Factors affecting the electron transfer properties of an immobilized cupredoxin

The ionic strength (I) dependence of the reduction thermodynamics (E°′, ΔHrc°′, and ΔSrc°′) and the kinetics of electron transfer (ET) for Pseudomonas aeruginosa azurin (AZ) adsorbed on a gold electrode coated with alkanethiolate SAMs has been investigated between pH 4.5 and 10.5 by cyclic voltammetry. The change in the reduction thermodynamics with I (sodium perchlorate) adheres to the Debye−Hckel model and allows the charges of the two redox states of AZ to be determined at different pH values. From pH 4 to 8 the protein charges are in agreement with those calculated considering the protonation states of the noncoordinating His35 and His83 residues and highlight that a single phosphate ion binds to both redox states of AZ, most likely at Lys122. A composite, Lys-based, equilibrium occurs at higher pH values, involving the loss of five protons at pH 10.5. The reduction thermodynamics extrapolated to zero I shows that the largely buried His35 dominates the electrostatic effects on E°′ for the equilibrium at around pH 7, whereas the residues involved in the high pH effect are more solvent exposed. At pH 10.5, the ET rate constants for AZ on all investigated SAMs are lower than the corresponding values at pH 4.5, probably due to a decrease in the tunneling efficiency at the AZ−SAM interface in terms of electronic coupling. It is suggested that Lys122 plays a distinctive role in this effect.