Electrostatic-field dependent activation energies modulate electron transfer of cytochrome c

Cytochrome c was electrostatically bound on Ag electrodes coated with self-assembled monolayers of carboxyl-terminated alkylthiols. Employing stationary and time-resolved surface enhanced resonance Raman spectroscopy, activation energies of the interfacial redox process were determined as a function of the electric field strength that was controlled by varying protein-electrode distance via the thiol alkyl chain length. At weak electric fields (long chain lengths), temperature- and overpotential-dependent measurements consistently yield a reorganization energy of 0.26 and 0.22 eV, respectively, which is distinctly lower than for cytochrome c in solution. This decrease is attributed to the lowering of the contribution of solvent reorganization for the reaction of the immobilized protein. At short alkyl chain length, high electric fields strongly raise the activation barrier for the structural reorganization of the protein and the rearrangement of the hydrogen bond network becomes rate limiting for the interfacial redox process as indicated by the H/D kinetic isotope effect, that increases with the electric field strength (Murgida, D. H.; Hildebrandt, P. J. Am. Chem. Soc. 2001, 123, 4062-4068). Thus, rate constants measured as a function of the temperature provide the activation enthalpy for the underlying proton-transfer steps. The values of 24.2 and 34.3 kJ mol(-1) determined in H2O and D2O, respectively, as well as the ratio of the preexponential factors A(H2O)/A(D2O) of ca. 0.8 cannot be reconciled within the semiclassical description of proton transfer but indicate thermally activated nuclear tunneling. The electric-field-induced alteration of the activation barrier that controls the dynamics of the interfacial electron transfer of cytochrome c may represent a general mechanism for modulating biological charge-transfer dynamics at membranes.