Abstract
Bacteriorhodopsin (bR) harnesses light energy to transport protons across the cell membrane of H. salinarium. Absorption of a photon by the protonated retinal chromophore initiates a cycle in which the chromophore releases a proton to an aspartate on the extracellular side and reprotonates from an aspartic acid on the cytoplasmic side. Vectorial proton transport depends on a switch in accessibility of the chromophore Schiff base nitrogen from the extracellular to cytoplasmic side. Changes in the retinal conformation are expected to be particularly important for understanding the pumping mechanism.