Formation and decay of P680 (PD1–PD2)+PheoD1− radical ion pair in photosystem II core complexes

Abstract

Under physiological conditions (278K) femtosecond pump-probe laser spectroscopy with 20-fs time resolution was applied to study primary charge separation in spinach photosystem II (PSII) core complexes excited at 710nm. It was shown that initial formation of anion radical band of pheophytin molecule (Pheo−) at 460nm is observed with rise time of ~11ps. The kinetics of the observed rise was ascribed to charge separation between Chl (chlorophyll a) dimer, primary electron donor in PSII (P680*) and Pheo located in D1 protein subunit (PheoD1) absorbing at 420nm, 545nm and 680nm with formation of the ion-radical pair P680+PheoDI−. The subsequent electron transfer from PheoD1− to primary plastoquinone electron acceptor (QA) was accompanied by relaxation of the 460-nm band and occurred within ~250ps in good agreement with previous measurements in Photosystem II-enriched particles and bacterial reaction centers. The subtraction of the P680+ spectrum measured at 455ps delay from the spectra at 23ps or 44ps delay reveals the spectrum of PheoDI−, which is very similar to that measured earlier by accumulation method. The spectrum of PheoDI− formation includes a bleaching (or red shift) of the 670nm band indicating that Chl-670 is close to PheoD1. According to previous measurements in the femtosecond–picosecond time range this Chl-670 was ascribed to ChlD1 [Shelaev, Gostev, Vishnev, Shkuropatov, Ptushenko, Mamedov, Sarkisov, Nadtochenko, Semenov and Shuvalov, J. Photochemistry and Photobiology, B: Biology 104 (2011) 45–50]. Stimulated emission at 685nm was found to have two decaying components with time constants of ~1ps and ~14ps. These components appear to reflect formation of P680+ChlD1− and P680+PheoD1−, respectively, as found earlier. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: Keys to Produce Clean Energy.