Kinetics of electron transfer in a mono-linked isoalloxazine-gold electrode

Abstract

A cyclic voltammetric investigation of the stability and redox properties of a mono-linked isoalloxazine-gold electrode in aqueous solution has been performed using repetitive cycles of potential. The effects of pH and the composition of the electrolyte solution on the kinetics of the anodic and cathodic electron transfer processes have been determined. A linear relationship between the formal potential and pH exists over the range pH 4.4–8.0 and the value of electron transfer rate constants for both of the anodic and cathodic processes are markedly pH dependent. The maximum values for these rate constants occur at about pH 6.0 where k anode = 1420 s −1 and where k cathode = 2240 s −1 although the cathodic rate is slower than the anodic process outside of the pH range 5.0–7.0. The electronic absorption spectra of the precursor N 3-( N-phenylthioureaylethyl)- N 10-phenethylisoalloxazine show pH-dependent changes in the extinction coefficient (ε) at 348 nm although simpler isoalloxazines, including riboflavin, do not. The maximum value of ε 348 also occurs at pH 6.0. The results indicate that the conformation of the substituted isoalloxazine changes with pH and that similar geometric changes in the molecule occur in the gold electrode system and alter its electrochemical properties.