Absence of isomerization of retinyl palmitate, retinol, and retinal in chlorinated and nonchlorinated solvents under...

Purified solutions of all-trans retinyl palmitate, retinol, and retinaldehyde in chloroform, methylene chloride, or hexane were exposed to white light or gold fluorescent light or were kept in the dark, and the resulting isomer distributions were determined by LC (liquid chromatography). No significant isomerization of any of the retinoids occurred either in the dark or on exposure to gold light in any of the solvents tested. However, a large amount of the 9-cis isomer and only much smaller amounts of other cis isomers were produced when retinol or retinyl palmitate in chloroform or methylene chloride solution was exposed to white light. The isomerization pattern of retinyl palmitate in chloroform was not altered by the addition of free-radical scavengers, addition of an organic base, or substitution of deuterochloroform for chloroform as solvent. Use of other polar solvents such as tetrahydrofuran, acetone, or methanol produced isomer distributions similar to those obtained in chloroform solution. Retinol and retinyl palmitate in hexane solution, on exposure to white light, were isomerized much less extensively than in chloroform or methylene chloride and produced a significant amount of the 13-cis, as well as the 9-cis, isomer. Isomerization of retinaldehyde in chloroform or in methylene chloride solution under white light yielded 13-cis, 11-cis, 9-cis, and 7-cis isomers, in order of decreasing amount, whereas in hexane solution, only the 13-cis and 9-cis isomers were produced in significant quantity.