Superexchange\nElectron Transfer and Protein Matrix\nin the Charge-Separation Process of Photosynthetic Reaction Centers

In type-II reaction centers, such as photosystem II (PSII)\nand\nreaction centers from purple bacteria (PbRC), light-induced charge\nseparation involves electron transfer from pheophytin (Pheo<sub>D1</sub>) to quinone (Q<sub>A</sub>), occurring near a conserved tryptophan\nresidue (D2-Trp253 in PSII and Trp-M252 in PbRC). This study investigates\nthe route of the Pheo<sub>D1</sub>-to-Q<sub>A</sub> electron transfer,\nfocusing on the superexchange coupling (|<i>H</i><sub>PheoD1···QA</sub>|) in the PSII protein environment. |<i>H</i><sub>PheoD1···QA</sub>| is significantly larger for the Pheo<sub>D1</sub>-to-Q<sub>A</sub> electron transfer via the unoccupied molecular orbitals of D2-Trp253\n([Trp]<sup>•–</sup>-like intermediate state, 0.73 meV)\ncompared to direct electron transfer (0.13 meV), suggesting that superexchange\nis the dominant mechanism in the PSII protein environment. While the\noverall impact of the protein environment is limited, local interactions,\nparticularly H-bonds, enhance superexchange electron transfer by directly\naffecting the delocalization of molecular orbitals. The D2-W253F mutation\nsignificantly decreases the electron transfer rate. The conservation\nof D2-Trp253/D1-Phe255 (Trp-M252/Phe-L216 in PbRC) in the two branches\nappears to differentiate superexchange coupling, contributing to the\nbranches being either active or inactive in electron transfer.