Vitamin A is adequately distributed within the body to maintain the biological function of retinoids in the peripheral tissues and the production of the visual chromophore, 11-cis-retinal, in the eye. One of the mysteries in our vision is that humans recognize color by use of a single chromophore molecule (11-cis-retinal), meaning that the chromophore is identical even between blue-absorbing and red-absorbing sensors. Humans have two different types of retinal containing light-sensitive proteins expressed in the retina, rhodopsin (Rh) achieving the twilight vision and three cone pigments, which mediate color vision. Each different chromophore-protein interaction allows preferential absorption of a selected range of wavelengths. While the structural basis for photoreaction and signal transduction of Rh has been well understood by the determination of its atomic-level structure, structural studies of cone pigments lag far behind those of Rh, mainly because of difficulty in sample preparation and lack of suitable methods in structural analysis.
We thus attempted to express monkey cone pigments in HEK293 cell lines for structural analysis using light-induced difference Fourier-transform infrared (FTIR) spectroscopy. The first structural information successfully elicited from the highly accurate spectra for each cone pigment showed that the retinal chromophore is structurally similar between Rh and cone pigments, but the hydrogen-bonding network around the retinal chromophore is entirely different between them. In addition, some spectral differences are observed between cone pigments, including protein-bound water molecules. These differences could be interpreted to play a role in spectral tuning.