Excitation energies of a water-bridged twisted retinal structure in the bacteriorhodopsin proton pump: a theoretical investigation.

The first proton transfer in the bacteriorhodopsin photocycle takes place during the L → M transition. Structural details of the pre proton transfer L intermediate have been investigated using experiments and computations. Here, we assess L-state structural models by performing hybrid quantum mechanical/molecular mechanical molecular dynamics and excitation energy calculations. The computations suggest that a water-bridged twisted retinal structure gives the closest agreement with the experimental L/bR shift in the excitation energy.