Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371.
J Phys Chem A. 2011 Aug 25;115(33):9308-13. doi: 10.1021/jp2052807. Epub 2011 Aug 2.
Controversy remains regarding the suitable density functionals for the calculation of vitamin B(12) systems that contain cobalt. To identify the optimum functionals, geometry optimization calculations were performed on a full-size model of methylcobalamin (MeCbl) using the B3LYP, B3LYP-D, BP86, and BP86-D methods in conjunction with the 6-31G* basis set. Single-point energy evaluations were also performed with the 6-311+G(2d,p) basis set. Consistent with previous studies, the BP86-optimized geometry showed fairly good agreement with the experimental geometry. Various factors that may influence the homolytic bond dissociation energy (BDE) of the Co-C bond of MeCbl were systematically evaluated with these methods. Our analysis demonstrated that dispersion was the largest correction term that influenced the magnitude of BDE. Previous studies have shown that B3LYP significantly underestimates BDE, whereas BP86 gives BDE values that are fairly close to the experimental values (36-37 kcal/mol). The same trend in the relative magnitudes of the BDEs was observed in the present calculations. However, BP86 underestimated the BDE for a full model of MeCbl. When the amount of Hartree-Fock exchange in the B3LYP functional was reduced to 15% and the dispersion correction was made (i.e., B3LYP*-D), the calculated BDE was in good accord with experimental values. B3P86-D also performed well. A detailed analysis was undertaken to determine which atoms in cobalamin have large dispersion interactions with a methyl fragment of MeCbl.
关于含有钴的维生素 B(12) 体系的合适密度泛函，仍存在争议。为了确定最佳泛函，使用 B3LYP、B3LYP-D、BP86 和 BP86-D 方法，结合 6-31G基组，对甲基钴胺素（MeCbl）的全尺寸模型进行了几何优化计算。还使用了 6-311+G(2d,p)基组进行单点能量评估。与先前的研究一致，BP86 优化的几何结构与实验几何结构相当吻合。使用这些方法系统地评估了可能影响 MeCbl 的 Co-C 键的均裂键离解能（BDE）的各种因素。我们的分析表明，色散是影响 BDE 大小的最大修正项。先前的研究表明，B3LYP 显著低估了 BDE，而 BP86 给出的 BDE 值与实验值（36-37 kcal/mol）相当接近。在本计算中，也观察到了 BDE 相对大小的相同趋势。然而，BP86 低估了 MeCbl 全模型的 BDE。当 B3LYP 泛函中的 Hartree-Fock 交换量减少到 15%并进行色散修正（即 B3LYP-D）时，计算出的 BDE 与实验值吻合较好。B3P86-D 也表现良好。进行了详细分析，以确定钴胺素中的哪些原子与 MeCbl 的甲基片段具有较大的色散相互作用。
