Use of Radioactive Carotene to Compare Absorption Coefficients of cis and trans β-Carotene

Abstract

Accurate values for the absorption coefficients of carotenoids can be obtained by correlation of specific radioactivities in comparison with the known coefficient of a standard substance. Tritium-labelled all-trans-β-carotene of known specific radioactivity was used as the standard. Comparison of absorbance values and specific radioactivity in 6 kinds of solvents (hexane, ethanol, ethyl acetate, acetone, benzene and dichloromethane) allowed the absorption coefficient to be determined in each solvent. After I₂-catalysed stereoisomerization, cis and trans isomers of β-carotene were separated by chromatography. Absorption coefficients for each isomer were determined by correlating recorded absorbance values with specific radioactivity, which was unchanged by the isomerization. The same approach can be used to compare the absorption coefficients of other groups of carotenoids that can be interconverted by simple chemical reactions.