OPTICAL ACTIVITY OF BIOL. MATERIAL 27 



plants ; consequently, the secondary origin of the racemic 

 form from initially active limonene appears to be probable. 

 The same conld be said also of racemic borneol. 



The last group of secondary substances to be considered 

 is that of the alkaloids. They seem to represent some 

 moditied fragments of protein molecules which perhaps are 

 some end products of metabolism. The question of the 

 optical purity of the alkaloids in plants has been repeat- 

 edly and extensively discussed. Apparently in a great 

 number of cases racemisation results from the process of 

 isolation. This seems to hold, in particular, for optically 

 inactive atropine, which represents the product of race- 

 misation of the laevorotatory hyoscyamine, the latter being- 

 found in plants in the optically active state (McKenzie 

 and Wood, 1919; Hess and Weltzien, 1920). It is known 

 that hyoscyamine is very easily racemised by w^eak alkalis 

 at room temperature. Some alkaloids, however, it was 

 suggested, might be present in plants in the racemic state, 

 for instance, coniine and scopoline. Since the racemisa- 

 tion of these alkaloids proceeds very slowly even at high 

 temperatures and pressures, an artificial racemisation in 

 the process of isolation seems excluded (Hess and Welt- 

 zien, 1920). The origin, in the plant, of racemic coniine 

 and scopoline is therefore still a mystery. 



In spite of the presence of a number of racemic forms 

 of alkaloids in plants, the majority of them are found in 

 the optically pure state, for instance, nicotine, anabasine, 

 etc. The alkaloids constitute, therefore, an exception 

 among the secondary substances which have severed their 

 connection with the primary complex. It is probable that, 

 owing to peculiarities of chemical structure, the mobility 

 of some groups in the molecule of several alkaloids is ex- 

 ceptionally low; their optical purity would be due, then, 

 to a too slow racemisation. In fact, it has not been pos- 

 sible to attain racemisation of the alkaloid heliotridane by 

 any of the means employed successfully in other cases 

 (Menshikov, 1937). 



5. Exclusiveness of the Asymmetry-Sign in Primary 



