Photolytic properties of the biologically active forms of vitamin B12

Abstract

The biologically active forms of vitamin B12, methylcobalamin (MeCbl) and adenosylcobalamin (AdoCbl), are important cofactors in a variety of enzymatic processes. In addition to their roles as cofactors, these B12 derivatives have unique photolytic properties based on the light-sensitivity of the organometallic CoC bond. Their photolysis is mediated by low-lying excited states, where photodissociation of the CoC bond leads to formation of singlet-born alkyl/cob(II)alamin radical pairs (RPs). Also, the geometric and electronic characteristics of cobalamins (Cbls) are different based on whether the 5,6-dimethylbenzimidazole base (DBI) is bound as the lower axial ligand (base-on) or replaced by water (base-off) in strongly acidic conditions. The focus of this review is to summarize the current understanding of these photolytic properties from a theoretical perspective. Potential energy surfaces (PESs) associated with low-lying excited states, constructed as functions of both axial bond lengths, provide the most reliable tool for understanding the photodissociation mechanisms. The primary computational method for calculating ground state properties is density functional theory (DFT), while time-dependent DFT (TD-DFT) is used for electronically excited states. Based on such constructed PESs, energy pathways connecting metal-to-ligand charge transfer (MLCT) and ligand field (LF) electronic states, can be associated with the light induced photo-homolysis of the CoC bond that is observed experimentally. Likewise, the crossing of the S1/S0 surfaces can be used to describe internal conversion (IC) to the ground state. Particular emphasis will be placed on the differences observed in the photodissociation mechanisms and the photolytic properties of the base-on versus base-off B12 cofactors. The possibility of intersystem crossing (ISC) and the formation of triplet RP is also presented based on semi-classical Landau-Zener theory.