OPTICAL METHOD 93 



tration may serve to make the method clear. 54 The tripeptid 

 d-Alanyl-glycyl-glycin shows in aqueous solution a specific 

 rotation of + 30. This substance can give rise in cleavage 

 to d-Alanyl and to glycylglycin. The former rotates light 

 but + 2.4, and the latter is optically inactive. When cleav- 

 age occurs therefore there is a sudden decrease in the rota- 

 tive power. Quite a different result is seen, however, if a 

 cleavage occurs which produces glycocoll and d-alanylglycin. 

 The latter of these two substances is characterized by a very 

 high rotative power, of + 50, and this type of cleavage 

 therefore shows itself by access of optical rotation. 



Emil Fischer and Abderhalden made the important state- 

 ment that racemic polypeptids are split asymmetrically by 



proteolytic ferments. Examining a dipeptid, NH 2-9 H - CO 



R 

 NH.CH.COOH, ^ mav j^ geen there are two asymmetrically 



R' 



placed carbon atoms. From this it follows, in accordance 

 with the well-known principle of van 't Hoff , there exist four 

 isomers, which group themselves into two racemoid bodies, 

 thus: 



d-Alanyl-1-leucin < > 1-Alanyl-d-leucin (racemic body A), 

 d-Alanyl-d-leucin << > 1-Alanyl-l-leucin (racemic body B). 



It has developed that the two racemic compounds behave 

 entirely differently toward pancreatic juice. Only that one 

 which presents the combination of the natural aminoacids 

 d-alanin and 1-leucin that is, racemic body A is split, and 

 this is attacked asymmetrically as only the combination 

 d-Alanyl-1-leucin is split ; while the combination of 1-Alanyl- 

 d-leucin remains unchanged. In this respect the proteolytic 

 ferments of animal tissues are shown to be analogous in their 

 influence to that of the pancreatic juice. 



M E. Abderhalden, Lehrb. d. physiol. Chem., 2d ed., pp. 266, 626, 1910. 



