Dissociative Excitation Energy Transfer in the Reactions of Protonated Cysteine and Tryptophan with Electronically Excited Singlet Molecular Oxygen (a1螖g)

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• 作者：Fangwei Liu ; Yigang Fang ; Yun Chen ; Jianbo Liu
• 刊名：Journal of Physical Chemistry B
• 出版年：2011
• 出版时间：August 18, 2011
• 年：2011
• 卷：115
• 期：32
• 页码：9898-9909
• 全文大小：1105K
• 年卷期：v.115,no.32(August 18, 2011)
• ISSN：1520-5207

文摘

We report a study on the reactions of protonated cysteine (CysH⁺) and tryptophan (TrpH⁺) with the lowest electronically excited state of molecular oxygen (O₂, a¹螖_g), including the measurement of the effects of collision energy (E_col) on reaction cross sections over the center-of-mass E_col range of 0.05 to 1.0 eV. Electronic structure calculations were used to examine properties of complexes, transition states and products that might be important along the reaction coordinate. For CysH⁺ + ¹O₂, the product channel corresponds to C^伪鈥揅^尾 bond rupture of a hydroperoxide intermediate CysOOH⁺ accompanied by intramolecular H atom transfer, and subsequent dissociation to H₂NCHCO₂H⁺, CH₃SH and ground triplet state O₂. The reaction is driven by the electronic excitation energy of ¹O₂, the so-called dissociative excitation energy transfer. Quasi-classical direct dynamics trajectory simulations were calculated for CysH⁺ + ¹O₂ at E_col = 0.2 and 0.3 eV, using the B3LYP/6鈥?1G method. Most trajectories formed intermediate complexes with significant lifetime, implying the importance of complex formation at the early stage of the reaction. Dissociative excitation energy transfer was also observed in the reaction of TrpH⁺ with ¹O₂, leading to dissociation of a TrpOOH⁺ intermediate. In contrast to CysOOH⁺, TrpOOH⁺ dissociates into a glycine molecule and charged indole side chain in addition to ground-state O₂ because this product charge state is energetically favorable. The reactions of CysH⁺ + ¹O₂ and TrpH⁺ + ¹O₂ present similar E_col dependence, i.e., strongly suppressed by collision energy and becoming negligible at E_col > 0.5 eV. This is consistent with a complex-mediated mechanism where a long-lived complex is critical for converting the electronic energy of ¹O₂ to the form of internal energy needed to drive complex dissociation.