Electrochemistry and photoelectrochemistry of photosystem I adsorbed on hydroxyl-terminated monolayers

Direct electrochemistry studies on Photosystem I (PSI) were performed using cyclic voltammetry and square wave voltammetry. PSI centers stabilized in aqueous solution by Triton X-100 surfactant were adsorbed on hydroxyl-terminated hexanethiol modified gold electrodes. We have identified the electron donor, P700, and the electron acceptor sites, FA/FB, based on the previously reported preferred orientation for P700 on hydroxyl-terminated self-assembled monolayers. The reported potential values (EP700 = +0.51 V vs. NHE; EFA/FB = 0.36 V vs. NHE) correlate very well with the established literature for P700, while the weaker signal for FA/FB lies within previous literature values. We were able to identify both redox centers on the same voltammogram. The P700 center clearly shows reversible electrochemical behavior. The expected FA/FB reduction is small in comparison, reflecting the dominant orientation of PSI with the FA/FB centers farther away and the P700 center nearer the electrode surface. As a molecular diode, PSI does not permit reverse direction conductivity to the FB so the small FA/FB peaks reflect other orientations besides the predominant one. PSI adsorbed on a hydroxyl-terminated hexanethiol modified gold substrate displays a photoenhanced reduction current for the P700 center in the presence of light and an electron acceptor, methyl viologen. This photoelectrochemical response demonstrates protein functionality after adsorption. 2006 Elsevier B.V. All rights reserved.