INTRODUCTION 5 



groups in the protein molecule is therefore very probable. Such groups 

 occur when amino acids, e.g., leucine and glutamic acid, are combined 

 together in the following way : 



CO NH CH CO NH CH CO NH 



C 4 H 8 (CH,), 



COOH 

 Leucine Glutamic acid 



and they are repeated when another amino acid is again combined in 

 this manner. 



(c) The combination of amino acids by the formation of 



CH, NH CO 

 groups is supported by the facts observed in the living body. 



Hippuric acid C 6 H 6 . CO NH . CH 2 . COOH is formed from 

 benzoic acid and glycine by the kidney, and the bile acids are pre- 

 sumably substances of similar constitution. 



(d) The condensation products of amino acids, obtained by Schaal, 

 Grimaux and by Curtius, many of which give the biuret reaction, 

 support the above contention. 



The Polypeptides. 



Our knowledge of the structure of the protein molecule is due to 

 the systematic researches of Emil Fischer and his pupils which were 

 begun in 1901 and are being continued at the present time. To them 

 also is due the proof of the accuracy of Hofmeister's view that the acid 

 amide form of combination of the amino acids is the principal one in 

 the protein molecule. 



Fischer has termed the combinations of the amino acids which he 

 has prepared, the poly peptides. This designation is in imitation of the 

 nomenclature adopted for carbohydrates where we differentiate between 

 mono-, di-, tri-, poly-saccharides according to the number of units, and 

 it contains the word peptone because these synthetical compounds 

 have very similar properties to the peptones derived from proteins by 

 hydrolysis by pepsin and weak acids and alkalies. The natural pep- 

 tones in all probability consist of a mixture of polypeptides. 



Fischer has devised three general methods for the synthesis of the 

 polypeptides : 



I. By boiling the anhydride of glycine with concentrated hydro- 

 chloric acid he found that the diketo- or diacipiperazine ring was split 

 and that the hydrochloride of a base C 4 H 8 NjO, was formed : 



CH, NH CO 



+ H,0 = HOOC . CH. . NH CO . CH, . NH, 

 CO HN CH, 



