134 PROTEINS AND AMINO-ACIDS [ch. 



strong bases as acids. The amino-acid, alanine, for instance, forms salts, 



sodium amino-propionate with a base, and alanine hydrochloride with 



an acid : 



CH3— CH— COONa CH3— CH— COOH 



I I 



NH2 NHg-HCI 



Substances behaving in this way have been termed "amphoteric" 

 electrolytes (see also p. 16). 



In the proteins, which are formed by condensation, as explained above, 

 there are always some NHg and COOH groups left uncombined. Hence 

 a protein must, in the same way, have the properties of both an acid 

 and a base. 



The amino-acids which are obtained by the hydrolysis of plant pro- 

 teins may be classified as follows: 



Aliphatic compounds. 



Mono-carboxylic mono-amino acids : 



Glycine or a-amino-acetic acid 



CH2(NH2)COOH 



Alanine or a-aniino-propionic acid 



CH3-CH(NH2)-COOH 



Valine or a-amino-iso-valeric acid 



CH3^ 



>CHCH(NH2)C00H 



CH3 



Leucine or a-amino-iso-caproic acid 



CH3V 



■ >CH • CH. • CH(NH2) • COOH 

 CH3^ 



Iso-leucine or a-amino-jS-methyl-^-ethyl-propionic acid 



CH3. 



^CH •CH(NH2) COOH 

 C2H6 



Serine or a-amino-jS-hydroxy-propionic acid 



CH20HCH(NH2)COOH 



Dicarboxylic mono-amino acids : 



Aspartic acid or a-amino-succinic acid 



COOH • CH2 • CH(NH2) • COOH 

 Glutaminic acid or a-amino-glutaric acid 



COOH •CH2-CH2 •CH(NH2)- COOH 



