Retinoids extinction coefficients

When retinol or its ester is mixed with a Lewis acid under anhydrous conditions, an intense transient blue color forms. Using antimony trichloride as the Lewis acid and anhydrous chloroform as the solvent, Carr and Price (1926) developed a quantitative assay for retinol. A large number of assay methods have subsequently been developed using a variety of Lewis acids, including tri-fluoroacetic acid (TFA) and trichloroacetic acid (TCA) (Dugan etaL, 1964). The stability of the color complex formed in various solvents has also been carefully studied (Subramanyam and Parrish, 1976). Some commonly used procedures based on this method are summarized in Table VI. The maximal intensity of the initial blue color complex formed is about threefold higher than the ultraviolet absorbancy of retinol. The maximal wavelengths and extinction coefficients of initial color complexes of various retinoids with trifluoroacetic acid are summarized in Table VII. 

Alternatively, the concentrations of retinoids in solutions can be expressed in mol/L by dividing the OD by the molar extinction coefficient e of the retinoid For example, in the above exercise ... 

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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