Collision Dynamics of Protonated N-Acetylmethionine with Singlet Molecular Oxygen (a1螖g): The Influence of the Amide Bond and Ruling Out the Complex-Mediated Mechanism at

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• 作者：Wenchao Lu ; Fangwei Liu ; Rifat Emre ; Jianbo Liu
• 刊名：Journal of Physical Chemistry B
• 出版年：2014
• 出版时间：April 10, 2014
• 年：2014
• 卷：118
• 期：14
• 页码：3844-3852
• 全文大小：420K
• 年卷期：v.118,no.14(April 10, 2014)
• ISSN：1520-5207

文摘

It has been proposed (J. Phys. Chem. B 2011, 115, 2671) that the ammonium group is involved in the gas-phase reaction of protonated methionine (MetH⁺) with singlet oxygen ¹O₂, yielding hydrogen peroxide and a dehydro compound of MetH⁺ where the -NH₃⁺ transforms into cyclic -NH₂-. For the work reported, the gas-phase reaction of protonated N-acetylmethionine (Ac-MetH⁺) with ¹O₂ was examined, including the measurements of reaction products and cross sections over a center-of-mass collision energy (E_col) range from 0.05 to 1.0 eV using a guided-ion-beam apparatus. The aim is to probe how the acetylation of the ammonium group affects the oxidation chemistry of the ensuing Ac-MetH⁺. Properties of intermediates, transition states, and products along the reaction coordinate were explored using density functional theory calculations and Rice鈥揜amsperger鈥揔assel鈥揗arcus (RRKM) modeling. Direct dynamics trajectory simulations were carried out at E_col of 0.05 and 0.1 eV using the B3LYP/4-31G(d) level of theory. In contrast to the highly efficient reaction of MetH⁺ + ¹O₂, the reaction of Ac-MetH⁺ + ¹O₂ is extremely inefficient, despite there being exoergic pathways. Two product channels were observed, corresponding to transfer of two H atoms from Ac-MetH⁺ to ¹O₂ (H2T), and methyl elimination (ME) from a sulfone intermediate complex. Both channels are inhibited by collision energies, becoming negligible at E_col > 0.2 eV. Analysis of RRKM and trajectory results suggests that a complex-mediated mechanism might be involved at very low E_col, but direct, nonreactive collisions prevail over the entire E_col range and physical quenching of ¹O₂ occurs during the early stage of collisions.