ixj PROTEINS AND AMINO- ACIDS 135 



Mono-carboxylic di-amino acids : 



Arginiiie or S-giianidine-a-araino- valeric acid 

 NH2 



HN=:C— NH • CH2 • CH2 • CH2 • CH(NH2) • COOH 



Lysine or a-f-di-aniino-caproic acid 



CHaCNHa) ' CHg ' CHg • CH2 ' CH(NH2) ' COOH 



Dicarboxylic di-amino acid : 



Cystine (dicysteine) or di-/3-thio-a-amino-propionic acid 



CH2 — S — S — CH2 



I I 



CH(NH2) CH(NH2) 



COOH COOH 



Aromatic compounds. 



Mono-carboxylic mono-amino acids : 



Phenyl-alanine or /3-phenyl-a-aniino-propionic acid 

 C6H5-CH2CH(NH2)COOH 

 Tyrosine or jo-hydroxy-phenyl-alanine 



OH • C6H4 • CH2 • CH(NH2) • COOH 



Heterocyclic compounds. 

 Proline or a-pyrrolidine-carboxylic acid 



CH2 CH2 



I I 



CH2 CHCOOH 



^NH^ 



IJistidine or /3-iminazole-alanine 

 CH 



NH N 



CH=C— CH2 • CH(NH2) ' COOH 

 Tryptophane or ^-indole-alanine CgHoN • CH2 ' CH (NH9) ' COOH 



C'CH2CH(NH2)COOH 



CH 



Different proteins are formed by various combinations of the above 

 acids and hence give different amounts on hydrolysis. 



There are certain properties and chemical reactions by means of 

 which proteins can be detected. These are illustrated in the following 

 experiment. 



