Environment-dependent photochemical behavior of adenosylcobalamin and its influence on the quenching of the 5'-deoxyadenosyl radical.

Adenosylcobalamin (AdoCbl) shows different photochemical behavior when free in solution compared to when it is bound to the photoregulatory protein CarH. In both cases, the initial photoproduct is cob(II)alamin and the 5'-deoxyadenosyl radical (Rios-Santacruz et al., Nature, 2026, 650, 1045-1052). However, in solution, the ultimate products are cob(II)alamin and the cyclic product, 5',8-cycloadenosine, while in the CarH protein, cob(I)alamin and the exocyclic methylene product 4',5'-anhydroadenosine are formed. This has led to speculation that alkene formation in CarH plays a protective role, preventing deleterious reactions of the 5'-deoxyadenosyl radical. Recently, we have found that the CarH-type reactivity (formation of alkene + cob(I)alamin) is not in fact unusual; we find this behavior across a range of alkylcobalamins (alkylCbls) (Mukherjee et al., Chem. Sci., 2026, 17, 5518-5531). In order to explore the factors driving these contrasting outcomes, we have characterized the photolytic reactivity of AdoCbl, both in a variety of solvents as well as in several AdoCbl-dependent proteins. In no case do we find evidence for CarH-type reactivity, even under conditions where other alkylCbls consistently generate cob(I)alamin + alkene. These findings indicate that the anomalous CarH behavior may be better understood not as CarH enforcing unusual reactivity on the cobalamin, but rather as CarH enabling the intrinsic alkyl-radical behavior that is otherwise quenched when the 5'-deoxyadenosyl radical is free in solution or bound in regular cobalamin-dependent proteins.