CHARGE RECOMBINATION REACTIONS IN PHOTOSYSTEM-II .1. YIELDS, RECOMBINATION PATHWAYS, AND KINETICS OF THE PRIMARY PAIR

Recombination reactions of the primary radical pair in photosystem II (PS II) have been studied in the nanosecond to millisecond time scales by flash absorption spectroscopy. Samples in which the first quinone acceptor (Q(A)) was in the semiquinone form (Q(A)(-)) or in the doubly reduced state (presumably Q(A)H(2)) were used. The redox state of Q(A) and the long-lived triplet state of the primary electron donor chlorophyll ((3)P680) were monitored by EPR. The following results were obtained at cryogenic temperatures (around 20 K). (1) The primary radical pair, P680(+)Pheo(-), is formed with a high yield irrespective of the redox state of Q(A) (2) The decay of the primary pair is faster with Q(A)(-) than with Q(A)H(2) and could be described biexponentially with t(1/2) approximate to 20 ns (approximate to 65%)/150 ns (approximate to 35%) and t(1/2) approximate to 60 ns (approximate to 35%)/250 ns (approximate to 65%), respectively. The different kinetics may be due to electrostatic and/or magnetic effects of Q(A)(-) on charge recombination or due to conformational changes caused by the double reduction treatment. (3) The yield of the triplet state 3P680 was high both with Q(A)(-) and Q(A)H(2) (4) The triplet decay was much faster with Q(A)(-) [t(1/2) approximate to 2 mu s (approximate to 50%)/20 mu s (approximate to 50%)] than with Q(A)H(2) [t(1/2) approximate to 1 ms (approximate to 65%)/3 ms (approximate to 35%)]. The short lifetime of the triplet with Q(A)(-) explains why it was not detected earlier. The mechanism of triplet quenching in the presence of Q(A)(-) is not understood; however it may represent a protective process in PS II. (5) Almost identical data were obtained for PS II-enriched membranes from spinach and PS LI core preparations from Synechococcus. Room temperature optical studies were performed on the Synechococcus preparation. In samples containing sodium dithionite to form Q(A)(-) in the dark, EPR controls showed that multiple excitation flashes given at room temperature led to a decrease of the Q(A)(-)Fe(2+) signal, indicating double reduction of Q(A). During the first few flashes, Q(A)(-) was still present in the large majority of the centers. In this case, the yield of the primary pair at room temperature was around 50%, and its decay could be described monoexponentially with t(1/2) approximate to 8 ns (a slightly better fit was obtained with two exponentials: t(1/2) approximate to 4 ns (x80%)/25 us (approximate to 20%). After 2000 flashes and subsequent dark adaptation for 20 min (in order to form the state P680 Pheo Q(A)H(2)), the yield of the primary pair was close to 100%, and its decay was slower [t(1/2) approximate to 13 ns or t(1/2) approximate to 5 ns (approximate to 50%)/20 ns (approximate to 50%). On the basis of these results and earlier work in the literature, we present a hypothesis providing a qualitative explanation for the photochemistry of PS II with regard to its dependence on temperature and the redox state of QA. This incorporates (a) an electrostatic effect of Q(A)(-) which increases the standard free energy of P680(+)Pheo(-) compared to centers containing Q(A) or Q(A)H(2) or lacking Q(A), (b) exergonic primary charge separation at cryogenic temperatures, even in the presence of Q(A)(-), and (c) an effective free energy for the excited stale (equilibrated between P680 and the antenna chlorophylls) which decreases with increasing temperature, this decrease being more pronounced the larger the antenna system. Under physiological conditions, factors a and c may conspire to diminish charge separation in PS II whenever Q(A)(-) is accumulated.