Counterion displacement during the molecular evolution of vertebrate visual pigments. Opsin binds to a retinal chromophore via a protonated Schiff base. The proton on the Schiff base is unstable, and the counterion, a negatively charged amino acid residue stabilizes the proton. Counterion displacement occurred during molecular evolution, and both the new (Glu113) and former (Glu181) counterions might facilitate the acquisition of new molecular properties of opsin‐based pigments.
Schiff base - Normally refers to composition of Methyl Anthranilate at equilibrium with an aldehyde. Schiff bases increase stability of aldehydic components as they are less reactive than their free counterparts.
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