Reaction between primary and secondary electron acceptors of Photosystem II of photosynthesis

Abstract

We studied some properties and interactions of intermediate ‘Q’, the fluorescence-quenching trapping center of the O 2-evolving photosystem, and its associated electron acceptor, ‘A’. 1. 1. Direct observation of the oxidation in darkness of photoreduced Q by the fully oxidized A pool revealed a half-time of the reaction of approx. 0.6 msec at room temperature. 2. 2. A study of the time course of the reduction of pool A in the light, using the chloroplast's capacity to reduce 2,6-dichlorophenolindophenol in the subsequent darkness as a measure of the pool, showed pool A to be unhomogeneous, 1 3 of it reacting more rapidly with Q - than the remainder. For this rapidly reacting fraction we estimated a bimolecular rate constant k 1 = 165 (pool units) −·sec −, with one pool unit corresponding to [Q] tot. 3. 3. From fluorescence-rise curves, effected by a series of brief flashes (converting Q only), we computed 18 for the ratio of the concentrations of A and Q. 4. 4. The shape of the rise curve in weak light was analyzed in terms of a bimolecular reaction between Q and A, varying the parameters: ratio [A]/[Q] and equilibrium constant K. Good fit with the experiment was obtained by assuming energy transfer between photosynthetic units, an [A]/[Q] ratio of approx. 20, and K < 10. 5. 5. It was concluded that the small initial rise of the total rise curve did not reflect the photoreduction of Q, but rather the activation of this trapping center.