Direct QM/MM simulation of photoexcitation dynamics in bacteriorhodopsin and halorhodopsin

Bacteriorhodopsin (bR) and halorhodopsin (hR) are membrane proteins of halobacteria. Both utilize the same chromophore, all-trans retinal protonated Schiff base (RPSB), for photo-activated ion transport. In spite of their structural similarities, the photoisomerization of RPSB in bR is much faster and with higher quantum yield compared to hR. Here, we couple a QM/MM approach with ab initio multiple spawning (AIMS) to simulate photoisomerization in bR and hR. The simulations reproduce the dramatically different time scales and quantum yields for these two proteins. The lifetime difference is attributed to electrostatic effects in the active site region surrounding the chromophore.