The Involvement of Water at the Retinal Binding Site in Rhodopsin and Early Light‐induced Intramolecular Proton Transfer

Extensive dehydration of air‐dried films of bovine rod outer segment membranes induces fully reversible changes in the absorption spectrum of rhodopsin, indicative of deprotonation of the retinylidene Schiff base in more than 50% of the rhodopsin molecules in the sample. This suggests that water is involved at the site of the Schiff base protonation in rhodopsin. In contrast, the spectrum of metarhodopsin I is resistant to similar dehydrating conditions, implying a significant difference in the mechanism for protonation in metarhodopsin I. The photochemistry of dehydrated membranes was also explored. Photoexcitation of deprotonated rhodopsin (λ max 390 nm) induces a large bathochromic shift of the chromophore. The major photoproduct at room temperature was spectrally similar to metarhodopsin I (λ max , 478 nm). These findings suggest that intramolecular proton transfer involving the Schiff base proton may occur in the earlier stages of the visual cycle, prior to or during the formation of metarhodopsin I.