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 Schiff base chromophore in the M(412) intermediate of bacteriorhodopsin photocycle relative to absorption in solution were investigated using a series of artificial pigments and studies of model compounds in solution. The artificial pigments derived from retinal analogs that perturb chromophore-protein interactions in the vicinity of the ring moiety indicate that a considerable part of the red shift may originate from interactions in the vicinity of the Schiff base linkage. Studies with model compounds revealed that hydrogen bonding to the Schiff base moiety can significantly red shift the absorption maximum. Furthermore, it was 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 that in M(412), the Schiff base linkage is hydrogen bonded to bound water and/or protein residues inducing a considerable red shift in the absorption maximum of the retinal chromophore.