Coenzyme B₁₂ (adenosylcobalamin = AdoCbl)-dependent enzymes catalyze complex molecular transformations by employing radical chemistry. The initial step in the native catalytic cycle, upon substrate binding, involves homolytic cleavage of the Co–C bond of AdoCbl to form the Co(II)/Ado^• radical pair (RP). Formation of Co(II)/Ado^• is subsequently coupled with H atom abstraction from the substrate. Interestingly, these same RPs can be generated upon light absorption without presence of a substrate. Herein, the photochemistry associated with the mechanism of Co–C bond photocleavage inside the AdoCbl-dependent ethanolamine ammonia-lyase (EAL) was investigated using a combined time-dependent density functional theory and molecular mechanics (TD-DFT/MM) approach. Excited state potential energy surfaces (PESs), constructed as a function of axial bond lengths, were used to understand the photocleavage of the Co–C bond and to elucidate the mechanism of photodissociation for AdoCbl inside the enzyme. The S₁ PES is characterized by two minima regions, namely, metal-to-ligand charge transfer (MLCT) and ligand field (LF) states, which are key minima regions along the reaction pathway. There are two possible routes for photolysis of AdoCbl inside EAL named Path A and Path B. Path B is slightly more energetically favorable than Path A and involves the elongation of the Co–N_ax bond followed by the elongation of both axial bonds Co–C and Co–N_ax. To further understand the effect of environment on the formation of RP, the photochemical data for AdoCbl-dependent EAL was also compared with base-on and base-off AdoCbl in solution.

中文翻译：

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辅酶B ₁₂依赖的乙醇胺氨解酶中光诱导自由基对形成的机理

辅酶B ₁₂（腺苷钴胺素= AdoCbl）依赖性酶通过采用自由基化学作用催化复杂的分子转化。底物结合后，天然催化循环的第一步涉及将AdoCbl的Co–C键均质裂解，形成Co（II）/ Ado ^•自由基对（RP）。Co（II）/ Ado的形成^•随后将H 2 O与从底物提取H原子耦合。有趣的是，在不存在底物的情况下，在吸收光时可以生成这些相同的RP。在本文中，使用结合时间依赖性密度泛函理论和分子力学（TD-DFT / MM）的方法研究了与AdoCbl依赖性乙醇胺氨裂解酶（EAL）内部Co-C键光裂解机理相关的光化学。激发态势能面（PESs）是轴向键长的函数，用于理解Co–C键的光裂解，并阐明了酶内部AdoCbl的光解离机理。S ₁PES的特征是两个最小区域，即金属到配体的电荷转移（MLCT）和配体场（LF）状态，它们是反应路径上的关键最小区域。存在用于EAL命名路径A和路径B.路径B的内部AdoCbl的光解两种可能的途径是稍微能量上高于路径A有利并涉及CO-N的伸长_斧随后两个轴向键CO-C的伸长键和Co–N _ax。为了进一步了解环境对RP形成的影响，还将AdoCbl依赖的EAL的光化学数据与溶液中的碱式和碱式AdoCbl进行了比较。