Polyenes retinyl—

Spectroscopic methods such as uv and fluorescence have rehed on the polyene chromophore of vitamin A as a basis for analysis. Indirectly, the classical Carr-Price colorimetric test also exploits this feature and measures the amount of a transient blue complex at 620 nm which is formed when vitamin A is dehydrated in the presence of Lewis acids. For uv measurements of retinol, retinyl acetate, and retinyl palmitate, analysis is done at 325 nm. More sensitive measurements can be obtained by fluorescence. Excitation is done at 325 nm and emission at 470 nm. Although useful, all of these methods suffer from the fact that the method is not specific and any compound which has spectral characteristics similar to vitamin A will assay like the vitamin... 

X = 0, CH2, CHCOOH, C(COOH)2, NH, NCH3 N(CH2CH=CH2), N(CHs)2 Cl Bobrowski and Das published a series of papers on the transients in the pulse radiolysis of retinyl polyenes31-37, due to their importance in a variety of biomolecular processes. They studied32 the kinetics and mechanisms of protonation reaction. The protons were released by pulse radiolysis, on a nanosecond time scale, of 2-propanol air-saturated solutions containing, in addition to the retinyl polyenes, also 0.5 M acetone and 0.2 M CCI4. Within less than 300 ns, the electron beam pulse results in formation of HC1. The protonated products of retinyl polyenes were found to absorb optically with Xmax at the range of 475-585 nm and were measured by this absorption. They found that the protonation rate constants of polyene s Schiff bases depend on the polyene chain... 

Bobrowski and Das33 studied the transient absorption phenomena observed in pulse radiolysis of several retinyl polyenes at submillimolar concentrations in acetone, n -hexane and 1,2-dichloroethane under conditions favourable for radical cation formation. The polyene radical cations are unreactive toward oxygen and are characterized by intense absorption with maxima at 575-635 nm. The peak of the absorption band was found to be almost independent of the functional group (aldehyde, alcohol, Schiff base ester, carboxylic acid). In acetone, the cations decay predominantly by first-order kinetics with half life times of 4-11 ps. The bimolecular rate constant for quenching of the radical cations by water, triethylamine and bromide ion in acetone are in the ranges (0.8-2) x 105, (0.3-2) x 108 and (3 — 5) x 1010 M 1 s 1, respectively. 

Structurally, vitamin A and many synthetic retinoids consist of a (3-ionone ring, a polyunsaturated polyene chain, and a polar end group. The polar end group can exist in several oxidation states, as retinol, retinal, or retinoic acid. Retinol and retinyl esters are the most abundant vitamin A forms found in the body (Blaner and Olson, 1994). Retinol can be esterified with long-chain fatty acids (mainly palmitate, oleate, and stearate) to form retinyl esters, which are the body s storage form of vitamin A. Retinol also can undergo oxidation to retinal, which can be oxidized further to retinoic acid. The active... 

Electronic Absorption Spectroscopy. The absorption spectra of some polyenes in the crystalline state have been studied. On adsorption of certain gases on the crystallite surfaces, a new band appears on the low-energy side of the long-wavelength band in the spectra of these polyenes, e.g. at 536, 537, and 375 nm for all- trans-ft-carotene, 15-cis-/3-carotene, and retinyl compounds, respectively.92 The surface pressure vs. area isotherms and absorption spectra of all-trans-, 9-cis-, and 13-cis-retinal, all-trans-retinol, and all- trans-retinyl acetate have been studied at air-water and air-solid interfaces, respectively. The spectra of monolayers of the isomeric retinals showed a red shift of 15 lnm compared with the solution spectra, whereas... 

Evidence showing that the retinyl polyene is polarized, not only in its vertically excited B state but also in the relaxed emitting state (most probably - -Ag), is provided by the photochemical behavior of retinol (ROH) and retinyl acetate (RAc) (163). 

With the exception of photodissociation to radical ions observed for retinol in polar solvents (144), cis-trans isomerization is the major photochemical transformation undergone by all forms of the free retinyl-polyene chromophore. [Unidentified photochemical damage has been reported to occur with very low quantum yields, e.g., 0.04 in the case of all-trans retinal (177).] We shall subsequently see that critical comparisons between the photochemical behavior of the biopigments and that of the opsin-free chromophore have led to the conclusion that the protein moiety plays a major role in governing the photochemical mechanism in rhodopsin (176). It is, therefore, natural that in parallel to spectroscopic and theoretical investigations, considerable attention has been devoted to the photoisomerization of model compounds, particularly to that of retinal isomers. 

