THE ORIGIN OF THE RED-SHIFTED ABSORPTION MAXIMUM OF THE M(412) INTERMEDIATE IN THE BACTERIORHODOPSIN PHOTOCYCLE

The factors that red shift the absorption maximum of the retinal Schif f base chromophore in the M(412) intermediate of bacteriorhodopsin pho tocycle relative to absorption in solution were investigated using a s eries of artificial pigments and studies of model compounds in solutio n. The artificial pigments derived from retinal analogs that perturb c hromophore-protein interactions in the vicinity of the ring moiety ind icate that a considerable part of the red shift may originate from int eractions in the vicinity of the Schiff base linkage. Studies with mod el compounds revealed that hydrogen bonding to the Schiff base moiety can significantly red shift the absorption maximum. Furthermore, it wa s demonstrated that although s-trans ring-chain planarity prevails in the M(412) intermediate it does not contribute significantly (only ca 750 cm(-1)) to the opsin shift observed in M(412). It is suggested tha t in M(412), the Schiff base linkage is hydrogen bonded to bound water and/or protein residues inducing a considerable red shift in the abso rption maximum of the retinal chromophore.