Tee CHES ROR: TRE: PROTEINS 97 
of these derivatives. With glycine the following reactions 
take place: 
CH,. COOH CH,. COOH 
| CI COs 
NH, NH.CO.NH.C,H; 
CH,. COOH CH, .CO 
l = H,0 4" | Nn. CH; 
NH.CO. NH. CH: 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 HypbrRotysis 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 
