Cationic State Distribution over the P700 Chlorophyll Pair in Photosystem I

Abstract

The primary electron donor P700 in photosystem I is composed of two chlorophylls, PA and PB. P700 forms the cationic [PA/PB]•+ state as a result of light-induced electron transfer. We obtained a PA•+/PB•+ ratio of 28:72 and a spin distribution of 22:78 for the entire PSI protein-pigment complex. By considering the influence of the protein components on the redox potential for one-electron oxidation of PA/PB monomers, we found that the following three factors significantly contributed to a large PB•+ population relative to PA•+: 1), Thr-A743 forming a H-bond with PA; 2), PA as a chlorophyll a epimer; and 3), a conserved PsaA/PsaB pair, the Arg-A750/Ser-B734 residue. In addition, 4), the methyl-ester groups of the accessory chlorophylls A−1A/A−1B significantly stabilized the cationic [PA/PB]•+ state and 5), the methyl-ester group orientations were completely different in A−1A and A−1B as seen in the crystal structure. When the methyl-ester group was rotated, the spin-density distribution over PA/PB ranged from 22:78 to 15:85.