INTRODUCTION 5 



These peptides contain several molecules of crystalline organic 

 radicles the amino-acids and break down into these on further 

 decomposition. Finally, a large number of simple end-products 

 of protein hydrolysis have been discovered, isolated, and, during 

 the last few years, many of them have been synthesized. These 

 bodies have been classified by v. Noorden into the following 

 groups : 



I. Monamido- Acids : 

 (a) Monobasic 



Glycocoll, alanine, serine, isoserine, amido- 

 valerianic acid, leucine, tyrosine, and 

 phenylalanine. 

 (6) Dibasic 



Aspartic acid and glutaminic acid. 

 II. Diamido- Acids : 



Lysine and arginine. 



III. Indol- and Skatol-Producing Group : 



Tryptophane, skatolamino-acetic acid, or indo- 

 lamino-propionic acid. 



IV. The Pyrrol Group : 



Pyrrolidine-carboxylic acid. 

 V. The Pyrimidine Group : 



Histidine. 

 VI. The Carbohydrate Group : 



Glucosamine. 



VII. The Sulphur-Carrying Group : 

 Cystine. 



The simplest of the amino-acids contain one organic radicle 

 and one basic radicle the acid character being due to the 

 carboxyl group COOH, and the basic character to the amidogen 

 group NH 2 . Amino-acids containing only one amidogen group 

 are classed as " monamino-acids," as CH 2 (NH 2 ).COOH, glycocoll, 

 or mono-amido-acetic acid ; others possessing two such basic 

 groups are termed " diamino-acids," as CH 2 (NH 2 ).(CH 2 ) 3 . 

 CH(NH 2 ).COOH, " lysine," or " ae-diamido-caproic acid." In 

 the majority of the amino-acids there is only one carboxyl group, 

 but sometimes there are two or more such groups, and then they 

 are classed as " monobasic," " dibasic," etc. One of the peculiar 

 characteristics of these amino-acids is their power of undergoing 



