Quantum dynamics of the femtosecond photoisomerization of retinal in bacteriorhodopsin

The membrane protein bacteriorhodopsin contains all-trans-retinal in a bind ing site lined by amino acid side groups and water molecules that guide the photodynamics of retinal. Upon absorption of light, retinal undergoes a su bpicosecond all-trans --> 13-cis phototransformation involving torsion arou nd a double bond. The main reaction product triggers later events in the pr otein that induce pumping of a proton through bacteriorhodopsin. Quantum-ch emical calculations suggest that three coupled electronic states, the groun d state and two closely lying excited states, are involved in the motion al ong the torsional reaction coordinate phi. The evolution of the protein-ret inal system on these three electronic surfaces has been modelled using the multiple spawning method for non-adiabatic dynamics. We find that, although most of the population transfer occurs on a timescale of 300 fs, some popu lation transfer occurs on a longer timescale, occasionally extending well b eyond 1 ps.