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Tyr 64, hydrogen-bonded to coenzyme phosphate in Treponema denticola cystalysin, was changed to alanine by site-directed mutagenesis. Spectroscopic and kinetic properties of the Tyr 64 mutant were investigated in an effort to explore the differences in coenzyme structure and kinetic mechanism relative to those of the wild-type enzyme. The wild type displays coenzyme absorbance bands at 418 and 320 nm, previously attributed to ketoenamine and substituted aldamine, respectively. The Tyr 64 mutant exhibits absorption maxima at 412 and 325 nm. However, the fluorescence characteristics of the latter band are consistent with its assignment to the enolimine form of the Schiff base. pKspec values of ∼8.3 and ∼6.5 were observed in a pH titration of the wild-type and mutant coenzyme absorbances, respectively. Thus, Tyr 64 is probably the residue involved in the nucleophilic attack on C4' of pyridoxal 5'-phosphate (PLP) in the internal aldimine. Although the Tyr 64 mutant exhibits a lower affinity for PLP and lower turnover numbers for α,β-elimination and racemization than the wild type, the pH profiles for their Kd(PLP) and kinetic parameters are very similar. Rapid scanning stopped-flow and chemical quench experiments suggest that, in contrast to the wild type, for which the rate-determining step of α,β-elimination of β-chloro-L-alanine is the release of pyruvate, the rate-determining step for the mutant in the same reaction is the formation of α-aminoacrylate. Altogether, these results provide new insights into the catalytic mechanism of cystalysin and highlight the functional role of Tyr 64. [ABSTRACT FROM AUTHOR]

(CpMoμ-S)2S2CH2, 2, and related derivatives serve as electrocatalysts for the reduction of protons with current efficiencies near 100%. The kinetics of the electrochemical reduction process has been studied, and the effects of varying the proton source, the solvent, the cyclopentadienyl substituents, and the sulfur substituents on the catalyst have been examined. The reduction of excess p-cyanoanilinium tetrafluoroborate under a hydrogen atmosphere in 0.3 M Et4NBF4/acetonitrile buffered at pH 7.6 is catalyzed by 2 at -0.64 V versus ferrocene, with an overpotential of 120 mV. Protonation of the sulfido ligand in 2 is an initial step in the catalytic process, and the rate-determining step at high acid concentrations appears to be the elimination of dihydrogen. The elimination may occur either from adjacent hydrosulfido sites or from a hydrosulfido-molybdenum hydride intermediate. [ABSTRACT FROM AUTHOR]

The mechanism of the spontaneous decomposition of 3-bromo-2,2-bis(bromomethyl)propanol (TBNPA) and the kinetics of the reaction of the parent compound and two subsequent products were determined in aqueous solution at temperatures from 30 to 70 °C and pH from 7.0 to 9.5. TBNPA is decomposed by a sequence of reactions that form 33-bis(bromomethyl)oxetane (B B MO), 3-bromomethyl-3- hydroxymethyloxetane (B MHMO), and 2,6-dioxaspiro[3.3]- heptane (DOH), releasing one bromide ion at each stage. The pseudo-first-order rate constant of the decomposition of TBNPA increases linearly with the pH. The apparent activation energy of this transformation (98 ± 2 KJ/mol) was calculated from the change of the effective second-order rate constant with temperature. The pseudoactivation energies of BBMO and BMHMO were estimated to be 109 and 151 KJ/mol, respectively. Good agreement was found between the rate coefficients derived from changes in the organic molecules concentrations and those determined from the changes in the Br- concentrations. TBNPA is the most abundant semivolatile organic pollutant in the aquitard studied, and together with its byproducts they posess an environmental hazard. TBNPA half-life is estimated to be about 100 years. This implies that high concentrations of TBNPA will persist in the aquifer long after the elimination of all its sources. [ABSTRACT FROM AUTHOR]

The application of photoactivated periodate (UV/I04-) to the degradation of 4-chlorophenol (4-CP) was explored in this study. Under low irradiation intensities (23 μW/ cm2), wavelength of 266 nm, and pH 3, 4-CR was observed to degrade by pseudo-first-order reaction kinetics. The small reduction in 4-CP degradation in the presence of tert-butyl alcohol (t-BuOH) suggests that degradation of 4-CP by UV/lO4- was not dominated by an OH pathway. 03 production was suppressed in the presence of t-BuOH under O2-limited environments and in the presence of 4-CP. Faster degradation of 4-CP in the presence of an OW scavenger under nitrogen purging as compared to air- saturated conditions indicates that O(3P) is an important reactive species with 4-CR in this system. When 4-CP is added to the system, 103 production is enhanced. On the basis of the elimination of potential reaction pathways, O(3P) and lO3- are suspected reactive species with 4-CP in this system. [ABSTRACT FROM AUTHOR]