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, F-A/F-B, based on the previously reported preferred orientation for P700 on hydroxyl-terminated self-assembled monolayers. The reported potential values (E-P700 = +0.51 V vs. NHE; E-FA/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 F-A/F-B reduction is small in comparison, reflecting the dominant orientation of PSI with the F-A/F-B centers farther away and the P700 center nearer the electrode surface. As a molecular diode, PSI does not permit reverse direction conductivity to the F-B so the small F-A/F-B 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. (c) 2006 Elsevier B.V. All rights reserved.