Primary Charge-Separation Rate at 5 K in Isolated Photosystem II Reaction Centers Containing Five and Six Chlorophyll a Molecules

Abstract

Although the primary charge-separation rate in the photosystem II (PSII) reaction center (RC) has been the subject of many time- and frequency-domain experiments, its value is still an unsettled issue, especially at or near room temperature. As a first step toward understanding its temperature dependence, it is important to have a reliable value for the rate at liquid helium temperatures. Presented are results from triplet bottleneck hole-burning (TBHB) experiments at 5.0 K on RCs isolated from spinach that contain either six or five chlorophyll a (Chl) molecules per two pheophytin (Pheo) molecules (referred to as RC-6 and RC-5). RC-5 possesses only one of the two peripheral Chls of the RC. The triplet state that serves as the population bottleneck is formed by charge recombination of the radical ion pair P680 + Pheo1- , where P680 is the primary electron donor and Pheo1 is the acceptor on the active (D1) branch. In the TBHB experiments, the laser burn intensity was varied over 2 orders of magnitude to assess the contribution from fluence broadening to the width of the zero-phonon hole (ZPH) burned into the P680 absorption band. The widths of the ZPH were also corrected for interference from a weak and sharp hole contribution from a state(s) that is unlikely to be involved in efficient charge-separation. The corrected ZPH widths for RC-5 and RC-6 were the same (2.3 ± 0.2 cm-1) and correspond to a charge-separation rate of (4.6 ± 0.4 ps)-1, in good agreement with a value recently measured for RC-6 by femtosecond pump−probe spectroscopy at 7 K (Greenfield, S. R.; Seibert, M.; Wasielewski, M. R. J. Phys. Chem. B 1999, 103, 8364). It appears that the removal of a peripheral Chl does not lead to structural changes in the core region of the RC that affect primary charge separation.