Distance and Potential Dependence of Charge Transport Through the P700 Reaction Center of Photosystem I

Photoinduced charge separation and transport through photosystem I (PSI) is an essential part of the photosynthetic electron transport chain. To investigate charge exchange processes mediated by the P700 reaction center of PSI, we have developed a strategy to functionalize gold electrodes with PSI complexes that orients and exposes their luminal side to the electrolyte. Bulk photoelectrochemical measurements demonstrate that PSI remains functional in a wide sample potential range around 0 mV/SSC. Electrochemical scanning tunneling microscopy (ECSTM) imaging of individual complexes shows lateral sizes in agreement with the dimensions of PSI and an apparent height that is gated by the probe potential of ECTSM as reported for smaller globular redox proteins. This experimental setup enables ECSTM current-distance spectroscopic measurements that unequivocally correspond to the P700 side of PSI. In these conditions, we observe that the spatial span of the current is enhanced (the distance-decay rate β is reduced) through the solution at sample potential 0 mV/SSC and probe potential 400 mV/SSC. This process corresponds to hole injection into an electronic state that is available in the absence of illumination. We propose that a pair of tryptophan residues located near P700 and known to integrate the hydrophobic recognition site for plastocyanin may have an additional role as hole exchange mediator involved in charge transport through PSI.