Bacteriorhodopsin.

Fourier transform infrared and Raman spectroscopy, solid-state NMR, and X-ray crystallography have contributed detailed information about the structural changes in the proton transport cycle of the light-driven pump, bacteriorhodopsin. The results over the past few years add up to a step-by-step description of the configurational changes of the photoisomerized retinal, how these changes result in internal proton transfers and the release of a proton to the extracellular surface and uptake on the other side, as well as the conservation and transformation of excess free energy during the cycle.