Ultrafast Electron Transfer in Photosynthesis:  Reduced Pheophytin and Quinone Interaction Mediated by Conical Intersections

The mechanism of electron transfer (ET) from reduced pheophytin (Pheo^-) to the primary stable photosynthetic acceptor, a quinone (Q) molecule, is addressed by using high-level ab initio computations and realistic molecular models. The results reveal that the ET process involving the (Pheo^- + Q) and (Pheo + Q^-) oxidation states can be essentially seen as an ultrafast radiationless transition between the two hypersurfaces taking place via conical intersections (CIs). According to the present findings, an efficient ultrafast ET implies that the Pheo^- and Q move toward each other in a given preferential parallel orientation, reaching the most effective arrangement for ET at intermolecular distances (R) around 5−3 Å, where the lowest CIs are predicted. Favored donor/acceptor interactions are related to orientations with some overlap between the lowest occupied molecular orbitals (LUMO) of the two systems, and they lead to state-crossings at an earlier stage of the movement (larger R). Furthermore, when the topology of the interacting moieties does not make possible the LUMOs overlap, the corresponding diabatic potential energy curves do not intersect. Thus, it is anticipated that large scale motions, which are difficult to monitor experimentally, are actually occurring in the photosynthetic reaction centers of bacteria, algae, and higher plants, to fulfill the observed ultrafast ET processes.