54 FATS, OILS, AND WAXES 



smell ; the solution contains sodium salts of fatty acids ; on 

 acidifying with sulphuric acid the fatty acids are precipitated. 



3. Lecithin on exposure to light and air absorbs oxygen 

 undergoing a change which reduces its solubility in alcohol 

 or ether and makes it increasingly soluble in water. 



4. Mixed with a little water, lecithin, in common with 

 some other lipins, swells up, forming slimy threads known as 

 myelin forms ; with excess of water these gradually produce 

 a sort of emulsion or colloidal solution from which they can 

 be precipitated by the addition of salts of barium or calcium. 



Lecithin like many other lipins is a yellow or yellowish- 

 white wax-like solid with a peculiar odour ; the lipins are 

 very hygroscopic, but some of them when carefully dried in 

 a vacuum can be obtained in form of powder. 



Lecithin is readily hydrolysed by boiling with alkalis, 

 notably baryta, and is also broken up by lipase, and, less 

 readily, by mineral acids. The products of its hydrolysis are 

 glycero-phosphoric acid- — 



CHjOHCHOHCHaOP^ (OH)2 



II 

 O 



choline HON(CH3)3CH2CH20H and fatty acids ; a similar 

 hydrolysis takes place in the germinating seed.* 



Originally it was considered that the fatty acids of lecithin 

 were either stearic, palmitic, or oleic, but it has since been 

 found that the more highly unsaturated acids, linolic and 

 linolenic, are also present. f The unsaturated arachidonic 

 acid X C20H32O2, containing four double bonds which occurs in 

 lipins of animal origin, has not hitherto been isolated from 

 plant lipins. 



To examine the products of the hydrolysis of lecithin, this 

 substance is heated with a solution of barium hydrate in 

 excess ; a baryta soap is formed, which may be filtered off. 

 The aqueous solution contains barium glycero-phosphate and 

 choline ; the latter may be extracted as follows : — § 



* Schulze: " Zeit. physiol. Chem.," 1887, li, 365 ; Schulze and Frank- 

 furt : " Ber. deut. chem. Gesells.," 1893, 26, 2151. 



t Levene and Rolf : " J. Biol. Chem.," 1925, 62, 759 ; 1926, 68, 285. 



X Ibid.. 1921, 46, 353 ; 1922, 54, 91. 



§Leathes: " The Fats," Monographs of Biochemistry, London, 1910. 



