Jensen Kasper P
Department of Chemistry, Yale University, New Haven, Connecticut 06511, USA.
J Phys Chem B. 2005 May 26;109(20):10505-12. doi: 10.1021/jp050802m.
The electronic structure and the ligand-field spectrum of cobalt(I) corrin is reported using complete active space multiconfigurational perturbation theory (CASPT2) to address some inconsistencies and the nature of the cobalt(I) "supernucleophile", cob(I)alamin. An assignment of six of the seven intense lines in the experimental spectrum is obtained at a root-mean-square accuracy of 0.14 eV and largest error of 0.21 eV. Agreement is significantly better for CASPT2 than density functional theory (DFT), but DFT does surprisingly well. The correlated wave function implies that the ground state of Co(I) corrin is severely multiconfigurational, with only 67% of the d(8) reference configuration and prominent contributions of 20% from open-shell metal-to-ligand charge-transfer configurations. The ground state exhibits a fascinating degree of covalency between cobalt and the nitrogen orbitals, described by the bonding and antibonding orbital pair of a cobalt d-orbital and a delta-orbital linearly combined from nitrogen orbitals. Thus, the standard description of the d(8) supernucleophile is not completely valid. From a biological perspective, the mixing in of Co(II) configurations in cob(I)alamin may be an important reason for the redox accessibility of the formal Co(I) state of the cofactor, which again provides the catalytic power for one half-reaction of enzymes such as cobalamin-dependent methionine synthase.
利用完全活性空间多组态微扰理论（CASPT2）报道了钴（I）钴胺素的电子结构和配体场光谱，以解决一些不一致问题以及钴（I）“超亲核试剂”钴胺素（cob(I)alamin）的本质。在均方根精度为0.14 eV且最大误差为0.21 eV的情况下，对实验光谱中的七条强线中的六条进行了归属。CASPT2的结果明显优于密度泛函理论（DFT），但DFT的表现也出奇地好。相关波函数表明，钴（I）钴胺素基态具有严重的多组态特征，只有67%的d(8)参考组态，开壳层金属到配体电荷转移组态的贡献显著，达20%。基态在钴和氮轨道之间表现出迷人的共价程度，由钴d轨道和由氮轨道线性组合而成的δ轨道的成键和反键轨道对来描述。因此，d(8)超亲核试剂的标准描述并不完全有效。从生物学角度来看，钴胺素（cob(I)alamin）中混入Co(II)组态可能是辅因子形式上的Co(I)态具有氧化还原可及性的一个重要原因，这再次为诸如钴胺素依赖性甲硫氨酸合酶等酶的一个半反应提供了催化能力。
