6 CHEMICAL STATICS 



fractional distillation in vacuo* The different fractions which 

 are thus obtained each contain the esters of only a few amino- 

 acids. The esters in each fraction are now reconverted into the 

 free amino-acids, and the individual acids are separated, identi- 

 fied and estimated by appropriate methods which differ some- 

 what for the different acids. 



The method, except so far as glutamic acid is concerned, is 

 by no means quantitative. The results which are obtained are 

 minimal yields. The extremely insoluble amino-acids, tyrosin, 

 cystin, and diaminotrioxydodecanic acid are separated from the 

 digest before esterification, and, so far as tyrosin is concerned, 

 our estimate is tolerably quantitative. 



By a combination of these methods it has been shown that the 

 protein molecule is built up of a series of amino-acids, and the 

 following amino-acids have been isolated from amongst the 

 products of the hydrolysis of various proteins.f 



A. Monoaminomonocarboxylic Acids 



1. Glycin: C 2 H 5 NO 2 , or amino-acetic acid: 



CH 2 .NH 2 .COOH. 



2. Alanin: C 3 HyNO 2 , or a-aminopropionic acid: 



CH 3 .CH(NH 2 ).COOH. 



3. Valin: C 6 HnNO 2 , or a-aminoisovalerianic acid 



8 X CH.CH(NH 2 )COOH. 



4. Leucin: C 6 Hi 3 NO 2 , or a-aminoisocaproic acid: 



CH.CH 2 .CH(NH 2 )COOH. 



CH 3X 



5. Isoleucin: C 6 Hi 3 NO 2 , or a-amino-/3-methyl-j8-ethylpropionic acid: 

 CH 3 , 



X CH.CH(NH 2 ).COOH. 



6. Phenylalanin : C9HnNO 2 , or /8-phenyl-a-aminopropionic acid: 



C 6 H 5 .CH 2 .CH(NH 2 ).COOH. 



7. Tyrosin: C 9 H U NO 3 , or 0-parahydroxyphenyl-a-aminopropionic acid: 



HO.C 6 H 4 .CH 2 .CH(NH 2 )COOH. 



* N. Zelinsky, A. Annenkov, and J. Kulikov (75) accomplish the separation 

 of the individual amino-acids by first neutralizing the excess of hydrochloric 

 acid with lead hydroxide and then subjecting the mixed hydrochlorides of the 

 ethyl esters to fractional distillation. 

 t Cited from Aders Plimmer (53). 



