CHARGE RECOMBINATION BETWEEN STABILIZED P-680+ AND REDUCED CYTOCHROME-B-559 IN QUINONE-RECONSTITUTED PS-II REACTION CENTER

Flash-induced absorbance changes in the nanosecond to millisecond time ranges have been measured in the Photosystem II reaction center complex consisting of D1 and D2 subunits and cytochrome b-559, in the presence of 2,5-dibromo-3-methyl-6-isopropylbenzoquinone (DBMIB). The results indicate that DBMIB largely quenches the primary radical pair (P-680+ Pheo-) and the formation of the triplet (3P-680). Long-lived absorption signals in the red and near-infrared (bleaching at 680 nm and broad increase at 740-830 nm) and in the green (peak at 560 nm) can be attributed to the oxidation of P-680 and to the reduction of cytochrome b-559. These data show that addition of DBMIB induces stabilization of P-680+ and a rapid (perhaps submicrosecond) reduction of cytochrome b-559. The signals attributed to P-680+ and to reduced cytochrome decay in parallel (t 1 2 = 2 ms), showing that the cytochrome reduces P-680+. The stabilization occurred also in the presence of plastoquinone-3 and (with DBMIB) at -29°C in a viscous solution containing 60% glycerol at a low concentration of quinone, suggesting that the quinone reconstitutes the function of QA and thus mediates electron transport from the reduced pheophytin a to the intrinsic cytochrome. © 1990.