Absence of electron-transfer-associated changes in the time-dependent X-ray free-electron laser structures of the photosynthetic reaction center.

Using the X-ray free-electron laser (XFEL) structures of the photosynthetic reaction center from that show light-induced time-dependent structural changes (Dods et al., (2021) Nature , 310-314), we investigated time-dependent changes in the energetics of the electron-transfer pathway, considering the entire protein environment of the protein structures and titrating the redox-active sites in the presence of all fully equilibrated titratable residues. In the dark and charge separation intermediate structures, the calculated redox potential () values for the accessory bacteriochlorophyll and bacteriopheophytin in the electron-transfer-active branch (B and H) are higher than those in the electron-transfer-inactive branch (B and H).

Mechanism of Asparagine-Mediated Proton Transfer in Photosynthetic Reaction Centers.

In photosynthetic reaction centers from purple bacteria (PbRCs), light-induced charge separation leads to the reduction of the terminal electron acceptor quinone, Q. The reduction of Q to Q is followed by protonation via Asp-L213 and Ser-L223 in PbRC from . However, Asp-L213 is replaced with nontitratable Asn-L222 and Asn-L213 in PbRCs from and , respectively.

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) and secondary (Q) quinones. Here, we investigate the electron transfer route, calculating the superexchange coupling () for electron transfer from Q to Q in the protein environment. is significantly larger in PbRC than in PSII.

Proton-mediated photoprotection mechanism in photosystem II.

Photo-induced charge separation, which is terminated by electron transfer from the primary quinone Q to the secondary quinone Q, provides the driving force for O evolution in photosystem II (PSII). However, the backward charge recombination using the same electron-transfer pathway leads to the triplet chlorophyll formation, generating harmful singlet-oxygen species. Here, we investigated the molecular mechanism of proton-mediated Q stabilization.

Conformational Changes and H-Bond Rearrangements during Quinone Release in Photosystem II.

In photosystem II (PSII) and photosynthetic reaction centers from purple bacteria (PbRC), the electron released from the electronically excited chlorophyll is transferred to the terminal electron acceptor quinone, Q. Q accepts two electrons and two protons before leaving the protein. We investigated the molecular mechanism of quinone exchange in PSII, conducting molecular dynamics (MD) simulations and quantum mechanical/molecular mechanical (QM/MM) calculations.

Mechanism of the formation of proton transfer pathways in photosynthetic reaction centers.

In photosynthetic reaction centers from purple bacteria (PbRCs) from , the secondary quinone Q accepts two electrons and two protons via electron-coupled proton transfer (PT). Here, we identify PT pathways that proceed toward the Q binding site, using a quantum mechanical/molecular mechanical approach. As the first electron is transferred to Q, the formation of the Grotthuss-like pre-PT H-bond network is observed along Asp-L213, Ser-L223, and the distal Q carbonyl O site.