Center for Modeling & Simulation Chemistry, Institute of Theoretical Chemistry, Shandong University, Jinan 250100, PR China.
J Phys Chem B. 2009 Dec 31;113(52):16681-8. doi: 10.1021/jp9077689.
Charge transfer between tyrosine and tryptophan residues in proteins is continuously a hot topic because of its important biological implication. On the basis of DFT calculations and ab initio molecular dynamics simulations, several possible proton/electron cooperative transfer mechanisms from tyrosine to a tryptophan radical (or cation) without or with the assistence of a base in proteins are proposed in this work which range from direct proton-coupled pi-electron pi-channel/sigma-channel transfers (PC(pi)E(pi)T versus PC(pi)E(sigma)T) to proton-coupled long-range hopping mechanisms depending on the peptide conformations. In general, because of a smaller ionization potential, tryptophan readily behaves as two oxidized states: dehydrogenated neutral radical and ionized radical cation. For the neutral radical, the proton/electron transfers between a tyrosine and a tryptophan radical prefer a cooperative direct coupling mode through the PC(pi)E(pi)T or PC(pi)E(sigma)T mechanism if two residues are proximal or can approach each other, while they cannot take place without assistence if two residues are far apart. The tryptophan radical cation prefers to form a complex with tyrosine to stabilize the hole when two residues are proximal or can approach each other. However, the electron transfer from tyrosine to tryptophan could occur via a hopping mechanism but is regulated by a base as a proton acceptor in the vicinity of tyrosine when two residues are separated. The dynamics properties and characters for these transfer events are also presented. The energetics comparison indicates that the energy barriers for the direct PCET (PC(pi)E(pi)T or PC(pi)E(sigma)T) mechanisms are higher than those of the base-assisting hopping mechanism (6.8-12.5 versus <or=4.9 kcal/mol), implying the latter is a favorable way for the proton/electron cooperative transfers. Hopefully, this work provides some helpful information for understanding the mechanisms of physiologically important electron transfer reactions.
在蛋白质中，酪氨酸和色氨酸残基之间的电荷转移因其重要的生物学意义而一直是一个热门话题。本工作基于密度泛函理论（DFT）计算和从头分子动力学（AIMD）模拟，提出了几种可能的质子/电子协同转移机制，包括酪氨酸向色氨酸自由基（或阳离子）的直接质子耦合π-电子π-通道/σ-通道转移（PC(pi)E(pi)T 与 PC(pi)E(sigma)T）和依赖于肽构象的质子耦合远程跳跃机制。一般来说，由于较小的电离势，色氨酸容易表现为两种氧化态：脱氢中性自由基和离子化自由基阳离子。对于中性自由基，当两个残基靠近或可以接近时，酪氨酸和色氨酸自由基之间的质子/电子转移通过 PC(pi)E(pi)T 或 PC(pi)E(sigma)T 机制优先发生协同直接耦合，而当两个残基相距较远时，则需要辅助才能发生转移。当两个残基靠近或可以接近时，色氨酸自由基阳离子更喜欢与酪氨酸形成复合物以稳定空穴。然而，当两个残基分离时，电子可以通过跳跃机制从酪氨酸转移到色氨酸，但由酪氨酸附近的碱基作为质子受体来调节转移。还展示了这些转移事件的动力学性质和特征。能量比较表明，直接 PCET（PC(pi)E(pi)T 或 PC(pi)E(sigma)T）机制的能垒高于碱基辅助跳跃机制（6.8-12.5 与 <或=4.9 kcal/mol），这意味着后者是质子/电子协同转移的有利途径。希望这项工作为理解生理上重要的电子转移反应机制提供了一些有价值的信息。
