Kacprzak Sylwia, Kaupp Martin, MacMillan Fraser
Institut für Anorganische Chemie, Universität Würzburg, Am Hubland, D 97074 Würzburg, Germany.
J Am Chem Soc. 2006 May 3;128(17):5659-71. doi: 10.1021/ja053988b.
Recent multifrequency EPR studies of the "high-affinity" quinone binding site of quinol oxidase (Q(H) site) have suggested a very asymmetric hydrogen-bonding environment for the semiquinone radical anion state. Single-sided hydrogen bonding to the O1 carbonyl position was one of the proposals, which contrasts with some previous experimental indications. Here density functional calculations of the EPR parameters (g-tensors, 13C, 1H, and 17O hyperfine tensors) for a wide variety of supermolecular model complexes have been used to provide insight into the detailed relations among structure, environment, and EPR parameters of ubisemiquinone radical anions. A single-sided binding model is not able to account for the experimentally observed low g(x) component of the g-tensor or for the observed magnitude of the asymmetry of the 13C carbonyl HFC tensors. Based on the detailed comparison between computation and experiment, a model with two hydrogen bonds to O1 and one hydrogen bond to O4 is suggested for the Q(H) site, but a model with one more hydrogen bond on each side cannot be excluded. Several general conclusions on the interrelations between EPR parameters and hydrogen bond patterns of ubisemiquinones in proteins are provided.
近期对醌氧化酶“高亲和力”醌结合位点（Q(H)位点）的多频电子顺磁共振（EPR）研究表明，半醌自由基阴离子态存在非常不对称的氢键环境。单侧氢键与O1羰基位置相连是其中一种推测，这与之前的一些实验迹象形成对比。在此，通过对多种超分子模型配合物的EPR参数（g张量、13C、1H和17O超精细张量）进行密度泛函计算，以深入了解泛半醌自由基阴离子的结构、环境和EPR参数之间的详细关系。单侧结合模型无法解释实验观测到的g张量低g(x)分量，也无法解释观测到的13C羰基超精细耦合（HFC）张量不对称性的大小。基于计算与实验的详细比较，提出了一种Q(H)位点模型，该模型中存在与O1的两个氢键和与O4的一个氢键，但两侧各多一个氢键的模型也不能排除。文中给出了关于蛋白质中泛半醌的EPR参数与氢键模式之间相互关系的几个一般性结论。
