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Schematic diagram of one model of spectral tuning by means of S308 in which a hydrogen bond between S308 and the protonated retinylidene Schiff base at K312 stabilizes the positive charge on the Schiff base nitrogen. The Schiff base counterion E129 is not shown. The helical wheel representation of an α-helical seventh transmembrane segment of opsin is shown, looking from the extracellular face.
