98 PHYSIOLOGICAL CHEMISTRY 



H 2 C CH 2 



19. Proline H 2 C C COOH 



\/ H 



N 

 H 



(OH) 

 H 2 C CH 



20. Oxyproline H 2 C C.COOH 



N 

 H 



The position of the hydroxyl is uncertain. 



General Properties and Reactions of Amino Adds. Solubil- 

 ity, Taste, Optical Activity. The amino acids obtained by 

 hydrolysis of protein are crystalline substances, which are 

 readily soluble in water, with the exception of cystine, which 

 dissolves with difficulty in both cold and hot water, and of 

 tyrosine, which is quite insoluble in cold water, but dissolves 

 more readily in hot water. Solutions of monoamino-monocar- 

 boxylic acids are neutral in reaction. Solutions of dicarboxylic 

 acids are acid, and of diamino acids are alkaline. The solu- 

 tions of monoamino-monocarboxylic acids in reality have both 

 acid and basic properties and should properly be classed as am- 

 photeric. They all dissolve in dilute acids or alkalies, except 

 cystin, which is not readily soluble in dilute ammonia. The 

 amino acids vary in taste. Glycocoll, alanine and caprine are 

 sweet, leucine is tasteless and isoleucine is bitter. With the 

 exception of glycocoll, all the amino acids are optically active 

 and exist in two forms, a dextro- and a levorotatory. Usually 

 only one of the two is found as a protein constituent, and this 

 is more often the levorotatory variety. If proteins are broken 

 down by hydrolysis with alkali, the resulting amino acids are 

 racemic, that is, they exist as equal amounts of the two optical 

 isomers. The form of the acids not present in the protein is 

 believed to be prodaccd daring the hydrolysis. Hydrolysis by 



