Electrochemical reduction of methylcobalamin and 5′-deoxyadenosylcobalamin on mercury in basic medium

Abstract

Electrochemical reduction of methylcobalamin and 5′-deoxyadenosylcobalamin (coenzyme B 12) was studied at Hg electrodes, in pH 11.7 basic media, by means of dc polarography, differential pulse polarography, cyclic voltammetry, controlled potential electrolysis, and chronoamperometry. A surfactant (Triton X-100) was used to minimize adsorption of the compounds on the mercury cathode. Methylcobalamin was found to exist as an equilibrium mixture of its base-off and base-on forms with K eq=3.6 calculated by an analysis of its two reduction waves at E 1/2=−1.20 V and E 1/2=−1.50 V (vs. SCE). The value of the forward homogeneous rate constant was found to be 2×10 3 s −1. A third, ill-defined, broad, catalytic H 2 reduction wave is found at E 1/2=−1.6 V. The first reduction wave is reversible and is assigned to the formation of a one-electron adduct intermediate from the base-off cobalamin which subsequently undergoes a cleavage of the Co-C bond (i.e., EC process), and the second wave is irreversible and is assigned to a one-electron reduction of the base-on cobalamin. The standard heterogeneous rate constant for the first reduction is 2×10 −2cm s −1 and the forward heterogeneous rate constant for the second reduction is 2×10 −6 cm s −1. The homogeneous rate constant for the follow-up chemical reaction of the first reduction process, as determined by chronoamperometric methods, is k=0.37 s −1 with a reversible half-wave potential of E 1/2 r=−1.24 V. In contrast, 5′-deoxyadenosylcobalamin, which appears to exist as the base-off form only, exhibits one irreversible reduction wave at E 1/2=−1.26 V and a broad, catalytic H 2 reduction wave in the same potential region as the methylcobalamin. The irreversible wave at −1.26 V is due to formation of one electron intermediate which undergoes Co-C bond cleavage much faster than that of methylcobalamin. Double step chronoamperometry showed this rate constant to be k=2.7×10 3 s −1.