Naturally occurring retinal-protein complexes (RPCs) have recently rec
eived much attention with regard to their potential use as light-sensi
tive elements for optical recording. The best-known RPC is bacteriorho
dopsin (BR), a photosensitive protein from the membrane of extreme hal
ophilic bacteria, which has been studied in great detail. The remarkab
ly robust nature of BR, coupled with its ability to reversibly change
color upon illumination and its high cyclicity of ground-to-photoinduc
ed state transitions, makes BR a promising material for optical inform
ation processing.