324 



PROTEINS 



CH 2 CHNH 2 COOH, are strong acids, owing to the predomi- 

 nating influence of the two carboxyl groups, while the diamino 

 derivatives of the monocarboxylic acids, such as lysine 

 CH 2 NH 2 CH 2 CH 2 CH 2 CHNH 2 COOH, ornithine CH 2 NH 2 CH 2 

 CH 2 CHNH 2 COOH, etc., have strongly marked basic charac- 

 teristics, the two amino groups here overpowering the single 

 carboxyl group. 



A class of substances which have to be carefully distin- 

 guished from the amino acids are the acid amides. These are 

 derived from carboxylic acids by replacing the hydroxyl group 

 of the carboxyl by NH 2 . Thus acetic acid CH 8 COOH 

 gives the amide CH 3 CONH 2 known as acetamide, while 

 aspartic acid COOHCH 2 CHNH 2 COOH gives the amide 

 CONH 2 CH 2 CHNH 2 COOH known as asparagine. 



AMINO ACIDS OBTAINED AS CLEAVAGE PRODUCTS OF 

 PROTEINS. 



Mono-carboxylic 

 mono-amino acids. 



Dicarboxylic 

 mono-amino acids. 



(i) Aliphatic Compounds. 



Glycine or a-amino-acetic acid CH 2 NH 2 COOH 

 Alanine or a-amino-propionic acid CH 3 CHNH 2 COOH 

 Amino-butyric acid CH 3 CH CHNH 2 COOH 

 Amino-caproic acid CH 3 CH 2 CH 2 CH 2 CHNH 2 COOH 

 Valine or o-amino-isovaleric acid 



CH 3 /~ 



Leucine or o-amino-isocaproic acid 

 CH \ 

 CH 3 \CHCH 2 CHNH 2 COOH 



Isoleucine or a-amino-j8-methyl -ethyl propionic acid 

 H 3 ^)CHCHNH 2 COOH 



Serine or a-amino 0-hydroxy propionic acid 

 CH 3 OHCHNH 2 COOH 



f Aspartic * or o-amino-succinic acid 



COOH CH.,CHNH 2 COOH 

 V Glutamic * or a-amino-glutaric acid 



COOH CH 2 CH 2 CHNH 2 COOH 



* The amides corresponding to these two acids, namely asparagine 

 CONH 2 CH 2 CHNH 2 COOH and glutamine CONH 2 CH 2 CH 2 CHNH 2 COOH are 

 of considerable importance in plants. The former occurs in asparagus and is 

 produced in seeds which are allowed to germinate in the dark (Schulze, 

 " Landwirtsch. Gahrb.," 1878, 411), while the latter has been found in the 

 seeds of Cucurbita and many other plants (Schulze and Barbieri, " Ber. deut. 

 chem. Gesells.," 1877, 10, 199; Schulze, id., 1896, 29, 1882). Asparagine and 

 glutamine being readily hydrolysed by mineral acids, are not obtained as cleavage 

 products of proteins by the ordinary methods of chemical hydrolysis, and for this 

 reason are not quoted in the above list of cleavage products. 



