Hofmann Matthias
Anorganisch-Chemisches Institut, Universität Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany.
J Biol Inorg Chem. 2009 Sep;14(7):1023-35. doi: 10.1007/s00775-009-0545-1. Epub 2009 May 30.
Mo(SSCH3)(S2C2(CH3)2)2 complexes with charges x between -3 and +3 were investigated by density functional theory computations as minimal nitrate reductase active-site models. The strongly reduced species (x = -2, -3) exist preferentially as pentacoordinate sulfo complexes separated from a thiolate anion. The oxidized extremes (x > 0) clearly prefer hexacoordinate complexes with an eta(2)-MeSS ligand. Among the neutral and especially for the singly negatively charged species structures with eta(2)-MeSS and eta(1)-MeSS ligands are energetically close to the sulfo methyl sulfide complex without SS bonding. For x = -1 the three isomers lie in a 1.5 kcal mol(-1) energy range. Putative mechanistic pathways for nitrate reduction from the literature were investigated computationally: (1) reduction at a pentacoordinate sulfo complex, (2) reduction at the ligand, and (3) reduction at the molybdenum center with an R-S-S ligand. All three pathways could be traced at least for some overall charges but no definite conclusion can be drawn about the mechanism. Complexes with larger dithiolato ligands were also computed in order to model the tricyclic metallopterin framework more accurately: the first heterocyclus (5,6-dihydro-2H-pyran) stabilizes the nitrate complex and the molybdenum oxo product complex by approximately 10 kcal mol(-1) and also reduces the activation barrier (by approximately 5 kcal mol(-1)). The effect of the second (1,2,3,4-tetrahydropyrazin) and third heterocyclus (2-amino-3H-pyrimidin-4-one) on the relative energies is relatively small. For bigger models derived from an experimental protein structure, nitrate reduction at a persulfo molybdenum(IV) complex fragment (mechanism 3) is clearly favored over the oxidation of a molybdenum-bound sulfur atom (mechanism 2). Mechanism 1 could not be investigated for the big models but seems the least favorable on the basis of the results from smaller models.
通过密度泛函理论计算研究了电荷x在-3到+3之间的Mo(SSCH3)(S2C2(CH3)2)2配合物，将其作为最小的硝酸还原酶活性位点模型。强还原态物种（x = -2，-3）优先以与硫醇盐阴离子分离的五配位硫配合物形式存在。氧化态极端情况（x > 0）明显更倾向于具有η(2)-MeSS配体的六配位配合物。在中性物种中，特别是对于单负电荷物种，具有η(2)-MeSS和η(1)-MeSS配体的结构在能量上与没有SS键的硫甲基硫醚配合物相近。对于x = -1，三种异构体处于1.5 kcal mol(-1)的能量范围内。对文献中推测的硝酸还原机理途径进行了计算研究：(1) 在五配位硫配合物处还原；(2) 在配体处还原；(3) 在具有R-S-S配体的钼中心处还原。至少对于某些总电荷，所有这三种途径都可以追踪到，但关于机理无法得出明确结论。还计算了具有更大二硫醇盐配体的配合物，以便更准确地模拟三环金属蝶呤框架：第一个杂环（5,6-二氢-2H-吡喃）使硝酸配合物和钼氧产物配合物稳定约10 kcal mol(-1)，并且还降低了活化能垒（约5 kcal mol(-1)）。第二个（1,2,3,4-四氢吡嗪）和第三个杂环（2-氨基-3H-嘧啶-4-酮）对相对能量的影响相对较小。对于从实验蛋白质结构衍生的更大模型，过硫钼(IV)配合物片段处的硝酸还原（机理3）明显优于钼结合硫原子的氧化（机理2）。对于大模型无法研究机理1，但根据较小模型的结果，它似乎是最不利的。
