Studies on the Retinal‐Protein Interaction in Bacteriorhodopsin

1 Different types of bacteriorhodopsin chromophores and their reactions are described. This includes intermediates of the reconstitution reaction of the purple complex, intermediates of the photochemical cycle and a photochemically active 500‐nm chromophore. In contrast to the native chromophore (purple complex) some of these species are reducible by borohydride yielding stable retinyl proteins. 2 Hydrolysis and/or extraction of the retinyl proteins reveals that in the corresponding non reduced parent chromophores the retinal is bound either noncovalently or covalently to lysyl residues of the polypeptide chain. 3 Retinol, retinyl lysine and their retro isomers isolated from the various reduced chromophores are identified by thin‐layer chromatography and mass spectrometry. 4 The absorption spectra of the retinyl proteins and binding studies with retinol and retinal indicate that the cyclohexene ring and the side chain of the retinyl moiety are forced into a coplanar conformation by interaction with the protein. The three peaked absorption band of the reduced chromophores is due to this planarized conformation and not to a retro configuration of the retinyl moiety. 5 Isomerization to retro compounds can be achieved by HCl as is known to be the case for retinyl compounds in solution. In addition photochemical isomerization is observed in the case of planarized retinyl moieties. Thus the native protein structure is responsible not only for a specific conformation of the retinyl moiety but also for its specific reactivity.