EVALUATION OF ELECTROCHEMICAL PARAMETERS FOR AN EC MECHANISM FROM A GLOBAL ANALYSIS OF CURRENT POTENTIAL TIME DATA - APPLICATION TO REDUCTIVE CLEAVAGE OF METHYLCOBALAMIN

Simultaneous evaluation of electron-transfer rate constant, k(o) follo wing chemical reaction rate constant, k(f), electron-transfer coeffici ent, alpha, and standard potential, E(o'), for electron transfer coupl ed to a following chemical reaction (EC mechanism) is described. A mat hematical model for the current response to a potential step is develo ped by incorporating the appropriate concentration terms into the Butl er-Volmer equation. Experimental current-potential-time (i-E-t) surfac es are fit to this model to evaluate the parameters. Fitting individua l i-t or i-E curves did not yield unique parameter values whereas an i -E-t surface constituted by several i-t or i-E curves could be fitted to obtain unique values. A generalized kinetic zone diagram for the EC reaction is drawn by examining the limiting forms of the expression f or current. Theoretical limits of measurable rate constants are estima ted from the zone diagram. The three-dimensional electrochemistry desc ribed above was used to study the reductive cleavage of methylcobalami n in dimethyl sulfoxide (DMSO) solvent and 0.1 M tetrabutylammonium pe rchlorate supporting electrolyte. The parameters estimated are as foll ows: alpha = 0.552 +/- 0.004; k(o) = 0.011 +/- 0.0015 cm s-1; k(f) = 1 500 +/- 140 s-1; E(o') = -1.54 +/- 0.01 V. The rate constant for the f ollowing reaction, k(f), in DMSO solvent is approximately 4000-fold fa ster than the similar process in aqueous medium. It is suggested that this enhancement is relevant to methyl group transfer in enzymatic rea ctions, e.g., methionine synthase, if the enzyme mechanism involves a reductive cleavage which produces a methyl radical.