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.