408 V. CHEMISTRY OF PHOSPHATIDES AND CEREBROSIDES 



insoluble in the latter, while lecithin readily dissolves in it. This same 

 investigator also recognized the existence of a third phospholipid, now 

 kno^vn as sphingomyelin, which separates from a warm alcohol extract 

 of brain on cooling; this contains no glycerol, but is a diaminophosphatide 

 with a N:P ratio of 2:1. The phosphatidic acids were first described as 

 their calcium salts by Chibnall and Channon.^'* 



Folch^* subsequently demonstrated that the compound formerly des- 

 cribed as "cephalin" is actually a mixture of several components, two of 

 which have been identified as phosphatidylethanolamine and phosphatidyl- 

 serine. Folch,^-^ as well as Folch and Woolley,^* proved that a third ceph- 

 alin fraction contains inositol. A new group of phosphorus-containing 

 lipids, called plasma.logcns, has been described by Feulgen and Bersin"; 

 these are shown to be acetals of fatty aldehydes with colamine glycero- 

 phosphate. 



3. The Lecithins 



(1) Structure of the Lecithins 



Apparently a number of lecithins exist in nature which differ in the 

 component fatty acids or in the position of such fatty acids in the molecule. 

 Diakonow^^ and Strecker-^ were in agreement that lecithin contains two 

 fatty acids in ester combination with glycerol, while a phosphoric acid 

 molecule is attached both to the third hydroxyl of glycerol and to a choline 

 molecule. These workers disagreed as to the method of combination of the 

 phosphoric acid and choline, Diakonow^^ insisting that the combination 

 was in the nature of a salt and took place through the hydroxyl group on 

 the choline nitrogen, while Strecker" believed that the union occurred 

 through the ethanol hydroxide. The results of Hundeshagen^^ and of 

 Gilson^* indicated that the latter ester form was the correct one. 



Two varieties of lecithin occur, depending upon whether the phosphate 

 is attached to the a- or to the j8-carbon of glycerol. These are referred to 

 as a- and /3-lecithins, respectively. That ordinary lecithin is a-lecithin is 

 indicated by the fact that most natural samples are optically active.^" 

 This is due to the unsymmetrical arrangement of the fatty acids on glycerol. 

 Thus, it has been sho^^^l that an optically active glycerophosphoric acid 

 can be isolated from the hydrolysis products of lecithin. -^^ That the /3- 

 variety of lecithin likewise occurs naturally was indicated by Fourneau 



" A. C. Chibnall and H. J. Channon, Biochem. J., 21, 233-24G (1927). 



26 J. Folch, J. Biol. Chem., 146, 35-44 (1942). 



26 J. Folch and D. W. Woolley, J. Biol Chem., Ify2, 963-964 (1942). 



''^ R. Feulgen and T. Bersin, Z. physiol. Chem., 260, 217-245 (1939). 



2« F. Hundeshagen, J. prakt. Chem., 28, 219-255 (1883). 



29 E. Gilson, Z. physiol. Chem., 12, 585^602 (1888). 



3» C. Ulpiani, Gazz. cMm. Hal, 31, 47-61 (1901). 



31 R. Willstiittcr and K. Ludecke, Ber., 37, 3753-3758 (1904). 



