The oxidation of the amino acids tyrosine and tryptophan by complexes based on M(bpy)33+ (M = Ru, Os) was studied by monitoring the cyclic voltammetry of the metal complex in the presence of the oxidizable amino acids. Addition of both amino acids to aqueous solutions of the metal complexes in phosphate buffer produced electrocatalytic enhancement in the oxidative wave observed at indium tin oxide electrodes. The kinetics for the oxidation by the Ru(III) and Os(III) forms was determined by digital simulation. The oxidation kinetics for tryptophan were consistent with outer-sphere electron transfer, giving an expected dependence of the oxidation rate constant on the reduction potential of the metal complex. In contrast, oxidation of tyrosine at pH 7.5 did not give an appreciable dependence on the metal complex potential. These results were explained by a kinetic model where proton transfer from tyrosine to phosphate can be the rate-limiting step in competition with a concerted, multisite electron-proton-transfer pathway that is observed at lower base concentrations. These results suggest that tyrosine oxidation in enzymes can access both pathways depending on the solvent accessibility of the oxidized residue and the availability of a suitable proton acceptor.
We report here corrected values for k -1 for back proton transfer in the mechanism in Scheme 1. The initial values did not include the total concentration of tyrosine in the calculated rate constants. We also report a corrected value for k red . The corrected Table 1 is given here.
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