Physical-Chemical Aspects of Synthetic Auxins 291 



through the reports indicating laikue to demonstrate the competitive 

 effects ot 2,4-D on enzymes (27, 28). While many enzymes have been 

 shown to be affected by the presence of this chemical, there seems to 

 be a lack of evidence of a clear-cut competitive effect of sufficient order 

 of magnitude to account for the rather magnificent biological re- 

 sponses induced by the chemical. 



BIOLOGICAL ACTIVITY OF CHLOROPHENOXYACETIC 



ACIDS 



In order to evaluate any physical differences that may be signifi- 

 cant in the relation of structure to activity and mode of action of the 

 chlorophenoxyacetic acids, it was necessary to assess and rate the 

 biological activity of members of this series. Such rating was done by 

 means of the root growth test using corn and lupine. The results of 

 this rating are to be found in Table 1. The values given in Table 1 

 are the molar activity in relation to 2,4-D which was taken as 100. 

 These values are in general accord with many published values given 

 in the literature. 



It is noted in this table that there is a very low order of activity 

 for those compounds substituted in the 2 and 6 position, which ob- 

 servation was reported by Muir and Hansch (22), Leaper and Bishop 

 (18), and Osborne et al. (26). It was suggested by Muir et al. (23) that 

 the 2,6-disubstituted compounds were relatively inactive because of 

 the requirement for free ortlw position with which to react with a sub- 

 strate molecule. Although it has been suggested several times that the 

 phenoxyacetic acids probably reacted with the protein to form a new 

 chemical species, the failure to find such a species by means of C^^- 

 labeled 2,4-D or a specific effect with any of the intermediate Krebs 

 cycle substrates (27) would appear to argue against specific chemical 



Table 1. The percentage molar activity of 

 some phenoxyacetic acids as measured in root- 

 growth tests with corn and lupine. 



