ALL known visual pigments have as chromophore the sterically hindered 11-cis isomer of retinal. Shortly after this was established, an unsuccessful attempt was made to prepare the much more hindered 7-cis-retinal by partial reduction of the corresponding 7,8-acetylene1. But, four new geometric isomers of retinal, all apparently 7-cis, have been synthesised by a six-step non-stereospecific sequence2. Structural assignments of the isomers were originally based on spectral data of two fractions of partially separated mixtures of isomers (7-cis and 7,9-dicis; 7,13-dicis and 7,9,13-tricis). Their configurations were, therefore, not completely firm. Since then two of the four isomers (7-cis and 7,9-dicis) have been isolated by high pressure liquid chromatography (HPLC). Their nuclear magnetic resonance (NMR) spectra unambiguously confirmed the tentative assignments made earlier. That of 7-cis-retinal is shown in Fig. 1 as a representative example. The remaining two 13-cis isomers remain unseparated in various HPLC conditions. Their structures, however, have since been confirmed through a steroselective synthesis of 7,9,13-tricis-retinal3. We report here preliminary results of the interaction of these 7-cis-retinal isomers with cattle opsin.
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