THE PROTEINS 79 



With bases the amino-acids form salt-like compounds such as potassium amino- acetate : 



CH 2 NH 2 



COOK 



Amino-acids also combine with one another. This power of combination much increases 

 the difficulty of separating the constituents from a mixture of amino-acids. Amino- 

 acids, which singly are extremely insoluble, are readily soluble when in the presence 

 of other amino-acids. 



On account of the dual nature of the amino -acid molecule, these substances act 

 as feeble conductors of the electric current, i.e. as electrolytes. The charge carried 

 by an amino -acid and its ionisation depends upon the conditions in which it is placed. 

 Since it may act either as the cation or the anion, it is spoken of] as an amphoteric 

 electrolyte. 



One reaction of the amino-acids is of special interest in connection with the respira- 

 tory functions of the body, namely, the formation of carbamino -acids. If a stream of 

 carbon dioxide be passed into a mixture of an amino-acid, e.g. glycine, with lime, the 

 carbon dioxide is taken up. On filtering the mixture a clear liquid passes through which 

 gradually in course of time deposits a precipitate of calcium carbonate. The filtrate 

 first obtained contains a compound of calcium, calcium glycine carbonate. The 

 formula is as follows : 



CH 2 .NH 



).CO 



COO Ca 



METHODS OF SEPARATING AMINO-ACIDS. By the hydrolysis of protein 

 by means of acid or of trypsin, we obtain a complex mixture of amino-acids. From 

 this mixture certain amino-acids are separated with ease. Thus tyrosine, which is ex- 

 tremely insoluble, crystallises out on concentrating the fluid, and further concentration 

 leads to the separation of leucine. The other acids, which keep each other mutually 

 in solution, are however very difficult to isolate. We owe to Fischer 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 con- 

 denser 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 glutarnic 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 evaporated to a syrupy consistence. By this 

 treatment all the amino-acids have been converted into the hydrochlorates of their 

 esters, e.g. : 



CH 2 NH 2 HC1 C 2 H 4 NH 2 H1 



I I 



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 aminoesters obtained in this way are hydrolysed the lower fractions by 



