Department of Chemistry, University of Louisville Louisville, KY, USA.
Department of Theoretical Chemistry, Institute of Chemistry, University of Silesia Katowice, Poland.
Front Chem. 2014 Feb 5;1:41. doi: 10.3389/fchem.2013.00041. eCollection 2013.
Coenzyme B12 (AdoCbl) is one of the most biologically active forms of vitamin B12, and continues to be a topic of active research interest. The mechanism of Co-C bond cleavage in AdoCbl, and the corresponding enzymatic reactions are however, not well understood at the molecular level. In this work, time-dependent density functional theory (TD-DFT) has been applied to investigate the photodissociation of coenzyme B12. To reduce computational cost, while retaining the major spectroscopic features of AdoCbl, a truncated model based on ribosylcobalamin (RibCbl) was used to simulate Co-C photodissociation. Equilibrium geometries of RibCbl were obtained by optimization at the DFT/BP86/TZVP level of theory, and low-lying excited states were calculated by TD-DFT using the same functional and basis set. The calculated singlet states, and absorption spectra were simulated in both the gas phase, and water, using the polarizable continuum model (PCM). Both spectra were in reasonable agreement with experimental data, and potential energy curves based on vertical excitations were plotted to explore the nature of Co-C bond dissociation. It was found that a repulsive (3)(σCo-C → σ(*) Co-C) triplet state became dissociative at large Co-C bond distance, similar to a previous observation for methylcobalamin (MeCbl). Furthermore, potential energy surfaces (PESs) obtained as a function of both Co-CRib and Co-NIm distances, identify the S1 state as a key intermediate generated during photoexcitation of RibCbl, attributed to a mixture of a metal-to-ligand charge transfer (MLCT) and a σ bonding-ligand charge transfer (SBLCT) states.
辅酶 B12（AdoCbl）是维生素 B12 中最具生物活性的形式之一，并且仍然是一个活跃的研究兴趣主题。然而，在分子水平上，对于 AdoCbl 中的 Co-C 键断裂机制以及相应的酶反应，人们了解得还不够。在这项工作中，我们应用含时密度泛函理论（TD-DFT）研究了辅酶 B12 的光解。为了降低计算成本，同时保留 AdoCbl 的主要光谱特征，我们使用基于核黄素钴胺素（RibCbl）的截断模型来模拟 Co-C 光解。通过在 DFT/BP86/TZVP 理论水平上进行优化，得到了 RibCbl 的平衡几何形状，并通过 TD-DFT 计算了低能激发态，使用相同的函数和基组。在气相和水中，使用极化连续体模型（PCM）模拟了计算得到的单重态和吸收光谱。两种光谱都与实验数据吻合良好，并绘制了基于垂直激发的势能曲线，以探讨 Co-C 键解离的性质。结果表明，一个排斥的（3）（σCo-C→σ*Co-C）三重态在较大的 Co-C 键距离处变得离解，类似于先前对甲基钴胺素（MeCbl）的观察结果。此外，作为 Co-CRib 和 Co-NIm 距离的函数获得的势能面（PES）确定 S1 态是在 RibCbl 光激发过程中产生的关键中间体，归因于金属-配体电荷转移（MLCT）和σ键-配体电荷转移（SBLCT）态的混合物。
