Molecular models of human visual pigments: Insight into the atomic bases of spectral tuning

The cycle of vision is a chain of biochemical reactions that occur after exposure of the pigments to the light. The known mechanisms of the transduction of the light pulse derive mainly from studies on bovine rhodopsin. The objective of this work is to construct molecular models of human rhodopsin and opsins, for which three-dimensional structures are not available, to analyze the retinal environment and identify the similarities and differences that characterize the human visual pigments. One of the main results of this work is the identification of Glu102 as the probable second counterion of the Schiff base in M opsin (green pigments) and L opsin (red pigments). Further, the analysis of the molecular models allows uncovering the molecular bases of the different absorption maxima of M and L opsins with respect to rhodopsin and S opsin. These differences appear to be due to both an increase in the polarity of the retinal environment and specific electrostatic interactions, which determine a reorganization of the electronic distribution of retinal by selectively stabilizing one of the two resonance forms.