The properties of bacteriorhodopsin in which the natural cofactor, retinal (Fig. 1, I), has been replaced by the synthetic analog, phenyl retinal (Fig. 1, II), have been studied. Phenyl retinal binds at the same site as retinal and supports light-dependent transmembrane proton translocation in phospholipid vesicles that contain bacterio-opsin. This result allows us to rule out proton abstraction from the beta-ionone ring of retinal as a step in the catalytic cycle. Furthermore, phenyl retinal...