Molecular dynamic, electrostatic, and quantum chemical calculations are applied in order to analyze in a model-independent approach the driving forces for the rise and decay of the M state in the bacteriorhodopsin photocycle. We find that a protein conformational change involving the reorientation of arginine R82 away from the chromophore binding site toward the extracellular region after the protonation of the primary acceptor aspartate D85 induces the development of several M subpopulations. They differ in the overall protein conformation and the total number and the distribution of protons and control the recovery of the ground state in different ways. This protein conformational change catalyzes extracellular proton release in the alkaline pH region and provides favorable electrostatic and structural features for speeding up the reprotonation of the retinal Schiff base, simultaneously slowing down its reisomerization. The de-and reprotonation steps are decomposed in single steps involving bound water molecules as intermediate proton binding sites. We show that, for each of the two overall translocations, the initial steps proceed near equilibrium, while further steps are unidirectional and fast.

We use molecular dynamics, electrostatic, and quantumchemical calculations to discuss chromophore and protein structural changes as well as proton transfer pathways in the first half of the bacteriorhodopsin photocycle. A model for the molecular mechanism is presented, which accounts for the complex pH dependence of the proton release and uptake pattern found for the M intermediates. The results suggest that transient transfer of the Schiff base proton to a nearby tightly bound water molecule is the primary step, which is accornpanyied by dissipation of free energy to the protein. From there, the energetically most favorable proton transfer is to aspartate D85. Arginine R82 is involved in the protein reorientation switch, which catalyzes the pK, reduction of glutamate E204. This residue is, therefore, identified as extracellular proton release group whose acid base equilibrium regulates the pH-dependent splitting of the photocycle.