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.     

Hydrocolloids in the digestive tract and related health implications

Hydrocolloids in the form of polymeric ingredients as well as natural biopolymer assemblies provide much of the macroscopic structure of foods. The controlled disassembly of hydrocolloid-structured foods in the digestive tract determines numerous nutritional properties driven by the rates of passage, digestion, absorption, and fermentation. Despite convincing evidence for health benefits of hydrocolloids (particularly dietary fibre) from epidemiology, and numerous in vitro model system studies, the detailed underlying mechanisms operating in the digestive tract are currently understood to only a limited extent. Distinct hydrocolloid-based processes occur in each of the gastric, small intestinal and large intestinal environments, with significant biological cross-talk between the sites. Hydrocolloids offer a major opportunity to tailor nutritional value and provide potential health benefits through control of gastric emptying and ileal brake mechanisms (satiety and potentially obesity), glycemic response (diabetes), plasma cholesterol levels (cardiovascular disease), and carbohydrate fermentation throughout the large intestine (colon cancer). There is often a parallel between the functionality of the plant-based foods which the human digestive tract evolved to digest and the use of extracted hydrocolloids in modern food structuring technology.     

Effect of cyclic stress on structural changes in polycarbonate as probed by positron annihilation lifetime spectroscopy

Positron annihilation lifetime spectroscopy (PALS) is used to probe structural changes in glassy polycarbonate in terms of changes in the hole volume and the number density of holes during fatigue (cyclic stress) aging. The ortho-positronium (o-Ps) pickoff annihilation lifetime τ₃, as well as the intensity I₃, were measured as a function of cyclic stresses and various previous thermophysical aging histories. It is found that τ₃, the longest of the three lifetime components resolved in the PALS of glassy polycarbonate, increases when a cyclic stress is applied. These results indicate that there is a structural change during fatigue aging. The “holes” where o-Ps can localize become larger upon fatigue aging. These results also suggest that a significant distinction exists between structural changes induced by thermophysical aging and fatigue aging. The o-Ps annihilation intensity, which is a relative measure of the hole density in a material, showed a continuous decrease upon fatigue aging, indicating the possibility of hole coalescence, which could be a precursor of crazing. The interaction between thermophysical aging and fatigue aging corresponds very well with the enthalpy relaxation behavior as reported previously, viz., a well-aged sample is much more sensitive to cyclic stress. More importantly, it is hypothesized that fatigue failure initiation is probably closely related to hole size and density fluctuation.     

Production and applications of monoenergetic polarized positron beams

The generation and applications of monoenergetic, high intensity, spin-polarized positron (e⁺) beams are reviewed. Techniques for obtaining highly polarized beams are discussed. Applications include studies of surface and bulk magnetism, studies of optically active molecules, tests of discrete symmetries, and polarized antiproton production.     

Evolution of nanometer voids in polycarbonate under mechanical stress and thermal expansion using positron spectroscopy

Positron Annihilation Lifetime Spectroscopy (PALS) measurements were conducted on polycarbonate subjected to either thermal expansion or to tensile and compressive strains. It was found that thermal expansion affected both the nanometer hole size and the hole number density, whereas mechanical stress affected mainly the size of existing holes, and did not generate or eliminate holes in the quasielastic deformation region. The effect of stress on yield and postyield behavior of this glassy material was also investigated. The deduced hole volume fraction of this polymer at 25°C was 6.8 ± 0.5% from the thermal expansion experiment and 7.2 ± 1.2% from the mechanical loading experiment. When the specimen was under compression, the hole volume fraction was found to continuously decrease. This can be considered as evidence of the inability of the free volume concept in explaining the yield behavior of glassy polymers. ©1995 John Wiley & Sons, Inc.