Model Compounds for the Study of Spectroscopic Properties of Visual Pigments and Bacteriorhodopsin

Article abstract of DOI:10.1021/ja00311a050

A series of modified retinals bearing nonconjugated positive charges along the polyene were synthesized.It was found that nonconjugated charges shifted the absorption maxima of retinal chromophore as well as protonated retinal Schiff bases.The magnitude of shift observed in bacteriorhodopsin (bR), 5170 cm^-1, could be found in our models by the additivity of two factors: (1) interaction through space with a positive charge located in the vicinity of the ring operating in nonprotic solvents, provided that the interaction between the charge and its counteranion is weakened by a homoconjugation effect; (2) weakening the interaction of the Schiff base positively charged nitrogen with its counteranion.A shift of ca. 5000 cm^-1 can also be achieved by interaction through space with two nonconjugated positive charges.The absorption maximum of protonated retinal Schiff base is influenced significantly by an interaction with a nonconjugated charge located in the vicinity of the ring moiety or carbon 9.The influence of a charge located in the vicinity of carbon 12 and carbon 14 is minor.Nonconjugated positive charges have a remarkable effect on C=C stretching frequencies as well.It is suggested that the different C=C stretching frequencies found in bR, visual pigments, as well as their photochemically induced intermediates, may originate from interaction with external charges, the C=N⁺ stretching frequency does no exhibit similar sensitivity to external nonconjugated charges, and it is practically unaffected by them.