CLASSIFICATION AND STRUCTURE OF FATTY ACIDS 33 



occur. This should not be too surprising, in view of the fact that several 

 of the well-known essential amino acids such as valine ((CH3)2CHCH(NH2)- 

 COOH), leucine ((CH3)oCHCH,CH(NH2)COOH), and isoleucine (CHj-- 

 CH2'C(CH3)CH(NH2)COOH), which occur in practically all animal and 

 plant proteins, contain branched chains. Moreover, many of the natural 

 products such as the carotenoids, vitamin A, vitamin E, and vitamin K, 

 have methyl groups replacing some of the hydrogen atoms in the side chain. 

 The presence of the methyl group usually results in an asymmetry of one of 

 the carbon atoms, and this accounts for an optical activity on the part of the 

 acid. However, since the substitution occurs at the tenninal carbon atom 

 in isovaleric acid, this product has no asymmetric carbon and does not ex- 

 hibit optical activity. 



a. Isovaleric Acid. As early as 1823 this acid was identified as a com- 

 ponent of dolphin and porpoise oils by Chevreul/^" who called it acide 

 phocenique. In the systematic nomenclature it is called 3-methylbutanoic 

 acid, and the fomiula is (CH3)2CHCH2C'OOH. Equally remarkable, in 

 addition to the presence of the branched-chain acid in these fats, is the 

 fact that such a short-chain aeid should be present. The only common fat 

 in which this has been observed is cow's milk fat, in which a considerable 

 proportion of butyric acid occurs. 



The identity of the isovaleric acid from dolphin and porpoise oil with the 

 product synthetically prepared was proved by Andre, ^^^ who also showed 

 that it was not a 50:50 mixture of butyric and caproic acids. Gill and 

 Tucker,'*- more recently, have reported as much as 26.7% of isovaleric 

 acid in a sample of porpoise jaw oil, while Klein and StigoF*^ in the same 

 year demonstrated its presence in the oil of the Black Sea dolphin. The 

 identification was satisfactory in both cases, being based in the first instance 

 upon the distillation temperature and neutralization value of the free acid 

 (coupled with a zero iodine number) and in the second case upon the boiling 

 point of the ethyl ester as well as upon certain other qualitative tests. 

 More recently, Tsujimoto and Koyanagi^^^ have identified cetyl isovalerate 

 as a distillation product of pilot-whale head oil {Globicephala melaena). 



b. Tuberculostearic Acid. Anderson and Chargaff^^^ were the first 

 to isolate this acid from human tubercle wax. It was also presumably 



1'" M. E. Chevreul, Recherches chimiques sur les corps gras d'origine animale, Levrault, 

 Paris, 1823, p. 115; cited by B. F. Daubert and C. G. King, Chem. Revs., 29, 269-285 

 (1941), p. 270. 



'31 E. Andr^, Bull. soc. chim., 35, 857-868 (1924); Compt. rend., 178, 1188-1191 (1924). 



132 A. H. Gill and C. M. Tucker, Oil & Fat Industries, 7, 101-102 (1930). 



'" A. Klein and M. Stigol, Pharm. Zentralhalle, 71, 497-500 (1930); Chem. AbsL, 2A, 

 5522 (1930). 



13^ M. Tsujimoto and H. Kovanagi, /. Soc. Chem. hid. Japan, 40, suppl., 272-274 

 (1937); Chem. AbsL, 31, 7685 (1937). 



'^ R. J. iVndcr.son and E. Gluirgaff, ./. liiol. Chem., So, 77-88 (1929). 



