PROTEINS 



73 



Obtained from protein substances, alanine is dextro-rotatory, is very 

 soluble in water, and possesses a sweet taste. Tyrosine, phenylalanine, 

 cystine, and serine are derivatives of alanine. This amino acid has been 

 obtained from nearly all proteins examined. Its absence from those 

 proteins from which it has not been obtained has not been proven. 

 Most proteins yield relatively small amounts of alanine. 



Serine, CH 2 (OH) -CH(NH 2 )- COOH. Serine is a-amino-p-hydroxy- 

 propionic acid and possesses the following structural formula: 



OH NH 2 



H C C COOH. 



H H 



Serine obtained from proteins is levo-rotatory, possesses a sweet taste, 

 and is quite soluble in water. Serine is not obtained in quantity from 



FIG. 22. SERINE. . 



most proteins, but is yielded abundantly by silk glue. Owing to the 

 difficulty of separating serine it has not been found in a number of 

 proteins in which it probably occurs. Serine crystals are shown in Fig. 

 22. 



Phenylalanine, C 6 H5-CH 2 -CH(NH 2 )- COOH. This product is 0- 

 phenyl-a-amino-propionic acid, and may be represented graphically as 

 follows: 



H NH 2 



C C COOH. 



H H 



