Premium
Mechanism of Cysteine Oxidation by a Hydroxyl Radical: A Theoretical Study
Author(s) -
Enescu Mironel,
Cardey Bruno
Publication year - 2006
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.200500585
Subject(s) - mechanism (biology) , chemistry , hydroxyl radical , cysteine , reaction mechanism , radical , computational chemistry , photochemistry , organic chemistry , catalysis , philosophy , epistemology , enzyme
Abstract 10.1002/cphc.200500585.abs Cysteine oxidation by HO . was studied at a high level of ab initio theory in both gas phase and aqueous solution. Potential energy surface scans in the gas phase performed for the model system methanethiol+HO . indicate that the reactants can form two intermediate states: a sulfur–oxygen adduct and a hydrogen bound reactant complex. However these states appear to play a minor role in the reaction mechanism as long as they are fast dissociating states. Thus the main reaction channel predicted at the QCISD(T)/6‐311+G(2df,2pd) level of theory is the direct hydrogen atom abstraction. The reaction mechanism is not perturbed by solvation which was found to induce only small variations in the Gibbs free energy of different reactant configurations. The larger size reactant system cysteine+HO . was treated by the integrated molecular orbital+molecular orbital (IMOMO) hybrid method mixing the QCISD(T)/6‐311+G(2df,2pd) and the UMP2/6‐311+G(d,p) levels of theory. The calculated potential energy, enthalpy, and Gibbs free energy barriers are slightly different from those of methanethiol. The method gave a rate constant for cysteine oxidation in aqueous solution, k =2.4×10 9 mol −1 dm 3 s −1 , which is in good agreement with the experimental rate constant. Further analysis showed that the reaction is not very sensitive to hydrogen bonding and electrical polarity of the molecular environment.