The Reductive Cleavage Mechanism and Complex Stability of Glutathionyl‐Cobalamin in Acidic Media

Abstract

AbstractThe mechanisms of the electrode reduction of vitamin B12a and the reductive cleavage of glutathionyl cobalamin (GSCbl) were investigated with digital simulations in acid media at pH 3.78 and 2.78. Even though the pH is quite acidic, the 1‐electron reduction of B12a goes through the base‐on form in a step‐wise sequence forming base‐on vitamin B12r which then is protonated to the base‐off form, and finally reduced in another 1‐electron step to vitamin B12s Unlike the case of B12a, the GSCbl is found to go by a 1‐electron overall mechanism. In this case, electronic structure calculations at the B3LYP/6‐311G(d,p) level for a model GSCbl complex show that the one‐electron adduct is a radical anion with the unpaired electron in a corrin ring π* orbital. When the axial 5,6‐dimethylbenzimidazole dissociates, the Co(III)‐S bond will break forming Vitamin B12s and the GS• radical. This reductive cleavage mechanism is the same as in the case of alkylcobalmins and is relevant to the biological mechanism of the B12 trafficking chaperone proteins, CblC, for processing GSCbl in B12 dependent enzyme reactions. Finally, we determined conditional stability constants for the formation of the GSCbl complex, Kobs, which are 4.4±0.5×103 M−1 and 9.9±0.1×102 M−1 at pH 3.78 and 2.78, respectively.