Three aspects are generally considered in the color-tuning mechanism of vision: (I) chromophore distortion, (II) electrostatic interaction between the protonated Schiff base and counterion, and (III) polarity around the β-ionone ring and polyene chain. Primate green- and red-sensitive proteins are highly homologous but display maximum absorption at 530 and 560 nm, respectively. In the present study, the N-D stretching frequency of monkey green-sensitive protein was identified by using C15-D retinal. The hydrogen-bonding strength between monkey green and red was identical. Together with a previous resonance Raman study, we conclude that the 30 nm difference originates exclusively from the polarity around the β-ionone ring and polyene chain. Three amino acids (Ala, Phe, and Ala in monkey green and Ser, Tyr, and Thr in monkey red, respectively) may be responsible for color tuning together with protein-bound water molecules around the β-ionone ring and polyene chain but not at the Schiff base region.
Keywords: color visual pigment; electrostatic interaction; isotope shift; protein-bound water molecule; protonated Schiff base; spectral tuning.