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The H-bond activation mechanism and enantioselectivity of hydroxyl-thiourea catalyst in conjugate amine addition of O-benzyl hydroxylamine to pyrazole crotonate is investigated using density functional theory (DFT) calculations. Two competing activation models are explored in detail. CN bond formation is stepwise in both of the two models. The enantioselective (S)-channel is more favorable than (R)-channel via the calculated barriers. The enantioselectivity originated from si face preferable than re face can be attributed to the H-bonded network provided by thiourea and hydroxyl groups in rate-determining step. The enantiomeric excess (ee) values predicted through ONIOM calculations are in line with the experiment. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011 [ABSTRACT FROM AUTHOR]

The disproportionation of superoxide radical anions catalyzed by copper-zinc superoxide dismutase was investigated in detail using density functional theory. The structures of each stationary point and the transition states were located so that the reaction pathways were determined. The results indicate that the reactions proceed by two steps both for the oxidized process of superoxide radical anion and the reduced one. The rate for the determining step of the reaction (2) is in very good agreement with the experimental value. The solvation effect on the reaction was also discussed. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [ABSTRACT FROM AUTHOR]

Abstract: The mechanism of the aminolysis of dimethyl phenylphosphinate with ammonia was investigated by the density functional theory at the B3LYP level with basis set 6-31G(d,p) in the gas phase. Single point energy evaluations were also performed for more precise energy predictions at the B3LYP and the BB1K levels with the 6-311++G(d,p) basis set. Solvent effects on the uncatalyzed process were assessed by the polarized continuum model with B3LYP/6-311++G(d,p) and BB1K/6-311++G(d,p) based on the gas-phase optimized geometries. Transition state structures and energies were determined for concerted and neutral stepwise pathways. The theoretical results show that the stepwise pathway is more favorable than the concerted process. The general base catalysis of the process was also examined. The catalytic role of a second ammonia molecule is executed by facilitating the hydrogen transfer processes and by decreasing all energy barriers. The results show that the most favorable pathway of the reaction is through the general-base-catalyzed neutral stepwise mechanism. [Copyright &y& Elsevier]

Abstract: The effect of water molecules on the enzyme-catalyzed decarboxylation of Orotidine 5′-monophosphate was investigated using hybrid Density Functional Theory (B3LYP functional). Different models were designed and the reaction barriers with these models were calculated. The results show that the indirect interactions between O-2 and O-4 of the pyrimidine ring and the positive charged residues through water bridges can reduce the barrier significantly; and that water molecules play a key role in the transition state stabilization of catalytic decarboxylation of Orotidine 5′-monophosphate. [Copyright &y& Elsevier]