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 r estricted 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, c onstitute the most realistic models used to date for DFT calculations on co factor B-12 and its homolysis product. The calculations prove that both the rmodynamics and kinetics of the homolytic bond cleavage of the Co-C bond ha ve very little dependence on the position of the axial benzimidazole ligand . The generality of these results is confirmed by additional calculations o n [Co(Cor)(Benz)(CH2R)] (R = tetrahydrofuran), [Co(Cor)(Im)-(CH3)] (Im = im idazole), and [Co(Cor-CH3)(Benz)(CH3)] (Cor-CH3 = methylated con-in).