Inscore Frank E, Knottenbelt Sushilla Z, Rubie Nick D, Joshi Hemant K, Kirk Martin L, Enemark John H
Department of Chemistry, University of Arizona, Tucson, Arizona 85721, USA.
Inorg Chem. 2006 Feb 6;45(3):967-76. doi: 10.1021/ic0506815.
X-ray crystallography and resonance Raman (rR) spectroscopy have been used to further characterize (Tp*)MoO(qdt) (Tp* is hydrotris(3,5-dimethyl-1-pyrazolyl)borate and qdt is 2,3-quinoxalinedithiolene), which represents an important benchmark oxomolybdenum mono-dithiolene model system relevant to various pyranopterin Mo enzyme active sites, including sulfite oxidase. The compound (Tp*)MoO(qdt) crystallizes in the triclinic space group, P1, where a = 9.8424 (7) A, b = 11.2323 (8) A, c = 11.9408 (8) A, alpha = 92.7560 (10) degrees, beta = 98.9530 (10) degrees, and gamma = 104.1680 (10) degrees. The (Tp*)MoO(qdt) molecule exhibits the distorted six-coordinate geometry characteristic of related oxo-Mo(V) systems possessing a single coordinated dithiolene ligand. The first coordination sphere bond lengths and angles in (Tp*)MoO(qdt) are very similar to the corresponding structural parameters for (Tp*)MoO(bdt) (bdt is 1,2-benzenedithiolene). The relatively small inner-sphere structural variations observed between (Tp*)MoO(qdt) and (Tp*)MoO(bdt) strongly suggest that geometric effects are not a major contributor to the significant electronic structural differences reported for these two oxo-Mo(V) dithiolenes. Therefore, the large differences observed in the reduction potential and first ionization energy between the two molecules appear to derive primarily from differences in the effective nuclear charges of their respective sulfur donors. However, a subtle perturbation to Mo-S bonding is implied by the nonplanarity of the dithiolene chelate ring, which is defined by the fold angle. This angular distortion (theta = 29.5 degrees in (Tp*)MoO(qdt); 21.3 degrees in (Tp*)MoO(bdt)) observed between the MoS2 and S-C=C-S planes may contribute to the electronic structure of these oxo-Mo dithiolene systems by controlling the extent of S p-Mo d orbital overlap. In enzymes, the fold angle may be dynamically modulated by the pyranopterin, thereby functioning as a transducer of vibrational energy associated with protein conformational changes directly to the active site via changes in the fold angle. This process could effectively mediate charge redistribution at the active site during the course of atom- and electron-transfer processes. The rR spectrum shows bands at 348 and 407 cm(-1). From frequency analysis of the normal modes of the model, [(NH3)3MoO(qdt)]1+, using the Gaussian03 suite of programs, these bands are assigned as mixed-mode Mo-S vibrations of the five-membered Mo-ditholene core structure. Raman spectroscopy has also provided additional evidence for an in-plane pseudo-sigma dithiolene S-Mo d(xy) covalent bonding interaction in (Tp*)MoO(qdt) and related oxo-Mo-dithiolenes that has implications for electron-transfer regeneration of the active site in sulfite oxidase involving the pyranopterin dithiolene.
X射线晶体学和共振拉曼（rR）光谱已被用于进一步表征（Tp*）MoO（qdt）（Tp为氢三（3,5-二甲基-1-吡唑基）硼酸盐，qdt为2,3-喹喔啉二硫烯），它代表了一个重要的基准氧钼单二硫烯模型体系，与包括亚硫酸盐氧化酶在内的各种吡喃蝶呤钼酶活性位点相关。化合物（Tp）MoO（qdt）结晶于三斜空间群P1中，其中a = 9.8424（7）Å，b = 11.2323（8）Å，c = 11.9408（8）Å，α = 92.7560（10）°，β = 98.9530（10）°，γ = 104.1680（10）°。（Tp*）MoO（qdt）分子呈现出具有单个配位二硫烯配体的相关氧钼（V）体系的扭曲六配位几何特征。（Tp*）MoO（qdt）中的第一配位层键长和键角与（Tp*）MoO（bdt）（bdt为1,2-苯二硫烯）的相应结构参数非常相似。在（Tp*）MoO（qdt）和（Tp*）MoO（bdt）之间观察到的相对较小的内球结构变化强烈表明，几何效应不是这两种氧钼（V）二硫烯所报道的显著电子结构差异的主要贡献因素。因此，在这两个分子之间观察到的还原电位和第一电离能的巨大差异似乎主要源于它们各自硫供体的有效核电荷的差异。然而，二硫烯螯合环的非平面性暗示了对Mo-S键的微妙扰动，这由折叠角定义。在MoS2和S-C=C-S平面之间观察到的这种角扭曲（在（Tp*）MoO（qdt）中θ = 29.5°；在（Tp*）MoO（bdt）中为21.3°）可能通过控制S p-Mo d轨道重叠程度对这些氧钼二硫烯体系的电子结构有贡献。在酶中，折叠角可能由吡喃蝶呤动态调节，从而通过折叠角的变化作为与蛋白质构象变化相关的振动能量直接传递到活性位点的转换器。这个过程可以在原子和电子转移过程中有效地介导活性位点的电荷重新分布。rR光谱在348和407 cm⁻¹处显示出谱带。使用Gaussian03程序套件对模型[（NH3）3MoO（qdt）]¹⁺的正常模式进行频率分析后，这些谱带被指定为五元钼-二硫烯核心结构的混合模式Mo-S振动。拉曼光谱也为（Tp*）MoO（qdt）和相关的氧钼-二硫烯中的面内伪σ二硫烯S-Mo d（xy）共价键相互作用提供了额外证据，这对涉及吡喃蝶呤二硫烯的亚硫酸盐氧化酶活性位点的电子转移再生有影响。
