Chemical structure and biological activity 



the ED 50's were well above the highest concentrations of these compounds 

 it was possible to test, and were estimated directly by extrapolation of the 

 probit lines. 



This phenomenon of alternation in growth-regulating activity has been 

 previously reported for other homologous series of growth substances. Thus 

 Thimann and Bonner (1938) noticed an alternation in activity (as measured 



Table 1 



Relative molar activities of a homologous series of oi-{2-naphthvloxy)-n-alkylcarboxylic acids in the 



tomato ovary test 



(All compounds tested as sodium salts) 



+ Two dimorphic foims of this compound were tested. 

 X ED 50 estimated by extrapolation of probit line. 



by the Avena test) in the indolyl series, C8HgN(CH2) „COOH (where n varies 

 from to 4) and Grace (1939) investigadng the naphthylacetic acids 

 CioH7(CH2) „COOH found greater root inducing activity with the acids 

 having an even number of carbon atoms in the side-chain, though a general 

 decrease in activity with increasing chain length was apparent. More 

 recently, Synerholm and Zimmerman (1947) reported a similar phenomenon 

 in the first seven members of the oj-(2:4-dichlorophenoxy) aliphatic series. 

 These authors also tested the first three members of the co- (2-naphthyloxy) 

 series and found /5- (2-naphthyloxy) -propionic acid to be inactive for cell 

 elongadon in the tomato leaf epinasty test. Synerholm and Zimmerman 

 (1947) put forward an hypothesis to account for these results based on an 

 analogy with the oxidation of fatty acids in animal metabolism, where it is 

 known that oxidation normally occurs at the /3-carbon atom, leading to the 

 formation of an acid with two fewer carbon atoms. Thus it was postulated 

 that a compound such as e-(2:4-dichlorophenoxy)-caproic acid (with 6 

 carbon atoms in the side-chain) was active as a growth regulator because by 



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