Thermal behaviour of polyetherimide peek blends

PEEK is characterised by high impact and fatigue resistance andT _g of 145C. Blends of PEEK and PEI have been made and scanning electron micro-graphs of the broken specimen show that the two polymers are completely miscible in all proportions. The study also shows that PEEKPEI 5050 blend, can be used as matrix for composite applications with appreciable enhancement ofT _g to 177C.     

Preorganization and protein dynamics in enzyme catalysis

Recently, an alternative has been offered to the concept of transition state (TS) stabilization as an explanation for rate enhancements in enzyme-catalyzed reactions. Instead, most of the rate increase has been ascribed to preorganization of the enzyme active site to bind substrates in a geometry close to that of the TS, which then transit the activation barrier impelled by motions along the reaction coordinate. The question as to how an enzyme achieves such preorganization and concomitant TS stabilization as well as potential coupled motions along the reaction coordinate leads directly to the role of protein dynamic motion. Dihydrofolate reductase (DHFR) is a paradigm in which the role of dynamics in catalysis continues to be unraveled by a wealth of kinetic, structural, and computational studies. DHFR has flexible loop regions adjacent to the active site whose motions modulate passage through the kinetically preferred pathway. The participation of residues distant from the DHFR active site in enhancing the rate of hydride transfer, however, is unanticipated and may signify the importance of long range protein motions. The general significance of protein dynamics in understanding other biological processes is briefly discussed.     

Pi-helix preference in unsolvated peptides

Ion mobility measurements have been used to examine helix formations in the gas phase for a series of alanine/glycine-based peptides that incorporate a glutamic acid (E) and lysine (K) at various positions along the backbone. Incorporation of an EK pair lowers the percent helix for all positions (presumably because hydrogen bonding between the backbone and the E and K side chains stabilize the nonhelical globular conformations). The largest percent helix is found when the EK pair is in an i,i+5 arrangement, which suggests that the preferred helical conformation for these peptides is a pi-helix. This conclusion is supported by comparison of cross sections deduced from the ion-mobility measurements to average cross sections calculated for conformations obtained from molecular dynamics simulations. The glutamic acid and lysine may form an ion pair that is stabilized by interactions with the helix macro-dipole.     

Adsorbing colloid flotation separation and polarographic determination of Mo(VI) in water

A method is described for the flotation and determination of Mo(VI) in water at ng/ml levels. Mo(VI) is preconcentrated and separated by adsorbing colloid flotation employing aluminium(III) hydroxide as collector and sodium lauryl sulphate as surfactant at pH 5.3 ± 0.1. The molybdenum content in the froth is estimated by using the catalytic wave of Mo(VI) in the presence of nitrate by charging current compensated d.c. polarography (CCCDCP) or differential pulse polarography (DPP). The effect of variables such as pH, ionic strength, concentration of collector and surfactant, time of stirring and gas flow-rate on the recovery of Mo by flotation is reported. The effects of various cations and anions on the flotation and determination of Mo are studied. This method is employed for the determination of molybdenum in natural fresh water samples.     

Reaction intermediates in the catalytic Gif-type oxidation from nuclear inelastic scattering

Nuclear inelastic scattering (NIS) of synchrotron radiation, also known as nuclear resonant vibrational spectroscopy (NRVS), has been shown to provide valuable insights into metal-centered vibrations at Mössbauer-active nuclei. We present a study of the iron-centered vibrational density of states (VDOS) during the first step of the Gif-type oxidation of cyclohexene with a novel trinuclear Fe₃(μ₃-O) complex as catalyst precursor. The experiments were carried out on shock-frozen solutions for different combinations of reactants: Fe₃(μ₃-O) in pyridine solution, Fe₃(μ₃-O) plus Zn/acetic acid in pyridine without and with addition of either oxygen or cyclohexene, and Fe₃(μ₃-O)/Zn/acetic acid/pyridine/cyclohexene (reaction mixture) for reaction times of 1 min, 5 min, and 30 min. The projected VDOS of the Fe atoms was calculated on the basis of pseudopotential density functional calculations. Two possible reaction intermediates were identified as [Fe^(III)(C₅H₅N)₂(O₂CCH₃)₂]⁺ and Fe^(II)(C₅H₅N)₄(O₂CCH₃)₂, yielding evidence that NIS (NRVS) allows to identify the presence of iron-centered intermediates also in complex reaction mixtures.