22 OPTICAL ACTIVITY OF BIOL. MATERIAL 



ill the process of isolation, but we shall see below that the 

 principle itself is definitely established. 



Pringsheim (1910) had shown on asparagine that the 

 optically pure form is gradually racemised by boiling with 

 water, a step in the isolation procedure. 



With appropriate treatment, amino-acids both of vege- 

 table and animal origin, always prove to be optically pure, 

 that is, one isomer only of each amino-acid is present, its 

 antipode being completely absent. In the case of leucine, 

 this was established by the elaborate investigations of 

 Ehrlich and Wendel (1908), the results of which are given 

 in Table 1. 



TABLE 1 



Specific Eotation of Preparations of Leucine of Different Origin, 

 IN Water at 20° (Ehrlich and Wendel, 1908) 



Synthetic, optically pure preparation - 10.3° 



From egg-wliite (chicken) - 10.4° 



From casein (cow's milk) -10.3° 



From yeast {Saccharomyces cerevisiae) - 10.8° 



The optical purity of tyrosine remained for a long time 

 questionable as a result of a number of old contradictory 

 observations (Lippman, 1884). But Schulze and Winter- 

 stein (1905) have definitely shown that, after careful 

 preparation from vegetable material, one obtains always 

 optically pure substances and that racemisation and the 

 consequent decrease of rotatory power are the result of 

 the application of coarse methods of isolation (Table 2). 



TABLE 2 



Optical Eotation of Preparations of Tyrosine of Different 



Origin, in Hydrochloride Solution 



Synthetic, optically pure prep- 

 " aration - 16.4° Fischer, 1900 



From Cow 's Milk ; hydrolysis | - 13.2° 



of casein by HCl \ - 11.6° ' ' 



From the bulbs of Dahlia 

 variabilis; 80% boiling 

 alcohol was used in the 

 isolation procedure 



- 12.5° Schulze and Winterstein, 1905 



- 12.9° 



From embryos of Liipinus 



albvs; autolysis - 16.2^^ 



