Reactions of Deprotonated Tyrosine and Tryptophan with Electronically Excited Singlet Molecular Oxygen (a1螖g): A Guided-Ion-Beam Scattering, Statistical Modeling, and Trajectory

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• 作者：Fangwei Liu ; Yigang Fang ; Yun Chen ; Jianbo Liu
• 刊名：The Journal of Physical Chemistry B
• 出版年：2012
• 出版时间：June 7, 2012
• 年：2012
• 卷：116
• 期：22
• 页码：6369-6379
• 全文大小：549K
• 年卷期：v.116,no.22(June 7, 2012)
• ISSN：1520-5207

文摘

The reactions of deprotonated tyrosine ([Tyr-H]^{鈭?/sup>) and tryptophan ([Trp-H]鈭?/sup>) with the lowest electronically excited state of molecular oxygen O₂[a1螖_g] have been studied in the gas phase, including the measurement of the effects of collision energy (E_col) on reaction cross sections over a center-of-mass E_col range from 0.05 to 1.0 eV. [Tyr-H]鈭?/sup> and [Trp-H]鈭?/sup> were generated using electrospray ionization, and both have a pure carboxylate anion structure in the gas phase. Density functional theory calculations and RRKM modeling were used to examine properties of various complexes, transition states, and products that might be important along the reaction coordinate. It was found that deprotonation of Tyr and Trp results in a large effect on their 1O₂-mediated oxidation. For [Tyr-H]鈭?/sup>, the reaction corresponds to the formation of a hydroperoxide intermediate, followed by intramolecular H transfer and subsequent dissociation to product ion 4-(2-aminovinyl)phenolate, and neutral H₂O₂ and CO₂. Despite that the reaction is 1.83 eV exothermic, the reaction cross section shows a threshold-like behavior at low E_col and increases with increasing E_col, suggesting that the reaction bears an activation barrier above the reactants. Quasi-classical, direct dynamics trajectory simulations were carried out for [Tyr-H]鈭?/sup> + 1O₂ at E_col = 0.75 eV, using B3LYP/4鈥?1G* level of theory. Trajectories demonstrated the intermediacy of complexes at the early stage of the reaction. A similar product channel was observed in the reaction of [Trp-H]鈭?/sup> with 1O₂, yielding product ion 3-(2-aminovinyl)indol-1-ide, H₂O₂ and CO₂. However, the reaction cross section of [Trp-H]鈭?/sup> is strongly suppressed by E_col and becoming negligible at E_col > 1.0 eV, indicating that this reaction proceeds without energy barriers above the reactants.}