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

•The excitation by 20-fs pulses at 710 nm was applied to core complexes of photosystem II.•Electron transfer with participation of chlorophyllD1, pheophytin and quinone was found in RC of photosystem II.•Time constants for three electron transfer reactions in photosystem II RC were found.•Time constant for reduction of pheophytinD1 molecule was found to be ~ 13 ps in photosystem II RC.•Time constant for oxidation of pheophytinD1 molecule was found to be ~ 250 ps in photosystem II RC.

Under physiological conditions (278 K) 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 710 nm. It was shown that initial formation of anion radical band of pheophytin molecule (Pheo−) at 460 nm is observed with rise time of ~ 11 ps. 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 420 nm, 545 nm and 680 nm 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 ~ 250 ps in good agreement with previous measurements in Photosystem II-enriched particles and bacterial reaction centers. The subtraction of the P680+ spectrum measured at 455 ps delay from the spectra at 23 ps or 44 ps 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 670 nm 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 685 nm was found to have two decaying components with time constants of ~ 1 ps and ~ 14 ps. 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.