PHOTOSYSTEM-I PHOTOCHEMISTRY UNDER HIGHLY REDUCING CONDITIONS - STUDY OF THE P700 TRIPLET-STATE FORMATION FROM THE SECONDARY RADICAL PAIR (P700+-A1-)

The photochemistry of spinach Photosystem I reaction centers has been studied under conditions of prereduced iron-sulfur centers Fe-S(A), Fe-S(B) and Fe-S(X) by flash absorption spectroscopy in the UV, blue and near-IR regions. Two kinetic phases are observed under such conditions, with t1/2 almost-equal-to 250 ns and 4-5 mu-s. From the spectral analysis of these two phases including the comparison with flash-induced absorption data obtained under moderately and strongly reducing conditions, the following conclusions were derived: the 250 ns phase corresponds to a recombination reacition between P700+ -A1- forming 3P700 with a yield close to one. This triplet state then decays with t1/2 almost-equal-to 4-5 mu-s. These results provide direct evidence for the reaction scheme: P700+ -A1- --> 3P700 under highly reducing conditions. The same reaction was proposed to occur in a preceding paper (Setif, P. and Bottin, H. (1989) Biochemistry 28, 2689-2697), but the halftime of the recombination reaction was then overestimated (750 ns). It is also found that the same reaction is occurring in reaction centers from the two cyanobacteria Synechococcus sp. and Synechocystic 6803.