ELECTROGENIC LIGHT REACTIONS IN PHOTOSYSTEM .1. RESOLUTION OF ELECTRON-TRANSFER RATES BETWEEN THE IRON-SULFUR CENTERS

Flash-induced voltage changes (electrogenic events) in photosystem I p articles from spinach, oriented in a phospholipid layer, have been stu died at room temperature on a time scale ranging from 1 mu s to severa l seconds. A phospholipid layer containing photosystem I particles was adsorbed to a Teflon film separating two aqueous compartments. Voltag e changes were measured across electrodes immersed in the compartments . In the absence of added electron donors and accepters, a multiphasic voltage increase, associated with charge separation, was followed by a decrease, associated with charge recombination. Several kinetic phas es were resolved: a rapid (<1 mu s) increase, ascribed to electron tra nsfer from the primary electron donor P700 to the iron-sulfur electron acceptor F-B, was followed by a slower, biphasic increase with time c onstants of 30 and 200 mu s. The 30-mu s phase is assigned to electron transfer from F-B to the iron-sulfur center F-A. The voltage decrease had a time constant of 90 ms, ascribed to charge recombination from F -A to P700. Upon chemical prereduction of F-A and F-B the 30- and 200- mu s phases disappeared and the decay time constant was accelerated to 330 mu s, assigned to charge recombination from the phylloquinone ele ctron acceptor (A(1)) or the iron-sulfur center F-X to P700.