Ishikita Hiroshi, Soudackov Alexander V, Hammes-Schiffer Sharon
Department of Chemistry, 104 Chemistry Building, Pennsylvania State University, University Park, Pennsylvania 16802, USA.
J Am Chem Soc. 2007 Sep 12;129(36):11146-52. doi: 10.1021/ja072708k. Epub 2007 Aug 18.
The mechanism for tyrosyl radical generation in the [Re(P-Y)(phen)(CO)3]PF6 complex is investigated with a multistate continuum theory for proton-coupled electron transfer (PCET) reactions. Both water and the phosphate buffer are considered as potential proton acceptors. The calculations indicate that the model in which the proton acceptor is the phosphate buffer species HPO(4)2- can successfully reproduce the experimentally observed pH dependence of the overall rate and H/D kinetic isotope effect, whereas the model in which the proton acceptor is water is not physically reasonable for this system. The phosphate buffer species HPO4(2-) is favored over water as the proton acceptor in part because the proton donor-acceptor distance is approximately 0.2 A smaller for the phosphate acceptor due to its negative charge. The physical quantities impacting the overall rate constant, including the reorganization energies, reaction free energies, activation free energies, and vibronic couplings for the various pairs of reactant/product vibronic states, are analyzed for both hydrogen and deuterium transfer. The dominant contribution to the rate arises from nonadiabatic transitions between the ground reactant vibronic state and the third product vibronic state for hydrogen transfer and the fourth product vibronic state for deuterium transfer. These contributions dominate over contributions from lower product states because of the larger vibronic coupling, which arises from the greater overlap between the reactant and product vibrational wave functions. These calculations provide insight into the fundamental mechanism of tyrosyl radical generation, which plays an important role in a wide range of biologically important processes.
采用质子耦合电子转移（PCET）反应的多态连续介质理论，研究了[Re(P - Y)(phen)(CO)₃]PF₆配合物中酪氨酸自由基生成的机制。水和磷酸盐缓冲液均被视为潜在的质子受体。计算结果表明，质子受体为磷酸盐缓冲物种HPO₄²⁻的模型能够成功再现实验观察到的总反应速率对pH的依赖性以及H/D动力学同位素效应，而质子受体为水的模型对于该体系在物理上是不合理的。磷酸盐缓冲物种HPO₄²⁻比水更适合作为质子受体，部分原因是由于其负电荷，磷酸盐受体的质子供体 - 受体距离约小0.2 Å。针对氢转移和氘转移，分析了影响总速率常数的物理量，包括各种反应物/产物振动态对的重组能、反应自由能、活化自由能和电子振动耦合。对于氢转移，速率的主要贡献来自基态反应物振动态与第三产物振动态之间的非绝热跃迁；对于氘转移，速率的主要贡献来自基态反应物振动态与第四产物振动态之间的非绝热跃迁。由于反应物和产物振动波函数之间的重叠更大，导致电子振动耦合更大，这些贡献比来自较低产物态的贡献更为显著。这些计算为酪氨酸自由基生成的基本机制提供了深入了解，酪氨酸自由基生成在广泛的生物学重要过程中起着重要作用。
