Spectral Tuning of Rhodopsin and Visual Cone Pigments

Retinal is the light-absorbing biochromophore responsible for the activation of vision pigments and light-driven ion pumps. Nature has evolved molecular tuning mechanisms that significantly shift the optical properties of the retinal pigments to enable their absorption of visible light. Using large-scale quantum chemical calculations at the density functional theory level combined with frozen density embedding theory, we show here how the protein environment of vision pigments tunes the absorption of retinal by electrostatically dominated interactions between the chromophore and the surrounding protein residues. The calculations accurately reproduce the experimental absorption maxima of rhodopsin and the red, green, and blue color pigments. We further identify key interactions responsible for the color-shifting effects by mutating the rhodopsin structure in silico, and we find that deprotonation of the retinyl is likely to be responsible for the blue-shifted absorption in the blue cone vision pigment.