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 ( E m ) values for the accessory bacteriochlorophyll and bacteriopheophytin in the electron-transfer-active branch (B L and H L ) are higher than those in the electron-transfer-inactive branch (B M and H M ). However, the stabilization of the charge-separated [P L P M ] •+ H L • - state owing to protein reorganization is not clearly observed in the E m (H L ) values in the charge-separated 5 ps ([P L P M ] •+ H L • - state) structure. Furthermore, the expected chlorin ring deformation upon formation of H L • - (saddling mode) is absent in the H L 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.