PROPERTIES OF LECITHINS 45 



3. Lecithin gives a purple coloration when added to a 

 mixture of strong sulphuric acid and sugar solution. 



The lecithins are yellow or yellowish-white wax-like solids 

 with a peculiar odour ; they are very hygroscopic, but some 

 of them when carefully dried in a vacuum can be obtained in 

 form of powder. They dissolve readily in the ordinary fat 

 solvents, such as ether, chloroform, carbon tetrachloride, ben- 

 zene, carbon disulphide, and also in oils and fats ; they are 

 also soluble in hot alcohol and ethyl acetate, but are only 

 sparingly soluble in acetone and methyl acetate, so that these 

 two substances may be used for the purification of the crude 

 product. They are precipitated from alcoholic solutions by 

 alcoholic solutions of platinic or cadmium chlorides. 



When mixed with a small quantity of water they swell up, 

 forming slimy threads, known as myelin forms ; with excess 

 of water they "dissolve," forming colloidal solutions which 

 are not coagulated by boiling, but from which they may be 

 precipitated by the addition of certain salts, such as those of 

 barium and calcium. 



As already stated, phosphatides dissolve in the same or- 

 ganic solvents as the fats, and are consequently liable to be 

 extracted from the tissues together with fats ; this fact must be 

 borne in mind in estimating the amount of fat in any sub- 

 stance by the method of weighing the residue remaining after 

 the evaporation of an ether extract. 



The chemical composition of the phosphatides differs from 

 that of the fats primarily in containing the two elements 

 nitrogen and phosphorus in addition to carbon, hydrogen, 

 and oxygen. According as they contain one or two atoms 

 of phosphorus in their molecules, they are classed as mono- 

 or di-phosphatides. The lecithins, like the fats, are esters of 

 glycerol with higher fatty acids, such as palmitic, stearic, and 

 oleic acids, but they differ from the fats in being at the same 

 time esters of phosphoric acid, as is shown by the following 

 formula of lecithin from egg yolk : — 

 CH2OCOC17H35 

 CHO-COCi7H,5 



CH./0-P(OH)-OCH.^CH2N(CHa)aOH 

 II 

 O 



