THE PROTEINS 89 



the first general method for their separation. We may take one 

 experiment as an example. 



Five hundred grammes of casein are heated for some hours under a reflux 

 condenser with 1^ litres of strong hydrochloric acid. The liquid is then saturated 

 with gaseous hydrochloric acid and allowed to stand for three days in the ice- 

 chest. Crystals of hydrochlorate of glutamic acid separate out. The filtrate 

 from these crystals is evaporated at 40 C. under diminished pressure to a syrupy 

 consistence, and is then dissolved in 1| litres of absolute alcohol. Hydrochloric 

 acid is then passed into the solution to complete saturation, the mixture being 

 warmed for a short time on the water bath, and the mixture is once more evapo- 

 rated to a syrupy consistence. By this treatment all the ammo-acids have been 

 converted into the hydrochlorates of their esters, e.g. : 



CHoNH,HCl C 2 H 4 NHoHCl 



| j" 



COOC 2 H 5 COOC 2 H 5 &c. 



From the hydrochlorates the esters are set free by the addition of potassium 

 carbonate, the mixture being cooled in a freezing mixture. By this means the 

 esters of aspartic and glutamic acids are separated and are extracted by shaking 

 with ether. The remaining esters are then liberated by the addition of 33 per 

 cent, caustic soda together with potassium carbonate, and are again extracted 

 by ether. The combined ethereal solutions are dried by standing over fused 

 sulphate of soda and then evaporated, when a residue containing the free esters 

 is obtained. These esters are then separated by fractional distillation under 

 a very low pressure obtained by means of the Fleuss pump, the second receiver 

 of the apparatus being cooled in liquid air. The various fractions of amino- 

 esters obtained in this way are hydrolysed the lower fractions by boiling for 

 some hours with water, the higher fractions by boiling with baryta. The acids 

 obtained by the hydrolysis can then be further purified by means of fractional 

 crystallisation. 



THE DISINTEGRATION PRODUCTS OF THE PROTEINS 



By the methods just described the following substances have been 

 isolated from proteins : 



A. FATTY SERIES 

 (1) Mono-amino-acids (Monobasic) 



GLYCINE or GLYCOCOLL. This, the simplest member of the 

 group, is amiuo-acetic acid : 



CH 2 NH 2 



COOH 



It occurs in considerable quantities among the disintegration products 

 of gelatin and to a slight extent among those derived from certain of 

 the proteins. Like the other -amino-acids, it has a sweetish taste, 

 whence its name was derived (yXvKoa- = sweet, KO\\IJ = glue). 



