442 PROTEINS 



known which have two amino groups attached to different 

 carbon atoms, such, for example, as a- 8- diamino valeric acid 

 CH2NH2CH2CH2CHNH2COOH derived from valeric acid 

 CH3CH0CH2CH2COOH. 



The dicarboxylic acids also can give rise to amino de- 

 rivatives such as aspartic acid COOHCH2CHNH2COOH de- 

 rived from the dicarboxylic acid succinic acid COOHCHgCHa 

 COOH and glutamic acid COOHCH2CH2CHNH2COOH de- 

 rived from glutaric acid COOHCH2CH2CH2COOH. 



It is important to note that all amino acids which are known 

 to take part in the building up of the protein molecule are a- 

 substituted acids, as will be seen from the list of protein 

 cleavage products given below. 



The presence of the — NHg group in amino acids confers 

 upon these substances basic properties, in addition to the acid 

 properties which they already possess. Thus, for example, 

 glycine CH2NH2COOH is able to react with hydrochloric acid 

 to produce glycine hydrochloride CHgNHaHClCOOH, just as 

 ammonia reacts with hydrochloric acid to give a hydrochloride ; 

 on the other hand, being an acid, it is also able to form metallic 

 salts, such as CH2NH2COOK. It is not surprising to learn 

 that the mono-amino acids, such as glycine and its homologues, 

 have no very pronounced acidic or basic properties. On the 

 other hand, the mono-amino derivatives of the dicarboxylic 

 acids, namely, aspartic acid COOHCH2CHNH2COOH and 

 glutamic acid COOHCH2 CH2CHNH2COOH, are strong acids, 

 owing to the predominating influence of the two carboxyl 

 groups, while the diamino derivatives of the monocarboxylic 

 acids, such as lysine CH2NH2CH2CH2CH2CHNH2COOH. 

 ornithine CH2NH2CH2 CH2CHNH2COOH, etc., have strongly 

 marked basic characteristics owing to the two amino groups. 



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 — NHg. Thus acetic acid CH3COOH 

 gives the amide CH3CONH2 known as acetamide, while 

 aspartic acid COOHCH2CHNH2COOH gives the amide 

 CONH2CH2CHNH2COOH known as asparagine. 



