88 PHYSIOLOGY 



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

 ami no- acetate : 



CH 2 NH 2 



COOK 



With neutral salts crystalline compounds may be also formed. With 

 sodium chloride glycine will form the double salt C 2 H 5 N0 2 .NaCl, 

 which may perhaps be represented : 



CH 2 NH 3 C1 



COONa 



Not only do the amino-acids form compounds with salts, but they 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 respiratory 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 cal- 

 cium, calcium glycine carbonate. The formula is as follows : 



CH 9 .NH 



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 extremely 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 



