Triplet states in a photosystem I reaction center complex. Inhibition of radical pair recombination by bipyridinium dyes and naphthoquinones

Flash-induced absorption changes between 400 and 570 nm were studied in a P700-chlorophyll a-protein complex from the thermophilic cyanobacterium Synechococcus sp. that lacked the bound secondary electron acceptors A₂ and P430. A positive peak at 520 nm, which increased linearly with the flash intensity and independently of the redox state of P700, is ascribed to a carotenoid triplet. Bleaching at 430 nm, which decayed with a half time of about 10 μs, was abolished when P700 was oxidized with ferricyanide and saturated at a high flash intensity, indicative of its dependence on the primary photochemistry of photosystem I.Several bipyridinium dyes and naphthoquinones suppressed the 10 μs decay of the 430 nm signal in a way indicating that the 10 μs component represents the P700 triplet generated by the back reaction between the reduced primary electron acceptor and oxidized P700 and that the added oxidants oxidize the reduced primary acceptor so rapidly that back electron transfer to oxidized P700 is prevented. Our results also show that the primary electron acceptor is located in a lypophilic environment in the chlorophyll-binding subunits of the photosystem I complexes. In a reaction center complex containing the secondary electron acceptors, the exogenous oxidants accept electrons only via P430.