On the Possibility of Protein–chromophore Charge Transfer in Visual Pigments

Quantum chemical calculations have been performed on the retinylidene Schiff base assumed to be protonated or hydrogen bonded at the nitrogen and perturbed by a carboxylate ion at its β‐ionone end. The results indicate that weak charge transfer type bands might exist due to electron transfer from the carboxylate ion to the Schiff base and that the intense (π, π*) band in the visible spectrum might acquire a significant degree of charge transfer character.