112 ASYMMETRIC ANALYfilS 



action of the two optic isomers is ditTerent. One could 

 not, then, speak of a single receptive protoplasmic sub- 

 stance which would simply react to a ditferent degree to 

 the two isomers, as is probably the case when the mech- 

 anism of action of the two isomers is the same. 



2. Dual Activity of Organic Acids. The mechanism of 

 toxic action of the optical isomers of organic acids can be 

 also investigated from another point of view. Heilbrunn 

 (1928), among others, called attention to the dual nature 

 of the action of organic acids upon living systems: (1) 

 Organic acids produce an electro-chemical effect upon 

 the surface of the cells, primarily due either to a destruc- 

 tion of the negative charge of the cell surface by posi- 

 tively charged hydrogen ions or to other physico-chem- 

 ical surface 'pJienomena; (2) Owing to their relatively 

 weak electrolytic dissociation, the solutions of organic 

 acids contain a considerable proportion of non-dissociated 

 molecules which penetrate into the interior of the cells 

 where they produce transformations of a chemical nature. 

 Koltzotf's experiments (1915) on the action of ditferent 

 acids on the feeding activity of fresh-water vorticellids 

 furnish an example of the first type of action. There the 

 biological effect of the organic acid depends only on the 

 pH and the mechanism of this action consists in electro- 

 chemical changes upon surfaces directly accessible to hy- 

 drogen ions. The sinniltaneous occurrence of the first 

 and second type of effects is illustrated in the experi- 

 ments of Stiles and Rees (1935) who showed that the 

 killing action of monobasic organic acids of the aliphatic 

 series first diminishes with the elongation of the chain 

 of carbon atoms in the molecule, then reaches a minimum 

 with valeric acid and finally again increases with the fur- 

 ther elongation of the chain. This phenomenon was ex- 

 plained on the idea that the degree of electrolytic disso- 

 ciation diminishes with the increase in the weight of the 

 molecule, while the killing action of the non-dissociated 

 molecules increases with the increase of molecular weight. 



