CH. VI.] LECITHIN. 163 



(B) Diaminophosphatides (N : P = 2 : i) 



Sphingomy elin . 



(C) Monaminodiphosphatides (N : P = i : 2) 



Cuorin. 



Their separation from other constituents of tissues is 

 dependent on the fact that though they are soluble in ether 

 they are insoluble in acetone. 



Lecithin is the best known member of the series. 

 The following account of its properties is abridged from 

 Maclean's valuable monograph. It is a yellowish-white 

 waxy substance, which on exposure to the air absorbs 

 oxygen and soon assumes a dark brown colour. It is very 

 hygroscopic and in the presence of moisture forms a soft 

 plastic mass. It dissolves very easily in alcohol, ether, 

 chloroform, benzene, petroleum ether and many other 

 organic reagents : also in aqueous solutions of bile salts. 

 It is insoluble in acetone and methyl acetate. In contact 

 with water it swells up and ultimately forms a slimy 

 emulsion or colloidal solution, from which it is readily 

 precipitated by salts with divalent cations, such as calcium 

 and magnesium ; salts containing monovalent cations, 

 such as sodium chloride, act in the same way, but more 

 slowly. In the presence of a small amount of sodium 

 chloride, acetone readily precipitates lecithin from its 

 emulsions with water. It is also readily precipitated from 

 ether or chloroform solution by this reagent. On treatment 

 with alkalies or acids it is hydrolysed, quickly on heating 

 and more slowly in the cold. Lecithin combines with 

 acids and bases. It also combines with certain salts of the 

 heavy metals, such as cadmium chloride, platinum chloride, 

 and mercuric chloride. Lecithin-cadmium-chloride is 

 almost insoluble in alcohol, but dissolves in a mixture of 

 carbon disulphide and ether or alcohol. It is probable 

 that the greater part of the lecithin of tissues exists in some 

 kind of combination with protein. Leithin is dextro- 

 rotatory. 



