Thermodynamics and kinetics of the electron transfer process of spinach plastocyanin adsorbed on a modified gold electrode

The reduction thermodynamics (ΔHrc∘′ and ΔSrc∘′) and the kinetics of electron transfer for spinach plastocyanin adsorbed on a polycrystalline gold electrode coated with a mixed SAM made of 11-mercapto-1-undecanol and 11-mercapto-1-undecanoic acid were determined through cyclic voltammetry. The adsorbed protein experiences a marked enthalpic stabilization of the oxidized state, likely due to the electrostatic interaction of surface lysine(s) with the negatively charged SAM. The kinetic data indicate that the electron transfer process occurs through a tunnelling mechanism and that the distance between the protein and the electrode surface can be calculated by the Marcus equation. The ionic strength of the solution remarkably affects both the thermodynamics and the kinetics of the electron transfer process in a fashion which, for the former parameters, adheres to the Debye–Hückel model.