How color visual pigments are tuned

The absorption maximum of the retinal chromophore in color visual pigments is tuned by interactions with the protein (opsin) to which it is bound. Rec ent advances in the expression of rhodopsin-like transmembrane receptors an d in spectroscopic techniques have allowed us to measure resonance Raman vi brational spectra of the retinal chromophore in recombinant visual pigments to examine the molecular basis of this spectral tuning. The dominant physi cal mechanism responsible for the opsin shift in color vision is the intera ction of dipolar amino acid residues with the ground- and excited-state cha rge distributions of the chromophore.