Centre for Theoretical and Computational Chemistry, Department of Chemistry, University of Tromsø, 9037 Tromsø, Norway.
J Phys Chem B. 2011 Feb 3;115(4):737-48. doi: 10.1021/jp109793r. Epub 2010 Dec 21.
Linear and quadratic response time-dependent density functional theory (TD-DFT) has been applied to investigate absorption (Abs), circular dichroism (CD), and magnetic CD (MCD) spectra of cyanocobalamin (CNCbl) and methylcobalamin (MeCbl). Although electronically excited states of both cobalamins have been probed by applying different experimental techniques, their exact nature remains poorly understood from an electronic structure point of view. Recent theoretical studies have revealed a lot of relevant information about their properties but also left some unresolved issues related to the nature of individual transitions. In this contribution, not only Abs but also CD and MCD spectra of both cobalamins were computed for direct comparison with experiment. The results were evaluated with respect to the choice of exchange-correlation functional, basis set, and the environment (gas phase or solvent) used in the calculation. Taking into account the complexity of the CNCbl and MeCbl systems, reliable agreement between theory and experiment was achieved based on calculations employing the BP86 functional, particularly for the low-energy α/β bands. This spectral range has been traditionally interpreted as a vibrational progression associated with a single electronic excitation, but according to the present analysis for both cobalamins, these bands are best interpreted as consisting of multiple electronic transitions.
已应用线性和二次响应时间相关的密度泛函理论（TD-DFT）来研究氰钴胺素（CNCbl）和甲钴胺素（MeCbl）的吸收（Abs）、圆二色性（CD）和磁圆二色性（MCD）光谱。尽管已经通过应用不同的实验技术研究了这两种钴胺素的电子激发态，但从电子结构的角度来看，它们的确切性质仍未得到很好的理解。最近的理论研究揭示了有关其性质的很多相关信息，但也留下了一些与个别跃迁性质有关的未解决的问题。在本研究中，不仅计算了 CNCbl 和 MeCbl 的 Abs，还计算了它们的 CD 和 MCD 光谱，以便与实验直接进行比较。结果是根据计算中使用的交换相关函数、基组和环境（气相或溶剂）的选择进行评估的。考虑到 CNCbl 和 MeCbl 系统的复杂性，基于采用 BP86 函数的计算，理论与实验之间实现了可靠的一致性，特别是对于低能量的α/β 带。这个光谱范围传统上被解释为与单个电子激发相关的振动进展，但根据对这两种钴胺素的分析，这些带最好被解释为由多个电子跃迁组成。
