Model systems for understanding absorption tuning by opsin proteins.

This tutorial review reports on model systems that have been synthesised and investigated for elucidating how opsin proteins tune the absorption of the protonated retinal Schiff base chromophore. In particular, the importance of the counteranion is highlighted. In addition, the review advocates for redefining the opsin shift, using the intrinsic absorption maximum (610 nm) of the protonated retinal Schiff base as a reference value, obtained by gas phase investigations, rather than the corresponding solvent value (441 nm). Hereby the necessity of correcting for solvent and counteranion effects is avoided, and it becomes clear that opsin proteins induce blueshifts in the chromophore absorption rather than redshifts.