Retinal derivatives

Resonance Raman Spectroscopy. A review of the interpretation of resonance Raman spectra of biological molecules includes a consideration of carotenoids and retinal derivatives. Another review of resonance Raman studies of visual pigments deals extensively with retinals. Excitation profiles of the coherent anti-Stokes resonance Raman spectrum of j8-carotene have been presented. Resonance Raman spectroscopic methods have been used for the detection of very low concentrations of carotenoids in blood plasma and for the determination of carotenoid concentrations in marine phytoplankton, either in situ or in acetone extracts. ... 

X-Ray Crystallography. The crystal and molecular structure of dl-2-cis-A-trans-abscisic acid (7-trans-9-cis, carotenoid numbering) has been determined by the X-ray method. X-Ray crystallographic data are presented for the retinal derivative (107). ... 

Spectroscopy and Physical Chemistry of Retinal and Visual Pigments. Several reviews and symposium proceedings discuss the spectroscopic, photochemical, or physicochemical properties of retinal and related compounds, and of natural and model visual pigments derived from them. " " In addition, many papers have been published dealing with specific aspects of the spectroscopy (u.v., n.m.r., resonance Raman) of retinals and rhodopsins" or with aspects of the photochemistry and physical chemistry of retinal derivatives which may be relevant to the functioning of rhodopsin and other visual pigments. The bacterial purple... 

Retinal Derivatives. The preparation of 2-methylretinol (83) by standard methods from /3-irone has been reported.42 A series of papers43-46 reports approaches to the synthesis of hindered 7-ds-isomers of ionylidene derivatives and hence retinal (84) isomers, the fourth paper46 describing a new stereoselective synthesis of 7,9-di-cis-/3-ionylideneacetaldehyde (97) and the preparation from this of 7,9-di-ds- and 7,9,13-tri-ds-retinal. A key step in this procedure was the photosensitized... 

Last are Car precursors for important metabolites. Only three examples shall be given. The first example is retinal (Fig. 3), which is the chromophore of the visual pigment rhodopsin (23) and is derived from P,P -carotene. Because the latter cannot be synthesized by mammals, they need it to be supplied as provitamin A. Retinal derivatives are also required for other regulatory functions. The second example is abscisic acid (Fig. 3), which is the plant hormone involved in the shedding of leaves in fall and in fruit ripening it is derived from violaxanthin. Finally, certain fragrances of roses are not synthesized directly, but they are breakdown products of the flowers Cars. 

The n.m.r. spectra of some retinal derivatives as visual pigment models have also been reported. " ... 

The retinal pool of 11 -cis-retinal is clearly critical for photon detection and visual signal transduction. There are two key aspects regarding this that are important to consider the precursor(s) from which 11 -cis-retinal derived, and, bioconversion of isomerized trans-retinal back to 11-cis-retinal that can recombine with opsin to re-form rhodopsin. The latter aspect is obligatory for restoration of the dark state, regenerating a photosensitive receptor capable of undergoing another cycle of photon detection and signal transduction. Considered together, these events represent the retinoid cycle in the visual process (22). The entire process of re-isomerization and formation of a new rhodopsin molecule occurs in two different retinal tissues and involves several enzymatic steps that are described below (see Fig. 7.14). 

Hypercarotenosis. This occurs from >ive doses of carotene that exceed the ca-city of the mucosa cells to cleave the mole-le to retinal derivatives. The excess carotene ccmes deposited in the body tissues. Except or the yellow or bronze-orange skin, there... 

Hendrickx et al. [41 2] have reported the first hyperpolarizabilities of retinal, retinal Schiff base and retinal protonated Schiff base at 1064 nm excitation wavelength. Retinal protonated Schiff base is responsible for the linear and NLO properties of bacteriorhodopsin protein. Their measured hyperpolarizabilities are 3600 X 10 ° esu for retinal protonated Schiff base and 470 x IO" esu for retinal Schiff base. They also investigated theoretical understanding of the first hyperpolarizabilities of retinal derivatives. Results are shown in Table 5. 

Table 5. Experimental and theoretical hyperpolarizabilities (in 10 esu) of various retinal derivatives. (Reprinted with permission from (E. Hendrickx, K. Clays, A. Persoons, C. Dehu and. 1.1,. Bredas, (1995) J. Am. Chem. Soc., 117, 3547). Copyright (1995) American Chemical Society...

Ray reviews some recent developments concerning the design of novel materials with large NLO effects. He considers a series of organic salts and various organometallic derivatives where it was found that metal-to-ligand charge-transfer has a dominant contribution to the second-order NLO response. He also discusses the first hyperpolarizability of several retinal derivatives, ionic octupolar molecules and zwitterionic derivativatives. Solvent effects on NLO properties are also reviewed. 

The arrival of the odorant or the odorant/OBP complex at the outer surface of the receptor protein induces a change in the latter. It is not possible to see directly what happens at this point. The receptor must be located in the cell wall and therefore we are dealing with single molecule events and these cannot be studied by techniques such as X-ray crystallography. Similarly, the environment is too complex to be amenable to elucidation by NMR. Therefore, ideas on the exact nature of the initial receptor event are based on calculated guesses extrapolated from what we know about other systems. One popular hypothesis is that the binding sites lie inside the cylindrical channel between the seven trans-membrane helices of the receptor protein. This would be consistent with hormone receptors and with the optical receptors in which the retinal-derived... 

A modified mechanism has been proposed for formation of the M—92 and M—106 ions in the mass spectra of carotenoids (see Scheme 1). Further resonance-enhanced laser Raman spectra of retinal derivatives have been reported.A comparison between various semi-empirical methods for the calculation of -carotene MO s has appeared. A better correlation between the u.v. spectrum and theoretical predictions is claimed by Suzuki et... 

Electronic Absorption Spectroscopy. Doping with iodine and SO3 had a significant effect on the absorption spectrum of /3-carotene. Triplet-triplet absorption spectra have been obtained for six carotenoids, e.g. canthaxanthin carotene-4,4 -dione (181)] in benzene, " and bimolecular rate constants for energy transfer from singlet oxygen to carotenoids evaluated. U.v. spectra of retinal, retinyl acetate, and axerophtene (182) in solid films have been determined. Several papers discuss the light absorption spectra of retinal derivatives as rhodopsin models. ... 

As you can see from the reaction scheme, the retinal derives from Vitamin A, which requires merely the oxidation of a —CH2OH group to a —CHO group to be converted to retinal. The precursor in the diet that is transformed to Vitamin A is )3-carotene. The )3-carotene is the yellow pigment of carrots and is an example of a family of long-chain polyenes called carotenoids. 

Harbinson, G.S., Mulder, P.P.J., Pardoen, H., Lugtenburg, J., Herzfeld, J., and Griffin, R.G. High-resolution carbon-13 NMR of retinal derivatives in the solid state. 

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