Electron Transfer Route between Quinones in Type-II Reaction Centers

[Image: see text] In photosynthetic reaction centers from purple bacteria (PbRCs) and photosystem II (PSII), the photoinduced charge separation is terminated by an electron transfer between the primary (Q(A)) and secondary (Q(B)) quinones. Here, we investigate the electron transfer route, calculating the superexchange coupling (H(QA–QB)) for electron transfer from Q(A) to Q(B) in the protein environment. H(QA–QB) is significantly larger in PbRC than in PSII. In superexchange electron tunneling, the electron transfer via unoccupied molecular orbitals of the nonheme Fe complex (Q(A) → Fe → Q(B)) is pronounced in PbRC, whereas the electron transfer via occupied molecular orbitals (Fe → Q(B) followed by Q(A) → Fe) is pronounced in PSII. The significantly large H(QA–QB) is caused by a water molecule that donates the H-bond to the ligand Glu-M234 in PbRC. The corresponding water molecule is absent in PSII due to the existence of D1-Tyr246. H(QA–QB) increases in response to the Ser-L223···Q(B) H-bond formation caused by an extension of the H-bond network, which facilitates charge delocalization over the Q(B) site. This explains the observed discrepancy in the Q(A)-to-Q(B) electron transfer between PbRC and PSII, despite their structural similarity.