Electron Transfer Route between Quinones in Type-II Reaction Centers

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.