Dealkylation of coenzyme B-12 and related organocobalamins: Ligand structural effects on rates and mechanisms of hydrolysis

Rates and mechanisms of dealkylations of coenzyme B-12, Ado-B-12, and of fi ve related organocobalamin compounds, including 2',5'-dideoxyadenosyl, 3',5 '-dideoxyadenosyl, 2',3',5'-trideoxyadenosyl, 1,5-dideoxyribofuranosyl, and tetrahydrofurfuryl complexes (2'dAdo-B-12, 3'dAdo-B-12, 2',3'ddAdo-B-12, 1 dRF-B-12, and THFF-B-12, respectively), were determined in acidic solutions . In each case, competitive homolytic and acid-induced hydrolytic cobalt-ca rbon bond decomposition pathways were identified. Two mechanisms were obser ved for Co-C bond hydrolysis: the first, involving initial depurination fol lowed by elimination from an organometallic intermediate, predominates for 2'dAdo-B-12 and 2',3'-ddAdoB(12); the second path, involving ring-opening p rotonation at the ribofuranosyl oxygen, analogous to hydrolyses of simple b eta-hydroxy and beta-alkoxy complexes, predominates for the other four comp lexes. The rates of both hydrolysis pathways exhibited a marked dependence on the ligand functional groups. Ado-B-12, the most substituted and most st able of the complexes, decomposes nearly 10 000-fold more slowly than the l east stable, unsubstituted THFF-B-12 complex. Systematic variation of the h ydroxy and adenine substituents on the furanosyl ring afforded insights int o the roles of these substituents in effecting this large stabilization tow ard hydrolysis. Because of the extreme hydrolytic stability of the coenzyme , biologically relevant homolytic dissociation of 5'-deoxyadenosyl radical is competitive with hydrolysis over a wide pH range where the unprotonated, base-on form is kinetically dominant. Determination of the pH dependence o f the dealkylation rates of Ado-B-12, 2'dAdo-B-12, and 3'dAdo-B-12 afforded quantification of the competition between homolytic and hydrolytic paths. In contrast to hydrolysis, the limiting homolytic Co-C bond dissociation ra te was found to be insensitive to hydroxy substitution. Finally, broader is sues relevant to organocobalamin chemistry and also bearing on earlier obse rvations, are considered, among them protonation equilibria and dealkylatio n kinetics of organocobalamins, and fitting procedures for axial base disso ciation equilibria.