Upon the absorption of a photon, BR undergoes a photocyclic mechanism that translocates a proton from the intracellular to the extracellular side of the molecule. The array of BR molecules, that construct the purple membrane, is capable of transporting enough protons to create a proton motive force across the membrane. This photophysical property is useful for biophotnic and bioelectronic device applications. Biophotonic devices make use of the discrete photostates of the native and/or branched photocycles that are produced by the absorption of light (left). The absorption maximum of each photostate is shown in parenthesis. Bioelectronic devices harness the electrical energy produced by the aformentioned proton motive force (below). Each of these devices is described elsewhere (see the Molecular Electronics page).

Regardless of the application, the all-trans retinal moiety is responsible for light absorption in BR. The retinal molecule is covalently bound to Lys-216, via a Schiff base nitrogen linkage, in the active site at the center of the molecule. Manipulation of the protein, by chemical or genetic means, changes the absorption properties of BR. Examples of how chemical (right) and genetic (below) manipulation affects the O-state lifetime are demonstrated. Thus, the photophysical properties of BR can be optimized for specific applications by changing the active site environment.