Guided-Ion-Beam Scattering and Direct Dynamics Trajectory Study on the Reaction of Deprotonated Cysteine with Singlet Molecular Oxygen

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• 作者：Yigang Fang ; Fangwei Liu ; Rifat Emre ; Jianbo Liu
• 刊名：The Journal of Physical Chemistry B
• 出版年：2013
• 出版时间：March 14, 2013
• 年：2013
• 卷：117
• 期：10
• 页码：2878-2887
• 全文大小：494K
• 年卷期：v.117,no.10(March 14, 2013)
• ISSN：1520-5207

文摘

We present a study on the gas-phase reaction of deprotonated cysteine with the lowest electronically excited state of molecular oxygen O<sub>2sub>[a<sup>1sup>螖<sub>gsub>], including the measurement of the effects of collision energy (E<sub>colsub>) on reaction cross sections over a center-of-mass E<sub>colsub> range from 0.1 to 1.0 eV. Deprotonated cysteine was generated using electrospray ionization, and has a carboxylate anionic structure (HSCH<sub>2sub>CH(NH<sub>2sub>)CO<sub>2sub><sup>鈥?/sup>) in the gas phase. Three product ion channels were observed. The dissociation of HSCH<sub>2sub>CH(NH<sub>2sub>)CO<sub>2sub><sup>鈥?/sup> to NH<sub>2sub>CH<sub>2sub>CO<sub>2sub><sup>鈥?/sup> and neutral CH<sub>2sub>S has the largest cross section over the entire E<sub>colsub> range. This product channel is driven by the electronic excitation energy of <sup>1sup>O<sub>2sub> (the so-called dissociative excitation transfer), and is strongly suppressed by E<sub>colsub>. Two minor channels correspond to the formation of HSCH<sub>2sub>C(NH)CO<sub>2sub><sup>鈥?/sup> + H<sub>2sub>O<sub>2sub> via abstraction of two hydrogen atoms from HSCH<sub>2sub>CH(NH<sub>2sub>)CO<sub>2sub><sup>鈥?/sup> by <sup>1sup>O<sub>2sub>, and the formation of OSCH<sub>2sub>CH(NH<sub>2sub>)CO<sub>2sub><sup>鈥?/sup> radical via elimination of 路OH from an intermediate complex, respectively. Density functional theory calculations were used to locate various complexes, transition states, and products. Quasi-classical direct dynamics trajectory simulations were carried out at E<sub>colsub> = 0.2 eV using the B3LYP/4-31G(d) level of theory. Trajectory results were used to guide the construction of a reaction coordinate, discriminate between different mechanisms, and provide additional mechanistic insights. Analysis of trajectories highlights the importance of complex mediation at the early stages of all reactions, and suggests a partially concerted mechanism for H<sub>2sub>O<sub>2sub> elimination.