THE. CHESISTRY OF -“PHE “PROTEINS 107 
This compound is glycylglycine, as termed by Fischer, the 
group NH,.CH,.CO being called the glycyl group. It might 
be mentioned that the benzoyl derivative of this compound 
was obtained by Curtius in 1882. 
The ester of this compound was formed when the glycine 
anhydride was boiled with alcoholic hydrochloric acid. Both 
free acid and the ester have a great tendency to change back 
into the anhydride, a property which has been utilised in 
isolating such compounds from the products of hydrolysis of 
proteins. The amino group is also very reactive, e.g. with 
chlorcarbonic acid ester it gives carbethoxyglycylglycinester, 
from which, on hydrolysis, glycylglycinecarboxylic acid is 
obtained. When heated with leucinester, carbethoxyglycyl- 
glycinester undergoes a similar condensation, forming carb- 
ethoxyglycylglycylleucine, where three molecules of amino 
acids are coupled together. 
The diketopiperazine of alanine, and that of leucine, the 
so-called leucinimide, both gave the corresponding dipeptides 
alanylalanine and leucylleucine, 
H,N . CH(C,H,). CO. NH . CH(C,H,). COOH. 
Difficulties were, however, experienced with the condensation 
of the other amino acid esters, and with the more complex 
esters of the dipeptides, as also with the condensation of 
dissimilar amino acids, where several products may result, 
so that progress by these means became almost impossible. 
The compounds so prepared, as expected from their constitu- 
tion, gave the red biuret reaction, which was shown by Schiff 
to be given by compounds having CO— NH groups joined 
together directly, or by a C atom or by a N atom. 
About this time (1902) Curtius and Benrath prepared benzoy!l- 
pentaglycinaminoacetic acid, and two years later Curtius showed 
that his biuret base was triglycylglycinester. This compound 
is the ester of Fischer's triglycylglycine. Triglycylglycin- 
ester, according to Curtius and Gumlich, changes into an 
anhydride, which is an octapeptide. 
The method of preparing dipeptides from the diketopipera- 
zines a few years later again proved useful, when it was found 
that by heating the esters of the amino acids to 150-180° combina- 
tion occurred, and instead of splitting open the ring by boiling 
with concentrated hydrochloric acid, dilute alkali at the ordinary 
