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.