A density functional study on the effect of the trans axial ligand of cobalamin on the homolytic cleavage of the Co-C bond

Density functional theory (DFT) Becke3LYP calculations including full and restricted geometry optimizations are carried out on the complexes [Co(Cor)(Benz)(CH3)] (Cor = corrin, Benz = benzimidazole), [Co(Cor)(Benz)], [Co(Cor)(CH3)], and [Co(Cor)]. These systems, despite the absence of side-chains, constitute the most realistic models used to date for DFT calculations on cofactor B12 and its homolysis product. The calculations prove that both thermodynamics and kinetics of the homolytic bond cleavage of the Co-C bond have very little dependence on the position of the axial benzimidazole ligand. The generality of these results is confirmed by additional calculations on [Co(Cor)(Benz)(CH2R)] (R = tetrahydrofuran), [Co(Cor)(Im)-(CH3)] (Im = imidazole), and [Co(Cor-CH3)(Benz)(CH3)] (Cor-CH3 = methylated corrin).