Rhodopsin analogs

If proton translocation were involved, it would be important to know where the proton is removed. The 9- and 13-methyl groups of retinal cannot be involved, as 9-and 13-desmethylrhodopsins did form batho intermediates [145]. Synthetic retinals 3 [145], 4 [146,147] and 5 [148] missing the crucial hydrogens assumed to intervene in the proton translocation hypothesis, were prepared and were found to form rhodop-sin analogs. Rhodopsin analogs from 3, 4 and 5 all formed red-shifted, batho... 

Fig. 14 shows further modifications of the structure of retinal, compounds 47-60, which however, yielded negative results, i.e., these structures, due to unfavorable steric and/or electronic interactions with the binding site did not form rhodopsin analogs. 

Both adrenaline and noradrenaline stimulate smooth muscles throughout the body and have a hypertensive effect. Their postsynaptic receptors are 7-helix transmembrane proteins (Fig. 11-6). A comparison of the effects of various analogs led to the classification of these receptors into classes a, a2, P, and P2, which are discussed briefly on pp. 553-555. The a receptors, which are structurally closely related to rhodopsin,753/754 are coupled via Gq /11 proteins to a phosphoinositide-activated phospholipase C (Figs. 11-9, 30-19).755 They usually provoke an excitatory response. However, in intestinal smooth muscles they are inhibitory. Adrenaline is usually more active at a receptors than is noradrenaline. A specific antagonist... 

Researchers have been trying to explain how retinal (or retinol) are chromophores in the visual system for more than 40 years with remarkably little success. Dartnall et. al. even claimed that the same retinal-based chromophore accounts for both the broad photopic spectral sensitivity of the eye as well as all three of the color-sensitive photodetectors found in the eye. There may be a problem of semantics related to this claim. In an attempt to synthesize an analog of rhodopsin, they formed a protonated azomethine using retinal and a simple amine, n-butylamine the resulting peak absorption was at 440 nm. It was hoped the material would have a peak near 500 nm (see further discussion below). Its absorption coefficient was not specified in Zollinger. Although this chromophore exhibits apeak in its absorption near that of the S-channel, it does not explain how the chromophores of the other spectral channels are formed. It is an irrelevant compound. 

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