FTIR EVIDENCE OF AN ALTERED CHROMOPHORE-PROTEIN-INTERACTION IN THE ARTIFICIAL VISUAL PIGMENT CIS-5,6-DIHYDROISORHODOPSIN AND ITS PHOTOPRODUCTS BSI, LUMIRHODOPSIN, AND METARHODOPSIN-I

FTIR studies of the BSI photoproduct and of the later intermediates lumirhodopsin and metarhodopsin-I of 5,6-dihydroisorhodopsin are reported on. Evidence is presented that in the BSI intermediate the retinal chromophore adopts a relaxed conformation in contrast to the bathorhodopsin intermediate of native rhodopsin. Whereas for the modified pigment, the Schiff base C = N stretching mode and its deuteration-induced isotopic shifts are similar to those of unmodified isorhodopsin, the corresponding values of the photoproducts differ. In addition, alterations in the carbonyl spectral range are observed (protonated carboxyl groups and amide-I). This shows that the chromophore-protein interaction is influenced by this modification. Some molecular events of the thermal decay of the bleached pigment occur at lower temperatures or even at an earlier intermediate of the photoreaction than in native rhodopsin.