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 fromBlastochloris viridisthat show light-induced time-dependent structural changes [Dods, R.et al. (2021) Nature589, 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 (E_m) values for the accessory bacteriochlorophyll and bacteriopheophytin in the electron-transfer active branch (B_Land H_L) are higher than those in the electron-transfer inactive branch (B_Mand H_M). However, the stabilization of the [P_LP_M]^•+H_L^•–state owing to protein reorganization is not clearly observed in theE_m(H_L) values in the charge-separated 5-ps ([P_LP_M]^•+H_L^•–state) structure. Furthermore, the expected chlorin ring deformation upon formation of H_L^•–(saddling mode) is absent in the H_Lgeometry of the original 5-ps structure. These findings suggest that there is no clear link between the time-dependent structural changes and the electron transfer events in the XFEL structures.