Computational modeling of electronically excited states in cobalamin-dependent reactions.

The current understanding of the photolytic properties of Vitamin B12 derivatives or cobalamins are summarized from a computational point of view. The focus is on two non-alkylcobalamins, cyanocobalamin (CNCbl) and hydroxocobalamin (HOCbl), two alkylcobalamins, methylcobalamin (MeCbl) and adenosylcobalamin (AdoCbl), as well as the stable cob(II)alamin radical. Photolysis of alkylcobalamins involves low-lying singlet excited states where photo-dissociation of the Co-C bond forms singlet-born alkyl/cob(II)alamin radical pairs (RPs). Potential energy surfaces (PESs) of low-lying excited states as functions of both axial bonds provide the most reliable tool for analysis of photochemical and photophysical properties. Due to the size limitations associated with the cobalamins, the primary method for calculating ground state properties is density functional theory (DFT), with time-dependent DFT (TD-DFT) mainly used for electronically excited states. The energy pathways on the lowest singlet surfaces of the alkylcobalamins, connect metal-to-ligand charge transfer (MLCT) and ligand field (LF) minima associated with photo-homolysis of the Co-C bond observed experimentally. Additionally, energy pathways between minima and seams associated with crossing of S1/S0 surfaces are the most efficient for internal conversion (IC) to the ground state. Depending on the specific cobalamin, such IC may involve simultaneous elongation of both axial bonds (CNCbl), or detachment of axial base coupled with corrin ring distortion (MeCbl). The possible involvement of triplet RPs is also discussed, and a mechanism of intersystem crossing based on Landau-Zener theory is presented