Mechanism and Reaction Coordinate of Directional Charge Separation in Bacterial Reaction Centers.

Using first-principles molecular dynamics, we predict the reaction coordinate and mechanism of the first charge-separation step in the reaction center of photosynthetic bacteria in a model including the special pair (P) and closest relevant residues. In the ground state, a dynamical localization of the highest occupied orbital is found to be a defining characteristic of P. This feature is linked to the tuning of the orbital energy levels by the coupling with two collective low-frequency vibrational modes.