Retinals with modified ring structures

Other retinals, in which the ionone ring was modified by an additional double bond, 38 [189,190], ring contraction, 39 [70], or a retinal with open polyene chain, 40 

Recently, even highly modified retinals, in which the trimethylcyclohexenyl ring was replaced with an aromatic ring, did form visual pigment analogs with cattle opsin [183,191], The pigment analogs from retinals 41, 42, 43 and 44, however, were 

The variety of structural modifications carried out on the ionone ring and the observation that pigment analogs could be made from all these derivatives strongly support the idea that the binding site of opsin is indeed very lenient as far as stereo-electronic requirements of the ring binding site are concerned. 

By the use of this type of photoaffinity-labeled retinals at various positions on the ring and side chain, and the use of radioactively labeled compounds, and by carrying out hydrolysis of the retinal from the protein into amino acid(s) or short peptides, it will be possible to determine which amino acid(s) or peptide sequences are close to the chromophore in the binding site. 

Models proposed to account for molecular changes in the primary event 

---