Highly unsaturated fatty acids. II. Fractionation by urea inclusion compounds

Summary 1. Urea segregation has been found to be a useful tool in concentration of highly unsaturated acids and esters. Fatty acids and esters derived from fish oils and methyl esters of hog adrenal fatty acids have been fractionated, and the polyunsaturated acids have been enriched. 2. The tendency for fatty acids and esters to combine with urea decreases with increasing unsaturation. Combination is favored by lowered temperature. Polyunsaturated acids are concentrated in the final filtrate whereas esters of these acids appear in the final inclusion compound fractions. Fatty acid concentrates have been obtained with iodine values ranging from 294 to 356. Bound esters exhibited iodine values as high as 338. 3. The increased chain length brought about by esterification has a minor role in increasing the yield of inclusion compounds. Likewise, shifting of the terminal polar group in an acid toward the center of the molecule by esterification does not explain the higher yield of inclusion compound from esters. 4. The improved tendency of esterified acids to form urea inclusion compounds is probably due to blocking of association of acid groups. This effect is found in unsaturated acids and in short chain saturated acids. In long chain saturated acids and esters this effect is not noticed. Both give high yields of inclusion compounds because the effect of the carboxyl group is overshadowed by the high energy of formation contributed by the long carbon chain.