Electron Transfer Route between Quinones in Type-II Reaction Centers

Abstract

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 (QA) and secondary (QB) quinones. Here, we investigate the electron transfer route, calculating the superexchange coupling (H QA–QB) for electron transfer from QA to QB 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 (QA → Fe → QB) is pronounced in PbRC, whereas the electron transfer via occupied molecular orbitals (Fe → QB followed by QA → 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···QB H-bond formation caused by an extension of the H-bond network, which facilitates charge delocalization over the QB site. This explains the observed discrepancy in the QA-to-QB electron transfer between PbRC and PSII, despite their structural similarity.

Decoding the mechanical conductance switching behaviors of dipyridyl molecular junctions

“…For the superexchange mechanism, the acceptor acts as a bridge for the electron to hop between donor sites and the donor acts as a bridge for the hole to hop between acceptor sites. [69][70][71][72][73][74] The superexchange model is also a simple atomistic model of tunneling through molecules. 75,76 However, for the small dipyridyl molecular junctions at lower bias voltages, there is no conductance channel entering the bias window and non-resonant electron transport dominates electron transport.…”

“…75,76 However, for the small dipyridyl molecular junctions at lower bias voltages, there is no conductance channel entering the bias window and non-resonant electron transport dominates electron transport. Although exact approaches for the calculation of local currents are available, [69][70][71][72][73][74] here we rely on a simpler approximate consideration of OTCTCA which should be enough for our purposes. For the OTCTCA method, the spatial distribution of the potential field around the molecular junction is first calculated which contains a series of potential wells and potential barriers as shown in Fig.…”

Mechanism of Asparagine-Mediated Proton Transfer in Photosynthetic Reaction Centers

“… 7 Ser-L223 not only accepts an H-bond from protonated Asp-L213 but also donates an H-bond to the distal carbonyl O site of Q B , 8 , 9 forming a Grotthuss-like proton conduit. 10 The formation of the proton transfer pathway [Asp-L213···Ser-L223···Q B ] leads to a significant increase in the electronic coupling between Q A and Q B , 11 facilitating proton-coupled electron transfer. Asp-L213 and Ser-L223 in the R. sphaeroides PbRC correspond to D1-His252 and D1-Ser264 in PSII, respectively, which also facilitate proton-coupled electron transfer toward Q B .…”