Isomerization takes place in a free or ion-paired retinylic cation, R+, where the double-bond character in the polyene chain is markedly reduced. 

RBP is relatively rich in aromatic amino acids, which create a deep hydrophobic pocket that is specific for the 8-ionone ring, polyene side chain, and polar end group. In addition to all- trans-retinol, RBP binds retinaldehyde, retinoic acid, and 13-c/s-retinol, but not retinyl esters or carotene. RBP shows considerable structural homology with 8-lactoglobulin from milk and other... 

In the synthesis of most glycoproteins containing mannose, the intermediate carrier of the mannosyl moiety is the polyene dolichol phosphate. However, in some systems, retinyl phosphate can act as the intermediate carrier between UDP-mannose and the acceptor glycoprotein. Retinyl phosphate maimose seems to be involved especially in the synthesis of hydrophobic regions of glycoproteins (DeLuca, 1977 Frot-Coutaz et al., 1985). 

Electronic Absorption Spectroscopy. Theoretical considerations of the light absorption spectra of /3-carotene (189) and related polyenes have been presented.A method has been devised for describing the shapes of the absorption spectra of polyenes such as -carotene and retinyl acetate as lognormal distribution curves.A new absorption band is seen in the spectrum of jS-carotene acting as electron donor in charge-transfer complexes.The triplet-triplet... 

Z-E isomerization via simple geometric inversion (one-bond flip, OBF, Fig. 2.3A) involves the torsional relaxation of the perpendicular excited state via an adiabatic mechanism which implies a non-volume-conserving process. This is not compatible with the ultrafast CTI in polyenes, in particular retinyl chromophores, and two other possible ways of photo-CTI have been proposed over the past 15 years [11]. 

The photochemistry of natural products, especially those with polyene and carbonyl chromophores85 and of C H10 hydrocarbons,88 has been reviewed. The sunlight photochemistry of retinyl acetate has been reported.87... 

Isomerization of ll-cw-retinal to the all-trans diastereomer is sterically favored because of the sterical repulsion between the 13-methyl and 10-H substituents in a planar polyene. In solutions of retinal or retinyl ester in which all stereoisomers are equilibrated by the reversible addition of iodine or trifluo-roacetic acid one finds only 0.1% of the W-cis diastereomer. The energy difference between all-trans- and ll-cis- retinal is 4.1 kcal/mol. The crystal structure of 11-cw-retinal shows a highly twisted polyene at C12-C13. The methyl group... 

A group of compounds described as vitamin A (retinol) includes those that, within their molecules, possess the characteristic (3-ionone ring and the isoprene chain. They are capable of forming molecules of polyene alcohol, aldehyde, acid, and ester (retinal, retinoic acid, and retinyl palmitate, respectively) (Figure 7.1). They can occur in many isomeric forms as groups of compounds possessing diversified prop-... 

The sensitivity of the fluorescence assay for retinol and its esters is comparable to that shown by other methods. In one of the cited methods, for example, a solution containing 10 ng retinol/ml gave a fluorescent yield at 480 nm of 30 arbitrary units when excited at 330 nm (Thompson et al., 1971). The assay reliability is influenced by many factors. One of these factors is the presence of other natural fluorescent compounds in the tissue extract. In the case of plasma or serum, the polyene pigment phytofluene, which shows absorption maxima at 331, 348, and 367 nm and emission maxima at 465, 490, and 520 nm, clearly is very similar to retinol. Inasmuch as the fluorescence intensity of phytofluene is 6.4 times higher than that of retinyl acetate, small amounts of phytofluene in plasma can markedly influence the presumed retinol value (Thompson et al.,... 

Vitamin A-active compounds [1] are present in foods of animal origin as retinoids (retinol, retinyl esters, retinylaldehyde, retinoic acid) and in those of plant origin as carotenoids (only carotenoids with one unsubstituted P-ionone ring and with an 11-carbon polyene chain at least are provitamins A). Retinyl palmitate is the main form used as a food supplement [4]. 

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