| The kinetics of oxidation of amino acids and nucleobases by the series of metal complex oxidants, M(bpy)33+ (M = Os, Fe, Ru) in aqueous solution at an ITO electrode, in the presence of added bases, was investigated utilizing catalytic cyclic voltammetry. A common mechanism involving initial formation of a hydrogen bond complex between the substrate and a base in solution was identified for all substrates. After association, oxidation can occur through two pathways - multiple site electron proton transfer (MS-EPT) or proton transfer followed by electron transfer (PT-ET). Kinetic isolation techniques were used to investigate each oxidation pathway individually, and have provided kinetic parameters and isotope effects for each step of the reaction mechanism for all substrates studied. Comparison between rate constants and isotope effects for each substrate provide insight into the role of proton transfer in biological system. The main emphasis of this study was to investigate the role of free energy change in MS-EPT reactions. We have found that the effects of free energy change on MS-EPT rate constant can be attributed to not only the base strength but also the oxidant reduction potential. Evaluation of rate constants and isotope effects for each system highlights the importance of the role of both the electron and the proton in redox active biological systems. |
