| Abstract
| - The involvement of protons in the heterogeneous electron transfer between cytochrome c and a gold electrode to which it is attached was studied by comparing the electron transfer rate constants for H2O and D2O solutions. Rate constants were measured as a function of the electrochemical cell solution and the protein incubant solution, i.e., k0(incubant, cell). Two separate isotope effects exist: a cell “isotope effect”, KIEcell = k0(H2O, H2O):k0(H2O, D2O), which is manifest at short time scales (<30 s) and arises from the viscosity difference between H2O and D2O, and an incubant isotope effect, KIEinc= k0(H2O, H2O):k0(D2O, H2O), which is manifest at longer times (>2 h) and results from H/D exchange. The two isotope effects are approximately equal (∼1.2) and a total isotope effect KIEtotal = k0(H2O, H2O):k0(D2O, D2O) can be constructed that is the product of KIEcell and KIEinc. The nature of the electron transfer process, possible coupling to a proton transfer process, and the involvement of specific hydrogens in the transfer mechanism are discussed.
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