The role of free energy change in coupled electron-proton transfer

The kinetics of oxidation of tyrosine by the series of metal complex oxidants, M(bPY)(3)(3+) (M = Os, Fe, Ru), in the presence of added bases (acetate, succinate monoanion, histidine, phosphate, and tris), were investigated in 0.5 M buffer with 0.8 M NaCl at 25 degrees C by utilizing a catalytic cyclic voltammetry technique. As reported in an earlier study, oxidation occurs by a series of pathways-electron transfer followed by proton transfer (ET-PT), proton transfer followed by electron transfer (PT-ET), and concerted electron-proton transfer (EPT). The latter two occur within H-bonded association complex between tyrosine and the added base. Kinetic isolation was used to focus on the EPT pathway, in which multiple site-electron proton transfer (MS-EPT) occurs with electron transfer to the oxidant and proton transfer to the base. Measured rate constants varied from k (EPT) = 5.0 x 10(3) to 9.8 x 10(7) M-1 s(-1). Systematic 0 variations in RT In k (EPT) with Delta G degrees 'were observed with RT Ink (EPT) increasing with -Delta G degrees 'with a slope of similar to 0.6 regardless of whether Delta G degrees 'was varied either by varying E degrees 'for the oxidant or the strength of the base. Over the similar to 0.7 eV variation in Delta G degrees 'for the complete data set the variation with Delta G degrees 'is consistent with the importance of quantum effects arising from the O-H transfer mode with possible appearance of a novel quantum beat effect at similar to 3000 cm(-1).