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. After electronic excitation, we demonstrate one specific mode that couples to P*, representing the reaction coordinate along which the excited state develops. The characteristic vibrational coordinate we predict to be the rotation of an axial histidine (HisM202), which selectively lowers the energy of one (P-M) of the two bacteriochlorophylls in P. This leads to a unidirectional displacement of electron density to establish PL+PM- charge-transfer character, a hypothesis well-supported by an extensive framework of experimental evidence.