PARENT SESSION
Posters P6A Type II reaction centres: Excited state dynamics and donor side. Abstracts (313-346)

Primary charge separation in reaction center is unique in Photosystem II, unistep from accessory chlorophylls by superexchange virtual mediation of P₆₈₀. Koichiro Mukai^*,1, Keisuke Saito¹, Hitoshi Sumi¹, ¹ Tennou-dai 1-1-1, Tsukuba, Ibaraki, Japan

ABSTRACT- Pigment arrangement in the reaction center (RC) looks similar among various photosynthetic organisms. In pigment-excitation energy, however, RC of photosystem II (PS II) is quite different from others since P680 composed of the central pair (P) of chlorophylls (Chls) is higher than both the accessory Chls (Bs) and the pheophytins (Hs) therein, according to recent experimental studies by Diner et al.. The surrounding around P seems constructed to be very hydrophobic only in PS II from a necessity that the P⁺ state produced after the primary charge separation must have a very high redox potential for extracting an electron from water. Moreover, the excited state B^* of Bs is the lowest in PS II RC, and the primary charge separation starts from B^* which is produced by excitation transfer from the core antenna enclosing RC. In this situation, the primary charge separation in PS II RC is thought to be different from that in other photosystems, where P^* is the lowest and the initial charge-separated state P⁺B^- is produced from P^*. We show by calculations, in fact, that in PS II RC, B^* is unistep converted to P⁺B^- by a superexchange mechanism, without real formation of P^*, where P^* works only as a quantum-mechanical virtual intermediate state between PB^* and P⁺B^-, the direct electronic coupling between them being negligibly small. Based on pigment excitation energies in PS II RC measured by Diner et al., we can reproduce the absorption spectrum of RC by assuming that P^* is a dimer and both B^*s and H^*s are monomers. Energy broadening of these states obtained by the absorption-spectrum reproduction was consistent with the assumption, justifying it posteriorly. The superexchange charge-separation mechanism mentioned above was derived from calculations based on this characterization of excited states in PS II RC. It reproduces well both the magnitude and the temperature dependence of the observed rate constant for charge separation after lowest-edge excitation of PS II RC.

KEY WORDS: excitation energy trapping to charge separation, superexchange virtual mediation by p680, reaction center in photosystem 2, accessory chlorophylls