84 II. CHEMISTRY OF FATTY ACIDS AND GLYCEROL 



It is obvious that such information can be used to establish or confirm the 

 configuration of unknown geometric isomers. 



McCutcheon et al.^^ conchided, on the basis of their data on the Raman 

 spectra, that a- and |S-Hnoleates are identical. This supports the earlier 

 results of McCutcheon''^ as well as those of Riemenschneider and co- 

 workers,-^- and also the findings of Kass and Burr.^^ They are, however, 

 at variance with the interpretation of Frankel and Brown. ^^^ 



(6) Isomerism of Fatty Acids 



Although isomerism is relatively less important in the lipids than in most 

 other organic compounds, there are some cases in which any one of the 

 several types of isomerism may play an important role in establishing the 

 physical and chemical properties, as well as the biological behavior of the 

 particular type of compound. Isomerism does not occur to any great ex- 

 tent in the saturated fatty acid series except in the case of the branched- 

 chain acids. On the other hand, a number of unsaturated acids are known 

 which have the same empirical formula as oleic acid, namely, C18H34O2. 

 These vary widely from each other in distribution in nature, in chemical 

 and in physical properties, and in metabolism, depending upon the position 

 of the ethylenic linkage as well as upon the geometric form of the double 

 bond. There is experimental evidence that such compounds may not be 

 interconvertible in the animal. The types of isomerism found in the fatty 

 acids are best understood by means of classification according to the follow- 

 ing scheme of Shriner, Adams, and MarveP^l 



a. Simple Structural Isomerism. This can be explained by the use of 

 simple structural formulas. 



(a) Nucleus or Chain Isomerism. This type is exemplified bj^ differences 

 in arrangement of the carbon atoms in the chain such as would be ex- 

 hibited by valeric (CH3(CH2)3COOH) and isovaleric acids ((CH3)£CHCH2- 

 COOH). It is of only slight significance in the case of fatty acids of bio- 

 chemical importance. 



(6) Positional Isomerism. In this case the substituted group or the 

 position of the double or triple bond varies in the different isomers. 

 This is frequently observed and is probably of the greatest importance. 



(c) Functional or Group Isomerism. Here the isomers have the same 

 empirical formulas but have different functional groups. 



b. Stereoisomerism or Space Isomerism. In this case, the isomers 

 have identical molecular formulas but the variation results from the dif- 

 ference in the three-dimensional space arrangement. 



282 R. W. Riemenschneider, D. H. Wheeler, and C. E. Sando, J. Biol. Chem., 127, 

 391^02 (1939). 



283 J. S. Frankel and J. B. Brown, J. Am. Chem. Soc, 65, 415-418 (1943). 



28^ R. L. Shriner, R. Adams, and C. S. Marvel, in H. Gilnian, Organic Chemistry, 2nd 

 ed., Vol. I, Wiley, New York, 1943, pp. 214-488. 



