Stabilizing intramolecular cobalt-imidazole coordination with a remote methyl group in the backbone of a cofactor B 12 -protein model.

This communication describes the stabilizing effect (ΔΔG° = -4 kJ mol-1 ) of a remote methyl group in the backbone of a cobalamin-enzyme mimic on intramolecular imidazole-cobalt coordination. For this purpose, two B12 derivatives with an appended imidazole base were synthesized and analysed with spectrophotometric pH titrations. Qualitative conformation analysis of the backbone structure suggests that a thermodynamically unfavoured gauche interaction in the base-off form of a model containing an (R)-configured CH3 group at position C176 of the linker between the corrin ring and the terminal imidazole ligand steers the base toward cobalt coordination.