Shining light on the mechanism of photochemical alkene formation in vitamin B12

Abstract

A wide range of proteins and enzymes depend on alkylcobalamins (alkylCbl or vitamin B₁₂) for their activities, owing to the unique, biologically relevant Co–C bond. CarH, a regulatory protein in the bacterial carotenoid biosynthetic pathway, utilizes the photosensitivity of the Co–C bond in adenosylcobalamin (AdoCbl) for gene regulation. B₁₂-dependent reductive dehalogenases rely on chemical Co–C bond cleavage to form a Co(III)–halide bond during catalysis. Ultrafast spectroscopy demonstrates that photolytic Co–C bond cleavage in cobalamins begins with the generation of a Co(II) species and an alkyl radical. Interestingly, both CarH and reductive dehalogenases are thought to generate a highly reactive Co(I) species as part of their reactivity. We have used time-resolved measurements of alkylCbls under single turnover conditions to better characterize this reactivity. We demonstrate that Co(I) can be generated in nearly quantitative yield during anaerobic photolysis of alkylCbls in aqueous solution. Remarkably, the addition of alkyl halide to this Co(I) species does not produce quantitative yield of Co(III)-alkylCbl species as would be expected given the “supernucleophilic” nature of the Co(I) center. Instead, we find a branching pathway which has significant implications in Cbl-dependent enzymes and vitamin B₁₂ based organometallic photochemistry. Finally, we demonstrate that both the final oxidation state of the cobalamin product and the fate of the organic radical formed are solvent-dependent, an observation that has implications for CarH photochemistry.