444 PROTEINS 



(2) Aromatic Compounds. 



Phenyl alanine or j3-phenyl a-amino propionic acid 



Mono-carboxylic „ . n t ^ -u ^ ■ ■ ■ 



., { Tyrosine or fi-parahydroxyphenyl a-amino-propionic 

 mono-amino acids, l-' r i- j j tr j r r 



acid 



I 



HOCsH^CHaCHNHgCOOH 



(3) Heterocyclic Compounds. 



CHo — Cri2 



I " I 

 Proline or a-pyrrolidine carboxylic acid CHg CHCOOH 



NH 



Hydroxyproline or hydroxy a-pyrrolidine carboxylic acid 

 Histidine or j3-imidazol a-amino propionic acid 



CH = C— CH2CHNH2COOH 



N NH 



CH 



Tryptophane or j3-indole a-amino propionic acid 



C— CH2CHNH2COOH 

 C«H,<^^CH 

 NH 



The above list comprises most of the more important 

 cleavage products of proteins, the constitution of which has 

 been definitely established. 



Since different proteins give rise to different amounts of 

 these various substances, it is obvious that a careful quantita- 

 tive determination of the amounts of these acids produced by 

 the hydrolysis of different proteins must be of considerable 

 value. 



To this end Fischer, in 1901, introduced his so-called 

 " Ester method," which consisted in converting the mixed 

 amino acids obtained by hydrolysis of proteins into their 

 corresponding esters, and then separating these by fractional 

 distillation. 



The method * is best illustrated by an example. Casein 

 was decomposed by hydrolysis with concentrated hydrochloric 

 acid, the hydrochloride of glutamic acid being separated by 

 filtration. The filtrate was then evaporated under reduced 

 pressure, taken up with alcohol and saturated with dry gaseous 



* Fischer : " Zeit. physiol. Chem.," 1901, 33, 151. 



