parameters a = 104.9, b = 80.0, c = 45.5 ? and a single subunit in the asymmetric unit. The elucidation of the three-dimensional structure of this enzyme will provide a structural framework for this novel class of dehydrogenases to enable a comparison to be made with other enzyme families and also as the basis for mutagenesis experiments directed towards the production of natural and synthetic opine-type compounds containing two chiral centres.     

Isothermal ice crystallization kinetics in the gas-diffusion layer of a proton-exchange-membrane fuel cell

Nucleation and growth of ice in the fibrous gas-diffusion layer (GDL) of a proton-exchange membrane fuel cell (PEMFC) are investigated using isothermal differential scanning calorimetry (DSC). Isothermal crystallization rates and pseudo-steady-state nucleation rates are obtained as a function of subcooling from heat-flow and induction-time measurements. Kinetics of ice nucleation and growth are studied at two polytetrafluoroethylene (PTFE) loadings (0 and 10 wt %) in a commercial GDL for temperatures between 240 and 273 K. A nonlinear ice-crystallization rate expression is developed using Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory, in which the heat-transfer-limited growth rate is determined from the moving-boundary Stefan problem. Induction times follow a Poisson distribution and increase upon addition of PTFE, indicating that nucleation occurs more slowly on a hydrophobic fiber than on a hydrophilic fiber. The determined nucleation rates and induction times follow expected trends from classical nucleation theory. A validated rate expression is now available for predicting ice-crystallization kinetics in GDLs.     

Impact of hygrothermal aging on structure/function relationship of perfluorosulfonic‐acid membrane

Perfluorosulfonic‐acid (PFSA) membranes are widely used as the solid electrolyte in electrochemical devices where their main functionalities are ion (proton) conduction and gas separation in a thermomechanically stable matrix. Due to prolonged operational requirements in these devices, PFSA membranes’ properties change with time due to hygrothermal aging. This paper studies the evolution of PFSA structure/property relationship changes during hygrothermal aging, including chemical changes leading to changes in ion‐exchange capacity (IEC), nanostructure, water‐uptake behavior, conductivity, and mechanical properties. Our findings demonstrate that with hygrothermal aging, the storage modulus increases, while IEC and water content decrease, consistent with the changes in nanostructure, that is, water‐ and crystalline‐domain spacings inferred from small‐ and wide‐angle X‐ray scattering (SAXS/WAXS) experiments. In addition, the impact of aging is found to depend on the membrane's thermal prehistory and post‐treatments, although universal correlations exist between nanostructural changes and water uptake. The findings have impact on understanding lifetime, durability, and use of these and related polymers in various technologies. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 570–581