Molecular dynamics study of early picosecond events in the bacteriorhodopsin photocycle: Dielectric response, vibrational cooling and the J, K intermediates

Molecular dynamics simulations have been carried out to study the J(625) and K-590 intermediates of bacteriorhodopsin's (bRs) photocycle starting from a refined structure of bR(568). The coupling between the electronic states of retinal and the protein matrix is characterized by the energy difference Delta E(t) between the excited state and the ground state to which the protein contributes through the Coulomb interaction. Our simulations indicate that the J(625) intermediate is related to a polarization of the protein matrix due to the brief (200 fs) change of retinal's charge distribution in going to the excited state and back to the ground state, and that the rise time of the K-590 intermediate is determined by vibrational cooling of retinal.