Effects of distance and driving force on photoinduced electron transfer between photosynthetic reaction centers and gold electrodes

The electron-transfer (ET) parameters for oriented and aligned monolayers of the bacterial photosynthetic reaction center (RC) from Rhodobacter sphaeroides formed on the top of self-assembled monolayers (SAMs) of alkanethiols of various lengths immobilized on gold electrode are estimated using cyclic voltammetry and photoelectrochemistry. Utilization of the unique polyHis tag in the protein and the Ni-NTA chelator complex in SAMs allows for specific protein orientation with the RC primary donor facing the electrode. To improve the efficiency of ET between the RC special pair and the electrode, an RC-Cyt complex was formed (J. Am. Chem. Soc. 2006, 128, 12044-12045). The results are analyzed in terms of integrated Marcus formalism, taking into account the density of electronic states in the metal. The dependence of the ET rate on the distance between RC and electrode demonstrates an adiabatic region up to 10 A, typical for other proteins, followed by a nonadiabatic area of electron tunneling with a P-factor of similar to 0.8 per methylene group of the alkanethiol. Reorganization energy of the system is rather low (0.23 eV) and indicates negligible (if any) system conformational change or protein tilting in the course of ET. Scanning probe microscopic (SPM) examination of the constructed surfaces confirmed a high density of surface coverage by the protein and the absence of RC structural deformation in the monolayers. Estimated reduction potential of the RC primary donor for the immobilized protein is +0.5 V (vs NHE), which is close to that observed for the protein in solution and in vivo. These results open the possibility for the measurements and detailed analysis of the mechanisms of ET in photosynthetic proteins in precisely organized monolayers on the surfaces of inorganic electrodes.