Retinal oxime

Fig. 2.162. Absorption spectra of Amphiopl expressed in HEK293s cells (a) and the HPLC patterns of retinal oximes (b). Absorption spectra and the HPLC patterns were measured before (a, curve 1, and b, top trace) and after irradiation at 520 nm for 2 min (a, curve 2, and b, middle trace). The HPLC pattern of retinal oximes extracted from a mixture of irradiated and non-irradiated bovine rhodopsin in equal amounts is indicated as a reference (b, bottom trace). The absorption maxima of the original pigment and its phoroproduct are shown in panel (a). Reprinted with permission from M. Koyanagi et al. [334].

Five retinol isomers, three. s -retinal oxime isomers, and a total of nine anti-retinal oxime and retinol isomers were baseline resolved using an 85.4/11.2/2/1.4... 

The compounds derived from retinaldehydc are named either as aldehyde derivatives or as compounds substituted by the bivalent retinylidene moiety, e.g., retinal oxime (26) or A -retinylidene-l-aminopropan-2-ol (27). 

Groenendijk et al. (13) were able to separate several geometric isomers of vitamin A compounds using silica gel plates. In their hands, activation of the plates by heating to 120 C rarely resulted in enhanced resolution. Mixtures of a low boiling alkane (hexane, petroleum ether, or cyclohexane) and a variable amount of a more polar solvent (8% diethyl ether, 50% diethyl ether or 20% ethyl acetate) were used. In these systems, retinol isomers move slower than the retinals, whereas the retinyl ester isomers move rather close to the solvent front. Moreover, for the retinyl ester isomers no separation is obtained. Also, it seems to be very difFicult to separate the different isomers of retinal. Modification to retinal oximes, however, facilitates the separation and identification since each isomer results in... 

Retinyl acetate is sometimes used as internal standard for retinol analysis it is not ideal because it is an ester, not a free alcohol (and is hydrolyzed by saponification processes), but superior in principle to the use of tocol or other nonretinoid forms for retinoid quantitation. Substituted retinal oximes have been used as internal standards (93). 15-Methylretinol (94) is conceptually an excellent internal standard for retinol analysis, because of its very similar chemical structure and properties. 

Biesalski and Weiser separated a -trans from l3-cis, W-cis, and 9-cis isomers of retinyl esters (retinyl palmitate, stearate, oleate, palmitoleate, and linole-ate) by isocratic adsorption HPLC (126). Bridges et al. used step gradients to separate geometric isomers of retinyl esters, retinal, retinal oximes, and retinal (both vitamin Ai and vitamin A2 forms), also by adsorption ( normal-phase ) HPLC (139,140). 

K Tsukida, M Ito, T Tanaka, I Yagi. High-performance liquid chromatographic and spectroscopic characterization of stereoisomeric retinal oximes. J Chromatogr... 

GM Landers, JA Olson. Rapid, simultaneous determination of isomers of retinal, retinal oxime and retinol by high-performance liquid chromatography. J Chromatogr 438 383-392, 1988. 

The work of Wald, Collins, Morton and Hubbard in this area was presented in a series of papers in 1953-5589. Subsequent authors have not always recognized that there were actually two different Schiff-bases involved. In the early work, Wald introduced the Oxime of Retinal, but only presumed that it was one of the products of his experiments90. This was a Schiff-base, an aldimine, involving a double bond between nitrogen and C15. It did not show an absorption peak near 500 nm. Bownds introduced the idea of a protonated Schiff-base in similar attempts to demonstrate a retinoid with an absorption near 500 nm. This also failed. Subsequently, calculations were performed based on a strained proton position in attempts to lower the theoretical energy of excitation and achieve an absorption peak near 500 nm. The results supported a theoretical peak in the area of 440 nm but held little hope for a peak near 500 nm or beyond. 

With hydroxylamine, al -trans, ll-c/5-, and 13-cw-retinal gave a mixture of the syn- and anti-oximes, whereas 11,13-di-cw-retinal gave only the syn-oxime/ The synthesis of the dansyl-lysyl-lysine-N-retinylidene Schiff base has been described/ The products of a colour reaction of retinoic acid (121) in 74% H2SO4 have been identified as (122) and (123)/ The oxidation and isomerization of retinoic acid by I2 and light have been used to prepare the a -trans- and 13-CI5-isomers of 4-oxoretinoic acid (124) which were separated by h.p.l.c/ The photoisomerization of the retinoid (125) has been studied. The many isomers produced were separated by h.p.l.c. and characterized by H and n.m.r. ... 

In a novel preparation of cm-isomers of retinoids, " thermal rearrangement of the vinylallene (87) gave an equimolar mixture of (IIZ)-, (11Z,13Z)-, and (9Z,llZ,13Z)-retinol. Synthesis of the vinylallene (87) is described. Another synthesis of (1IZ)-, (13Z)-, and (1 lZ,13Z)-retinal used organosilane protecting groups syn- and ant/-oximes of these retinol isomers were prepared. 

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