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

Abstract

Unrestricted 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 Habstraction channels, and for the overall temperature-dependent rate constants, respectively: k α = (3.42 ± 0.11) × 10 8 exp[(−1118 ± 9)/T], k β = (1.13 ± 0.03) × 10 8 exp[(−1070 ± 8)/T], k γ = (2.11 ± 0.26) × 10 7 exp[(2049 ± 34)/T], and k tot = (2.12 ± 0.26) × 10 7 exp [(2047 ±

Protein Stability and Unfolding Following Glycine Radical Formation

“…Considering the multiple locations from where hydrogen can be abstracted, the ΔE for the reactions can vary. The lowest ΔE for the hydrogen abstraction from the side chain of methionine is −138.7 kJ·mol −1 , serine is −167.3 kJ·mol −1 , and asparagine is −183.8 kJ·mol −1 [ 44 , 45 , 46 ]. The ΔE for the reaction of the C α from glycine is comparable to these residues at −144.7 kJ·mol −1 [ 34 ].…”

Kinetics and mechanism of the β-alanine + OH gas phase reaction: A quantum mechanical approach

“…Each reaction path was modeled taking into account the results of previous work. According to these results, [43][44][45][46][47] it is a reasonable assumption that the amino acids þ OH reactions occur via a two step mechanism, with a reversible first step leading to a pre-reactive complex and a second step yielding the corresponding radical and H 2 O (for more details on the mechanism see ref. 48).…”

Site specificity of OH α‐H abstraction reaction for a β‐hairpin peptide: An ab initio study

“…First, the to‐be‐abstracted α‐H should be reachable from the aqueous phase. Second, the occurrence of a α‐H abstraction site is determined by the competition between the α‐H abstraction and other reactions to the side chains of the residues in a protein 12–14. The to‐be‐abstracted α‐H is also determined by the geometry required by the associated transition state.…”

“…The to‐be‐abstracted α‐H is also determined by the geometry required by the associated transition state. Recently, the mechanism and kinetics of OH radical reacting with various amino acids were investigated by Galano et al12–16 to determine the branching ratio of α‐H abstraction reactions taking place between the backbone and side chain. These branch ratios and associated transition states can be used to estimate the occurrence of a particular α‐H abstraction site.…”

“…It is common in modern theoretical/computational practice to use model peptides or peptide fragments to reflect the local protein environment 12–23, 26. For example, various single amino acid residues12–16, 26 and methyl‐substituted glycine anion26 with or without structural restriction to mimic secondary structures17–23 have been studied for the α‐H abstraction reaction. To obtain more realistic results by extending the system size, we examined the preferential α‐H to be abstracted by an OH radical with various secondary structural motifs 9.…”