ALBUMINS OR PROTEINS. 147 



I. ALIPHATIC SERIES. 



1 . Mono-amino-mono-carboxylic acids . 



Glycocoll. 



Alanine. 



Amino-isovaleric acid. 



Leucine. 

 Isoleucine. 



2. Mono-amino-hydroxy-mono-carboxylic acids: Serine. 



3. Mono-amino-di-carboxylic acids . . I Aspartic acid. 



[ Glutamic acid. 



4. Di-amino-carboxylic acids ... . I L y sine - 



[ Argmme 



5. Di-amino-hydroxy-monocarboxylic acids. Di-amino-tri-hydroxy-dodecylic acid. 



6. Amino acids containing Sulphur: Cysteine and Cystine. 



II. AROMATIC SERIES. 



Mono-amino-mono-carboxylic acids: Phenylalanine. 

 Mono-amino-hydroxy-mono-carboxylic acids : Tyrosine. 



m. HETERO-CYCLIC COMPOUNDS. 



Mono-a-amino-mono-carboxylic acids . 



Pyrrolidine-carboxylic acid. 



Tryptophane. 



Hydroxy-mono-amino-mono-carboxylic acids: pyrrolidine-carboxylic acid. 



Histidine. 



The carbohydrate group should also be included. These occupy a pecu- 

 liar position, because they are absent from a large number of the proteins: 

 in others their occurrence is questioned; while in still another group of 

 proteins they appear in larger amount, but only in part as a direct constitu- 

 ent of the albumin molecule. Many authors classify all these proteins con- 

 taining carbohydrates as compound albumins. As previously indicated, 

 we do not consider this as justifiable. We shall subsequently return to 

 this carbohydrate group. 



Let us turn to the individual amino acids. We shall consider their 

 distribution when we return to the composition of the individual proteins. 

 For the moment we will only classify them according to their constitution, 

 as this is necessary in order to understand their biological significance. 



The mono-amino-mono-carboxylic acids can be derived from the normal 

 fatty acid series: C n H2 n O 2 . The simplest member of this series is glyco- 

 coll, also called glycine or amino-acetic acid: 



COOH 



We shall use this formula, but the following is also possible 



COO 



CH,-NH 3 . 



