Department of Chemistry and Biochemistry, Queens College and the Graduate Center of the City University of New York, 65-30 Kissena Boulevard, Flushing, New York 11367, United States.
J Phys Chem B. 2011 Aug 18;115(32):9898-909. doi: 10.1021/jp205235d. Epub 2011 Jul 28.
We report a study on the reactions of protonated cysteine (CysH(+)) and tryptophan (TrpH(+)) with the lowest electronically excited state of molecular oxygen (O(2), a(1)Δ(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(+) + (1)O(2), the product channel corresponds to C(α)-C(β) bond rupture of a hydroperoxide intermediate CysOOH(+) accompanied by intramolecular H atom transfer, and subsequent dissociation to H(2)NCHCO(2)H(+), CH(3)SH and ground triplet state O(2). The reaction is driven by the electronic excitation energy of (1)O(2), the so-called dissociative excitation energy transfer. Quasi-classical direct dynamics trajectory simulations were calculated for CysH(+) + (1)O(2) at E(col) = 0.2 and 0.3 eV, using the B3LYP/6-21G 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 (1)O(2), 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 O(2) because this product charge state is energetically favorable. The reactions of CysH(+) + (1)O(2) and TrpH(+) + (1)O(2) 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 (1)O(2) to the form of internal energy needed to drive complex dissociation.
我们报告了关于质子化半胱氨酸 (CysH(+)) 和色氨酸 (TrpH(+)) 与分子氧的最低电子激发态 (O(2)，a(1)Δ(g)) 反应的研究，包括测量碰撞能量 (E(col)) 对反应截面的影响在质心 E(col) 范围为 0.05 到 1.0 eV。电子结构计算被用于研究沿反应坐标可能重要的复合物、过渡态和产物的性质。对于 CysH(+) + (1)O(2)，产物通道对应于过氧化物中间体 CysOOH(+) 的 C(α)-C(β) 键断裂，伴随着分子内 H 原子转移，随后解离为 H(2)NCHCO(2)H(+)、CH(3)SH 和基态三重态 O(2)。该反应由 (1)O(2) 的电子激发能驱动，即所谓的解离激发能转移。在 E(col) = 0.2 和 0.3 eV 下，使用 B3LYP/6-21G 方法对 CysH(+) + (1)O(2)进行了准经典直接动力学轨迹模拟。大多数轨迹形成具有显著寿命的中间体复合物，这意味着在反应的早期阶段复合物形成很重要。在 TrpH(+)与 (1)O(2)的反应中也观察到了解离激发能转移，导致 TrpOOH(+)中间体的解离。与 CysOOH(+)不同，TrpOOH(+)除了基态 O(2)之外，还解离成甘氨酸分子和带电荷的吲哚侧链，因为这种产物电荷状态在能量上是有利的。CysH(+) + (1)O(2)和 TrpH(+) + (1)O(2)的反应具有相似的 E(col)依赖性，即碰撞能强烈抑制，在 E(col) > 0.5 eV 时变得可以忽略不计。这与一个复杂介导的机制一致，其中长寿命的复合物对于将 (1)O(2)的电子能转化为驱动复合物解离所需的内部能的形式是至关重要的。
