Vitamin B₁₂ derivatives (Cbls = cobalamins) exhibit photolytic properties upon excitation with light. These properties can be modulated by several factors including the nature of the axial ligands. Upon excitation, homolytic cleavage of the organometallic bond to the upper axial ligand takes place in photolabile Cbls. The photosensitive nature of Cbls has made them potential candidates for light-activated drug delivery. The addition of a fluorophore to the nucleotide loop of thiolato Cbls has been shown to shift the region of photohomolysis to within the optical window of tissue (600–900 nm). With this possibility, there is a need to analyze photolytic properties of unique Cbls which contain a Co–S bond. Herein, the photodissociation of one such Cbl, namely, N-acetylcysteinylcobalamin (NACCbl), is analyzed based on density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. The S₀ and S₁ potential energy surfaces (PESs), as a function of axial bond lengths, were computed to determine the mechanism of photodissociation. Like other Cbls, the S₁ PES contains metal-to-ligand charge transfer (MLCT) and ligand field (LF) regions, but there are some unique differences. Interestingly, the S₁ PES of NACCbl contains three distinct minima regions opening several possibilities for the mechanism of radical pair (RP) formation. The mild photoresponsiveness, observed experimentally, can be attributed to the small gap in energy between the S₁ and S₀ PESs. Compared to other Cbls, the gap shown for NACCbl is neither exactly in line with the alkyl Cbls nor the nonalkyl Cbls.

中文翻译：

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维生素B ₁₂硫醇衍生物的光介导性质

维生素B ₁₂衍生物（Cbls =钴胺素）在光激发下表现出光解特性。这些性质可以通过包括轴向配体的性质在内的几个因素来调节。激发后，在光不稳定的Cbls中有机金属键均被裂解成上轴向配体。Cbls的光敏性质使其成为光活化药物递送的潜在候选者。已显示在硫脲Cbls的核苷酸环上添加荧光团可将光均溶区域移至组织的光学窗口内（600–900 nm）。有这种可能性，需要分析包含Co-S键的独特Cbls的光解特性。在此，一种这样的Cbl，即N的光解离-乙酰半胱氨酰钴胺素（NACCbl）基于密度泛函理论（DFT）和时变DFT（TD-DFT）计算进行了分析。计算S ₀和S ₁势能面（PESs）作为轴向键长的函数，以确定光解离的机理。像其他Cbls一样，S ₁ PES包含金属到配体的电荷转移（MLCT）和配体场（LF）区域，但存在一些独特的区别。有趣的是，NACCb ₁的S ₁ PES包含三个不同的极小区域，为形成自由基对（RP）的机理打开了几种可能性。通过实验观察到的轻度光响应性可以归因于S ₁和S之间的能量小间隙_0个PES。与其他Cbl相比，NACCbl所示的间隙既不与烷基Cbl也不与非烷基Cbl一致。