Electronic and Steric Influence of Trans Axial Base on the Stereoelectronic Properties of Cobalamins

Density functional theory (DFT) has been applied to investigate the relationship between steric and electronic properties of the trans axial base and energetics of Co−C bond cleavage in models of coenzyme B 12 . By using structurally reliable six-coordinate models, B-[Co III (corrin)]-R +, it was shown that for a given base (B) the energy of homolytic cobalt−carbon bond cleavage correctly follows the Co−C R bond lengthening. For a given axial ligand (R) the dissociation energy is very weakly dependent on the trans axial base and correlates with its basicity. Analysis of the five-coordinate homolysis products, B-[Co II (corrin)] +, shows that the Co II −N B bond length is in the range of ∼2.2 Å, slightly shorter in comparison to six-coordinate analogues, while five-coordinate heterolysis products, B-[Co III (corrin)] +2, have the Co III −N B bond significantly shorter ∼1.9 Å. This noticeable difference suggests that controlling the Co−N B bond length could be an effective way to promote biologically important homolysis of the Co−C bond in B 12 -dependent enzymes over abiological heterolysis.