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

Abstract

Using the X-ray free-electron laser (XFEL) structures of the photosynthetic reaction center from <i>Blastochloris viridis</i> that show light-induced time-dependent structural changes (Dods et al., (2021) Nature <i>589</i>, 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 (<i>E</i>_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_LP_M]^•+H_L^•<b>^-</b> state owing to protein reorganization is not clearly observed in the <i>E</i>_m(H_L) values in the charge-separated 5 ps ([P_LP_M]^•+H_L^•<b>^-</b> state) structure. Furthermore, the expected chlorin ring deformation upon formation of H_L^•<b>^-</b> (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.