Opsin analogs

Eq. 23-37.499,509-510a Constrained analogs of 11-czs retinal also combine with opsin to form rhodopsinlike molecules with absorption maxima near 500 nm. 

Based on some early work in the 1930 s, the assumption has been that the chromophores of vision are based on retinol or retinal coupled to a protein, opsin, in such a way as to form a chromophore. The variation in the location and possibly the style of couphng has been assumed to account for the actual spectral absorption of the material. After massive efforts, this premise has still not been demonstrated. However, it has caused a great deal of concentration on retinol and retinal as the fundamental structure of the chromophores. To this day, no laboratory experiments have shown either of these two retinoids exhibit a spectral absorption characteristic matching those of vision, human or otherwise. The retinols are neither chromophores nor analogs of the chromophores of vision. 

Studies of retinal isomers and retinal analogs in solution were useful in that they permitted to speculate on the conformation of the opsin-bound 1 l-c/s-retinal, an important problem in the stereochemistry of vision. Although there was a priori no reason to believe that the conformation of 1 l-cis-retinal when bound to opsin will be identical either to its solution conformation or to its conformation in the crystalline... 

The study of visual pigment analogs serves to clarify the nature of chromophore-opsin interactions in visual pigments. Visual pigment analogs are modified retinals bound to opsins, which can be investigated by spectroscopic and biochemical methods. 

A. Kropf, unpublished work), all combined with cattle opsin to give visual pigment analogs. 

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. 

A series of hydroretinals were synthesized and were bound to bovine opsin to form visual pigment analogs, the hydrorhodopsins [92]. Model protonated Schiff bases were also prepared from each of these retinals, and the magnitudes of the opsin shifts were determined. Table 1 summarizes the data obtained and shows the opsin shift for bovine rhodopsin for comparative purposes. Binding studies were also carried out to ascertain that the hydroretinals occupy the same binding site as 11-cK-retinal in rhodopsin. This is required to show the relevance of the data to the natural system. 

The 13-c/j retinal-chromophore in dark-adapted bacteriorhodopsin exhibits a very different photocycle, whose predominant intermediate has an absorption maximum at 610 nm [199], and which contains no intermediate [202,238] analogous to M. The 610 nm intermediate will decay to either the 13-c/s chromophore or the dW-trans form, the latter pathway being responsible for the phenomenon of light-adaptation [199]. This pathway does not explain, however, why monomeric bacteriorhodopsin shows poor light-adaptation [168,239]. The chromophore in the 13-c/s configuration is not associated with proton translocation [240]. Indeed, reconstitution of bacterio-opsin with 13-demethyl retinal, which traps the retinal moiety in the 13-c/s configuration, results [241] in a non-transporting photocycle. 

Land and marine animals possess rhodopsin. Certain freshwater fish and some amphibians have the pigment porphyropsin in place of rhodopsin. The function of porphyropsin is analogous to that of rhodopsin, and it contains 3-dehydroretinal in place of retinal. Cyanopsin (dehydroretinal + cone-type opsin) is found in the retinas of most freshwater fish. 

The carbon skeleton of the naphthalene compounds (563) and (564), which are structurally analogous to ethyl retinoate (24) and retinaldehyde (2), was obtained via the ketone (562) (Akhtar et al., 1982). The aldehyde (564), which binds, as the chromophore, to opsin, was obtained by reducing the ethyl ester (563) with lithium aluminum hydride/aluminum chloride and finally oxidizing the product with manganese(IV) oxide. The trans double bond of methyl (all- )-... 

The following is a summary of the 9-cis or 11-cis (sometimes 1-cis) analogs that form pigments with bovine opsin. 

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