Retinaldehyde geometric isomers

The absorption spectrum of retinaldehyde (2) in the visible and near-ultraviolet region shows three bands a main one at about 370 nm and two weaker transitions at 280 nm and 250 nm. It has been concluded from a number of theoretical calculations that the main 370-nm band in all geometric isomers of retinaldehyde (2) is attributable to the allowed transition in the lowest state (Wiesenfeld and Abrahmson, 1968 Langlet et aL, 1969a Christensen and Kohler, 1973 Schulten et a/., 1976). 

Geometric isomers of retinaldehyde (2) formed adducts with Eu(fod)3, and these adducts underwent isomerization when they were irradiated. It was particularly striking that photolysis of the adduct of (2) with Eu(fod)3 in a nonpolar solvent gave a substantial amount of (1 lZ)-retinaldehyde (377) (Ellis et al.,... 

When the mixture of these geometric isomers of (2) was chromatographed twice, two fractions were obtained whose compositions were determined on the basis of their H-NMR spectra. The first fraction contained (7Z, 13Z)-reti-naldehyde (353) and (7Z, 9Z, 13Z)-retinaldehyde (354), while the second fraction comprised (all- )-retinaldehyde (2), (7Z)-retinaldehyde (355), and (7Z, 9Z)-retinaldehyde (356). The 7-cis aldehydes (353), (354), (355), and (356) are remarkably stable. An absorption spectrum of an ethanolic solution of these aldehydes, kept under nitrogen at about 0 C, showed no change over a period of 6 months. Of course, such 7Z isomers are extremely sensitive to light. 

Retinal (retinaldehyde) occurs in significant quantities only in ocular tissue of mammals. Inasmuch as ll-cw and all-Iran isomers of retinal and retinol play important roles in the visual cycle, the separation of geometric isomers of both retinoids is discussed here. All retinoids (and carotenoids) are isomerized by heat and light, so samples should be handled with extreme precautions if information on the original isomer distribution is desired. Because the m-isomers generally have lower molecular extinction coefficients than the -trans forms (Table 1), the presence of large amounts of c -isomers may result in underestimation of retinoid (or carotenoid) quantities. 

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