Kinetics of charge separation and A0-→A1 electron transfer in photosystem reaction centers

The charge separation P700*A(0) --> P700(+)A(0)(-) and the subsequent electron transfer from the primary to secondary electron acceptor have been studied by subtracting absorption difference profiles for cyanobacterial photosystem I (PS I) complexes with open and closed reaction centers. Samples were excited at 660 run, which lies toward the blue edge of the core antenna absorption spectrum. The resulting PS I kinetics were analyzed in terms of the relevant P700, P700(+), A(0), and A(0)(-) absorption spectra. In our kinetic model, the radical pair P700+A(0)(-) forms with 1.3 ps rise kinetics after creation of electronically excited P700*. The formation of A(1)(-) via electron transfer from A(0) requires similar to 13 ps. The kinetics of the latter step are appreciably faster than previously estimated by other groups (20-50 ps).