Primary charge separation in photosystem I: a two-step electrogenic charge separation connected with P700+A0- and P700+A1- formation.

A PS I membrane preparation from a PS II deficient mutant of Synechocystis sp. PCC 6803 (psb DI/DII/C) was investigated by picosecond photovoltage and fluorescence measurements. The photovoltage kinetics show two distinct phases. At low excitation energies the fast phase correlates with the fluorescence decay time constant of 22 +/- 4 ps. This phase is ascribed to the trapping of excitons as described by the reaction (AntiP700)* A0-->AntiP700+A0-. In addition to this phase, the photovoltage displays a second rising phase of smaller amplitude with a time constant of 50 +/- 15 ps. We assign the latter phase to further electron transfer from A0 to the secondary acceptor, A1. Assuming the protein as a homogeneous dielectric, our results suggest that the transmembrane distance A0-A1 spans only a small part (20 +/- 8%) of the distance P700-A1.