Chemical structure and biological activity 



tomato parthenocaipy test (see Table 1). The solubility of the lower 

 melting point dimorph of this compound is, in fact, somewhat less than that 

 of the almost inactive dodecanoic acid. 



In compounds with eight and ten carbon atoms in the side-chain a certain 

 amount of oxidation may take place at the naphthyloxy end of the chain 

 (i.e. on the co-carbon atom). This phenomenon of (o-oxidation is known from 

 studies on animal metabolism to occur with those fatty acids having 8 to 1 1 

 carbon atoms, and has also been postulated as possibly occurring in plant 

 tissues by Fawcett et al. (1952). It would thus be possible, on the assumption 

 of co-oxidation, to account for the reduced activity of the octanoic, decanoic, 

 and dodecanoic acids. It seems more probable, however, that the falling-off 

 in activity of the 8, 10, and 12 carbon acids is due primarily to restrictions on 

 the penetration of these molecules through the plasma membranes imposed 

 by the length of their fatty side-chains, rather than to any differences in their 



Table 2 



Relative molar activities of certain o)-{'i-chloro-2-naphthyloxy)-rv-alkylcarboxylic acids in the tomato 



ovary test 



(All compounds tested as sodium salts.) 



primary activity at the site of action in the cytoplasm. If this is so, the very 

 sudden drop in activity between the 6 and 8 carbon acids would imply that 

 there is some critical side-chain length below which penetration is not 

 affected and above which it becomes the factor limiting the over-all biological 

 activity of the molecule. 



Studies on the effect of nuclear substitution in 2-naphthyloxyacetic acid 

 have shown that the biological activity of this molecule in the tomato 

 parthenocarpy test is unimpaired by the introduction of a chlorine atom in 

 position 3. In the higher homologues of this series, however, the activity of 

 the butyric, caproic, octanoic, and decanoic acids is completely destroyed by 

 a chlorine atom in this position (see Table 2), which must presumably block 

 the ^-oxidation process. A similar phenomenon has previously been 

 reported, though without comment, by Synerholm and Zimmerman (1947). 

 These authors found that y- (2 :4-dichlorophenoxy) -butyric acid, as would be 



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