Recombination Kinetics of the Radical Pair, P680+I‒, in Closed Reaction centers of PS II as Function of Temperature

Abstract

The primary charge separation in photosystem II takes place from the excited singlet state of the primay donor, a specialized Chl-a (P680 or Chl-aII). The electron is transfered to a pheopytin a molecule (I) and, subsequently, to the first stable acceptor, a special plastoquinone (QA). The formation of P630+QA ‒ with a quantum yield close to 1 takes only about 400 ps. A charge recombination between P680+I− occurs, if electron transfer from I− to QA is blocked (closed reaction center); e.g., by chemical reduction of QA. The study of the charge recombination reaction is complementary to the investigation of forward electron transfer in order to clarify the mechanism of the primary reactions in PS II. Recent reports (e.g, 1–5) concerning the lifetime and quantum yield of P+I− in closed RC’s are rather contradictory to each other. In a simplified model of the primary processes (5,6) one expects that the free energy of P+I− might be only slightly below or even above that of the excited state in the antenna (Chl-a*) due to the negative charge located on QA and due to the presence of antenna chlorophylls (Chl-a).