Dynamical effects in metalloprotein heterogeneous electron transfer

Abstract

In this work we assess the influence of physiological viscosities on metalloprotein electron transfer reactions. To that end we investigated the direct electrochemistry of the copper proteins azurin and CuA adsorbed on SAM-coated electrodes. The experimental results show a change of ET regime from non-adiabatic to friction control upon shortening tunneling distances, which is paralleled by a sigmoidal increase of the apparent ET reorganization energies. These data could be accurately described using novel Matyushov’s model, thereby validating this recent theoretical development, which however did not describe viscosity effects in its original formulation. We demonstrate that ET rate constants and effective relaxation times vary with viscosity following power laws. Moreover, the crossover parameter g that determines the ET regime could be redefined in terms of the exponents of these power laws. The magnitude of g, i.e. the extent of frictional control, was found to be protein specific as it is determined by the dynamical features of the protein milieu. Interestingly, Stokes shift and diffusional relaxation times were found to be of similar magnitude, thus resulting in non-negligible frictional control even at tunneling distances as long as 19 Å, and this effect is amplified by physiologically high viscosities, thus highlighting the influence of intracellular macromolecular crowding in modulating protein ET reactions.