title: Temperature dependent coordination effects in base-off adenosyl and methylcobalamin by X-ray edge spectroscopy.
abstract: 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 Co-C bond may be both decreasing the tendency for homolytic cleavage while increasing the tendency for hetrolytic Co(I) B12 formation.
authors: Department of Chemistry, Georgetown University, Washington, DC.
authors: M D
authors: MD
authors: Wirt
authors: M R
authors: MR
authors: Chance
mesh_terms: D003038:Cobamides / Q000737:chemistry*; D015394:Molecular Structure; D013057:Spectrum Analysis; D013696:Temperature; D014805:Vitamin B 12 / Q000031:analogs & derivatives* / Q000737:chemistry; D014965:X-Rays
chemical_list: D003038:Cobamides; C019476:mecobalamin; C000913:cobamamide; D014805:Vitamin B 12