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Examination of the role of base-off cobalamin species (where the 5,6-dimethylbenzimidazole ligand coordinated to cobalt is detached by protonation of the imidazole nitrogen) in differentiation between homolytic and heterolytic cobalt-carbon bond cleavage mechanisms is a primary step in better understanding B12-dependent enzyme catalysis. X-ray absorption edge spectroscopy provides the first direct structural evidence of five-coordination in base-off adenosyl- and base-off methylcobalamin complexes at room temperature. Integration of 1s-3d pre-edge transitions of the base-off species reveals the dependence of coordination number on temperature. Gradual increases in 1s-3d transition intensities, as the temperature is increased from 180 K to 298 K, reflect a change in the coordination number from six (where a water molecule is presumed to occupy the coordination site vacated by the 5,6-dimethylbenzimidazole ligand) to primarily five-coordinate. Base-off configurations that strengthen the CoC bond may be both decreasing the tendency for homolytic cleavage while increasing the tendency for hetrolytic Co(I) B12 formation.
Journal of Inorganic Biochemistry
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Michael D. Wirt, Mark R. Chance. Temperature dependent coordination effects in base-off adenosyl and methylcobalamin by X-ray edge spectroscopy. Journal of Inorganic Biochemistry, Volume 49, Issue 4, 1993, Pages 265-273, https://doi.org/10.1016/0162-0134(93)80062-E.