53 



The table shows most substances to be inactive, but also shows 

 some of them to be highly active and to cause a maximal shift in 

 0.001 AI concentration and still be active in 0.0001 M. Among 

 the most active ones we find dinitrophenol and dinitronaphthol, 

 both of which uncouple oxidative phosphorylation. That the two 

 actions might actually have some relation to one another is sug- 

 gested by the activity of dicumaroi (33) and pentachlorophenol 

 (27), both of which are also known to uncouple. Owing to its 



O-CH2-COOH 

 ,C1 



a 



(a) 0>) 



Fig. 10, a: 2,4-Dichlorophenol. b: 2,4-DichIorophenoxyacetic acid. 



insolubility, dicumaroi could not be tested in a higher concentra- 

 tion than 0.0001 Ai. The abolition or substitution of the phenolic 

 hydroxy group, which is essential for the uncoupling activity, also 

 deprived the molecule of its action on rhodamin. 



A point worth mentioning is the great difference in the activity 

 of two chemically closely related substances, 2,4-dichlorophenol 

 and 2, 4-dichlorophenoxy acetic acid (Fig. 10). Both substances 

 are highly active biologically, but their activity is very different, 

 so it is intriguing to find them different also in this simple /"« vitro 

 experiment ( see Chapters 1 2 and 16). In the present experiment 

 with rhodamin, a tentative explanation for the differences in the 

 activity of the dichlorophenol and 2-4 dichlorophenoxy acetic acid 

 (2,4D) can be found in steric relations. If the rhodamin molecule, 

 as well as the two chlorophenols and the nitrophenols are built of 

 an atomic model, it is found that the active nitrophenol and nitro- 

 naphthol can easily be brought into a position in which the phen- 

 olic hydroxide and one NO2 group touch the two resonating N's 

 of the dye. The same holds for the carboxylic O" and one CI (4) 



