SPECTRAL SHIFTS IN RETINAL SCHIFF BASE COMPLEXES

BRIDGES,(~) and Dartnall and Lythgoecl) have reported on the A,,, values of the visual pigments of a large series of fish species. Furthermore, Dartnall and Lythgoec2) summarize the A,,, values for animals other than fishes. Both grotkps have observed a marked tendency of the A,,, values to cluster in certain wavelength regions (512, 523.5, 534 and 543 nm for retinal, based fish pigments, at 4785,486.5, 494, 500.5, 506, 51 1.5 and 519 nm for retinal, based fish pigments, and at 492, 501.5, 519 and 528 nm for retinal, based pigments from other animals). Bridges has suggested that these discontinuous clusters reflect a discontinuity in a series of available proteins (opsins) that couple with the retinal to form the visual pigment. Dartnall and Lythgoe suggest, as an extension of Dartnall’s earlier hypothesis for the structure of rhodopsin,(ll) that transitory dipoles on the prosthetic group could be stabilized by electrostatic interaction with charged groups on the protein, and that these dipoles may be located in at least twenty different positions on the chromophore-protein complex. We report here a series of measurements which indicate that this discontinuity of spectra may be the result of a discrete number of substituents near the protein chromophore, which by inductive or field effects can change the positive charge on the nitrogen atom of the chrornophore and shift the A,,, value. It is known that rhodopsin, the visual pigment extracted from rod cells, is composed of one molecule of retinal (vitamin A aldehyde) and a molecule of a large protein, opsin. In forming rhodopsin the two components appear to undergo a condensation reaction between the aldehyde and a free amine group on opsin, to form a Schiff base linkage. Then in a subsequent step the nitrogen is ~rotonated.(~* 4,