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 chlor ophyll ((3)P680) were monitored by EPR. The following results were obt ained 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 (a pproximate to 65%), respectively. The different kinetics may be due to electrostatic and/or magnetic effects of Q(A)(-) on charge recombinat ion or due to conformational changes caused by the double reduction tr eatment. (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 (approxima te to 50%)] than with Q(A)H(2) [t(1/2) approximate to 1 ms (approximat e to 65%)/3 ms (approximate to 35%)]. The short lifetime of the triple t with Q(A)(-) explains why it was not detected earlier. The mechanism of triplet quenching in the presence of Q(A)(-) is not understood; ho wever it may represent a protective process in PS II. (5) Almost ident ical data were obtained for PS II-enriched membranes from spinach and PS LI core preparations from Synechococcus. Room temperature optical s tudies were performed on the Synechococcus preparation. In samples con taining sodium dithionite to form Q(A)(-) in the dark, EPR controls sh owed that multiple excitation flashes given at room temperature led to a decrease of the Q(A)(-)Fe(2+) signal, indicating double reduction o f 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 descr ibed monoexponentially with t(1/2) approximate to 8 ns (a slightly bet ter 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 w as 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 pr oviding a qualitative explanation for the photochemistry of PS II with regard to its dependence on temperature and the redox state of QA. Th is incorporates (a) an electrostatic effect of Q(A)(-) which increases the standard free energy of P680(+)Pheo(-) compared to centers contai ning Q(A) or Q(A)H(2) or lacking Q(A), (b) exergonic primary charge se paration at cryogenic temperatures, even in the presence of Q(A)(-), a nd (c) an effective free energy for the excited stale (equilibrated be tween P680 and the antenna chlorophylls) which decreases with increasi ng temperature, this decrease being more pronounced the larger the ant enna system. Under physiological conditions, factors a and c may consp ire to diminish charge separation in PS II whenever Q(A)(-) is accumul ated.