Light isomerizes the chromophore of bacteriorhodopsin

Abstract

The primary photochemical event in the two light-transducing pigments whose chromophore is retinal, rhodopsin or bacteriorhodopsin, is a source of controversy. It was originally proposed that the primary photoevent in the bleaching of rhodopsin is the photoisomerization of the chromophore from 11-cis to all-trans retinal^1,2. Photochemical considerations suggested that a photoisomerization is the primary event in both rhodopsin and bacteriorhodopsin^3–7. However, this description of bacteriorhodopsin's photochemistry has been questioned^8–10. To elucidate this problem, we determined the isomeric conformation of retinal for two of the photolytic intermediates of bacteriorhodopsin, using a method that enables us to extract chromophores from the photocycle intermediates L and M at low temperatures (−74 °C), and have determined the isomeric conformation of the extracted retinals by HPLC^11,12. Here we provide direct evidence that isomerization of the chromophore has taken place in two of the early photocycle intermediates (L and M) of bacteriorhodopsin.