THE CHEMISTRY OF THE PROTEINS 97 



of these derivatives. With glycine the following reactions 

 take place : 



CH.,.COOH CH-.COOH 



I ' + C b H s .NCO= I 



NH, NH.CO.NH.QHj 



CH...COOH CH 2 .CO\ 



I = H 2 0-f- | >N. G CH 3 



NH.CO. NH.C 8 Hs NH.CO/ 



They are similar in the cases of the other amino acids. 



The most important of the derivatives of the amino acids are 

 their esters, which are easily obtained by the action of gaseous 

 hydrochloric acid and methyl or ethyl alcohol. These com- 

 pounds were first prepared by Curtius, who has used them for 

 his researches upon the aliphatic diazo compounds. As obtained 

 on esterification, the esters are in the form of their hydro- 

 chlorides ; from these Fischer obtained the free esters by the 

 action of alkali at a low temperature and extraction with ether. 

 The esters are liquids capable of distillation under diminished 

 pressure, and have served as the means of the separation of the 

 amino acids as they result on the hydrolysis of proteins. They 

 have also been of great use in their synthesis. 



Many attempts have been made to prepare the acid chlorides 

 of the amino acids, and eventually it was found that they could 

 be prepared by the combined action of phosphorus pentachloride 

 and acetylchloride upon the amino acid. By their means the 

 synthesis of the proteins was materially advanced, as all possible 

 combinations of the amino acids together could be thus made. 



II. The Hydrolysis of the Proteins 



Of the various methods which have been employed in the 

 study of the composition of the proteins, that .'of hydrolysis has 

 been made use of most frequently, on account of the better and 

 more certain results which are obtained thereby. The proteins 

 can be hydrolysed by the action of acids, alkalies, and the 

 proteoclastic enzymes ; by either of these means a complex 

 mixture of amino acids results. They are of two kinds — the 

 monoamino and the diamino acids. To Drechsel, Hedin, and 

 especially[Kossel, we owe an excellent method for the separation 

 of the diamino acids, which lends itself to a quantitative estima- 

 tion of these compounds as they result on hydrolysis. The 



7 



