Bacteriorhodopsin.

High-resolution maps from X-ray diffraction of bacteriorhodopsin and some of its photointermediates have yielded insights into how the isomerization of the bound retinal drives ion transport. Although important mechanistic details are still undecided, the events of the photochemical cycle are now understood to reflect changes in specific hydrogen bonds of protein groups and bound water molecules in response to motions of the retinal chain.