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 Blastochloris viridis that show light-induced time-dependent structural changes (Dods et al., (2021) Nature 589, 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 (Em) values for the accessory bacteriochlorophyll and bacteriopheophytin in the electron-transfer-active branch (BL and HL) are higher than those in the electron-transfer-inactive branch (BM and HM). However, the stabilization of the charge-separated [PLPM]•+HL•– state owing to protein reorganization is not clearly observed in the Em(HL) values in the charge-separated 5 ps ([PLPM]•+HL•– state) structure. Furthermore, the expected chlorin ring deformation upon formation of HL•– (saddling mode) is absent in the HL geometry 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.