Prytkova Tatiana R, Kurnikov Igor V, Beratan David N
Departments of Chemistry and Biochemistry, Duke University, Durham, NC 27708, USA.
J Phys Chem B. 2005 Feb 3;109(4):1618-25. doi: 10.1021/jp0457491.
A long-standing challenge in electron-transfer theory is to compute accurate rates of long-distance reactions in proteins. We describe an ab initio Hartree-Fock approach to compute electronic-coupling interactions and electron-transfer rates in proteins that allows the favorable comparison with experiment. The method includes the following key features; each is essential for reliable rate computations: (1) summing contributions over multiple tunneling pathways, (2) averaging couplings over thermally accessible protein conformations, (3) describing donor and acceptor electronic structure explicitly, including solvation effects, and averaging coupling over multiple energy-level crossings of the nearly degenerate donor-acceptor ligand-field states, and (4) eliminating basis set artifacts associated with diffuse basis functions. The strong dependence of coupling on donor-acceptor distance and on pathway interferences causes large variations of the computed electron-coupling values with protein geometry, and the strongest coupled conformers dominate the electron-transfer rate. As such, averaging over thermally accessible conformers of the protein and of the redox cofactors is essential. This approach was tested on six ruthenium-modified azurin derivatives using the high temperature nonadiabatic rate expression and compared with simpler pathways, average barrier, and semiempirical INDO models. Results of ab initio Hartree-Fock calculations with a split-valence basis set are in good agreement with the experimental rates. Predicted rates in the longer-distance derivatives are underestimated by 3-8-fold. This analysis indicates that the key ingredients needed for quantitatively reliable protein electron-transfer rate calculations are accessible.
电子转移理论中一个长期存在的挑战是计算蛋白质中长距离反应的准确速率。我们描述了一种从头算哈特里 - 福克方法，用于计算蛋白质中的电子耦合相互作用和电子转移速率，该方法能够与实验进行良好的比较。该方法包括以下关键特征；每个特征对于可靠的速率计算都是必不可少的：（1）对多个隧穿途径的贡献进行求和，（2）对热可及的蛋白质构象上的耦合进行平均，（3）明确描述供体和受体的电子结构，包括溶剂化效应，并对近简并供体 - 受体配体场态的多个能级交叉处的耦合进行平均，以及（4）消除与弥散基函数相关的基组伪影。耦合对供体 - 受体距离和途径干扰的强烈依赖性导致计算出的电子耦合值随蛋白质几何结构有很大变化，并且耦合最强的构象主导电子转移速率。因此，对蛋白质和氧化还原辅因子的热可及构象进行平均是必不可少的。使用高温非绝热速率表达式对六种钌修饰的天青蛋白衍生物测试了该方法，并与更简单的途径、平均势垒和半经验INDO模型进行了比较。使用分裂价基组的从头算哈特里 - 福克计算结果与实验速率吻合良好。在较长距离衍生物中的预测速率被低估了3 - 8倍。该分析表明，定量可靠的蛋白质电子转移速率计算所需的关键要素是可获取的。
