304 PROTEINS 



place two or even three atoms of hydrogen in acetic acid 

 by the — NH2 group to produce diamine acetic acid 

 CH(NH2)2COOH and triamino acetic acid C(NH2)3COOH ; 

 these compounds are, however, not known, and appear to be 

 incapable of existing. 



The next homologue after acetic acid, namely, propionic 

 acid CHgCHoCOOH, can give two mono - amino acids 

 CH3CHNH2COOH and CH2NH2CH2COOH known re- 

 spectively as a- and yS- amino propionic acids, according as 

 the amino group is attached to the a- carbon atom, adjacent 

 to the carboxyl group, or to the /3- carbon atom, which is 

 next but one from the carboxyl. 



In the case of the higher homologues, diamine acids are 

 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 

 CH3CH2CH2CH2COOH. 



The dicarboxylic acids also can give rise to amino de- 

 rivatives such as aspartic acid COOHCH2CHNH2COOH de- 

 rived from the dicarboxylic acid succinic acid COOHCHgCHg 

 COOH and glutamic acid COOHCH2CH2CHNH2COOH de- 

 rived from glutaric acid COOHCHgCHgCHaCOOH. 



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 CH2NH2HCICOOH, just as 

 ammonia reacts with hydrochloric acid to give a hydrochloride ; 

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

 lic 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 ; they be- 

 long, in fact, to the class of bodies known as amphoteric elec- 

 trolytes (see p. 301). On the other hand, the mono-amino 

 derivatives of the dicarboxylic acids, namely, aspartic acid 

 COOHCH2CHNH2COOH and glutamic acid COOHCHg 



