Kinetics and mechanism of the gas‐phase OH hydrogen abstraction reaction from methionine: A quantum mechanical approach

Abstract

AbstractUnrestricted density functional theory (BHandHLYP) calculations have been performed, using the 6‐311G(d,p) basis sets, to study the gas‐phase OH hydrogen abstraction reaction from methionine. The structures of the different stationary points are discussed. Ring‐like structures are found for all the transition states. Reaction profiles are modeled including the formation of prereactive complexes, and negative net activation energy is obtained for the gamma H‐abstraction channel. A complex mechanism is proposed, and the rate coefficients are calculated using transition state theory over the temperature range 250–350 K. The rate coefficients are proposed for the first time and it was found that in gas phase the hydrogen abstraction occurs almost exclusively from the gamma site. The large overall rate coefficient for the methionine + OH reaction compared to other free amino acids could explain the significant role of methionine in the oxidative processes. The following expressions in [L/(mol s)] are obtained for the alpha, beta, and gamma H‐abstraction channels, and for the overall temperature‐dependent rate constants, respectively: kα = (3.42 ± 0.11) × 108 exp[(−1118 ± 9)/T], kβ = (1.13 ± 0.03) × 108 exp[(−1070 ± 8)/T], kγ = (2.11 ± 0.26) × 107 exp[(2049 ± 34)/T], and ktot = (2.12 ± 0.26) × 107 exp[(2047 ± 34)/T]. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 212–221, 2003