What makes red visual pigments red? A resonance Raman microprobe study of retinal chromophore structure in iodopsin

We have obtained resonance Raman spectra of iodopsin, a red-sensitive (lambda max 571 nm) pigment from chicken cone cells, to investigate the molecular mechanism of the opsin shift in visual pigments. Detergent-solubilized iodopsin samples were examined with a Raman microprobe to obtain spectra from a 77-K photostationary steady-state mixture composed of 11-cis-iodopsin and its 9-cis-isoiodopsin and all-trans-bathoiodopsin photoproducts. The vibrational modes of these species have been assigned by comparison with spectra of the corresponding bovine pigments. The single bond stretching frequencies of the bovine, toad, and chicken pigments are found to exhibit a regular correlation as a function of the pigment absorption maxima that is consistent with the expected effects of increased electron delocalization. The C = NH stretching frequencies of iodopsin and bathoiodopsin are at 1644 and 1638 cm-1, respectively, and shift down to 1621 and 1617 cm-1, respectively, when the nitrogen is deuterated. The C = ND stretching frequencies of the various pigments are found to decrease linearly with increasing absorption maxima, suggesting that at least part of the opsin shift in visual pigments results from weakened electrostatic interaction between the retinal chromophore and its protein counterion. The Raman data are inconsistent with the idea that a charged protein residue is shifted along the chromophore to regulate the opsin shift. Taken together with the mutagenesis and model compound results, these resonance Raman data suggest that the opsin shift between the green and red cone visual pigment arises from two effects. First, Tyr-274 provides increased electrostatic stabilization of the Schiff base-counterion ion pair. Second, the opsin shift is enhanced by the dipolar residues Ser-177 and Thr-282 that interact with the chromophore near the ionone ring to preferentially stabilize the highly dipolar charge distribution of the electronically excited retinal chromophore [Mathies, R., & Stryer, L. (1976) Proc. Natl. Acad. Sci. U.S.A. 73, 2169-2173].