The primary electron donor P700 in photosystem I is composed of two chlorophylls, P A and P B. P700 forms the cationic [P A/P B] •+ state as a result of light-induced electron transfer. We obtained a P A•+/P B•+ 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 P A/P B monomers, we found that the following three factors significantly contributed to a large P B•+ population relative to P A•+: 1), Thr-A743 forming a H-bond with P A; 2), P A 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 [P A/P B] •+ 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 P A/P B ranged from 22:78 to 15:85. © 2011 Biophysical Society.
Saito, K., & Ishikita, H. (2011). Cationic state distribution over the p700 chlorophyll pair in photosystem i. Biophysical Journal, 101(8), 2018–2025. https://doi.org/10.1016/j.bpj.2011.09.010