138 THE PROTEINS AND THE AMINO-ACIDS 



The amino-acids, since they contain trivalent nitrogen and a car- 

 boxyl-group, are simultaneously bases and acids, in other terminology 

 are Amphoteric Acids. They form crystalline salts with metallic bases 

 and with mineral acids. 



On treatment with nitrous acid the amino-acids lose their amino- 

 group, which is replaced by a hydroxyl-group as follows : 



CH 2 .NH 2 CH 2 OH 



+ HNO 2 =| + N 2 + H 2 O 



COOH COOH 



It will be observed that all of the amino-acids obtained in the 

 hydrolysis of proteins are a-amino-acids, that is, an ami no-group is 

 attached to the carbon atom immediately adjacent to the carboxyl- 

 group. This is probably a fact of great significance, since, as we shall 

 see, the proteins are formed by the union of long chains of amino-acids, 

 linked together by means of their amino- and carboxyl -groups. These 

 groups being closely adjacent, the resultant chains are shorter, and the 

 weight of the other radicals in the molecule more evenly distributed 

 than would be the case if the carboxyl- and amino-groups were separated 

 by a long chain of carbon linkages, and the possibility of such heavy 

 compounds as the proteins possessing sufficient stability to permit 

 their formation probably resides in this device for shortening the chain 

 of serial linkages. Corresponding to this view we find that the oo-amino- 

 group which is also present in lysine, is not united in proteins to any 

 carboxyl-group but remains free and reacts with nitrous acid just as 

 the amino-acid does. 



THE SYNTHESIS OF PROTEINS. 



The marked predominance of amino-acids among the products of 

 protein hydrolysis long ago led biological chemists to surmise that the 

 amino-acid structure, or some derivative of that structure, must be 

 represented in a high degree in the protein molecule, and it was in 

 following up this clue that Schiitzenberger in 1888 carried out one of 

 the earliest and most successful attempts to synthesize bodies of a 

 protein character. Recognizing that the decomposition of proteins 

 into amino-acids is essentially a phenomenon of hydrolysis, he regarded 

 .dehydration as an essential feature of any attempt at protein syn- 

 thesis, while the abundance of amino-acids among the products of 

 protein hydrolysis, and the presence therein, as he thought, of bodies 

 related to urea, led him to believe that protein synthesis must consist 

 in the linkage of amino-acids with molecules of urea and the elimination 

 of water. Accordingly amino-acids were mixed with urea and phos- 

 phorus p'entoxide and heated to 125 C. The product was a pasty 

 solid, soluble in water and readily coagulated by alcohol. It was 

 furthermore precipitated from aqueous solutions by the usual protein 

 precipitants and gave the biuret- and xanthoproteic reactions. 



