448 C. Hansch and R. M. Muir 



we might be able to help finish this argument by making a p-chloro- 

 phenoxyacetic acid which didn't have the usual type of ortlto posi- 

 tions. ^Ve did this by putting nitrogens adjacent to the side chain. 

 Considering that our bioassays w^ere carried on in acid solution, we 

 could conclude that these nitrogens should be positively charged to 

 some degree. Consequently, they would have the tetrahedral con- 

 figuration as do the carbon atoms, and so we would have, in effect, 

 a p-chlorophenoxyacetic ester which had no ortho positions open to 

 electrophyllic attack. We bioassayed this compound in four different 

 tests and found that it was not only completely inactive as a stimu- 

 lant of grow^th, but that it was a competitive inhibitor of />chloro- 

 phenoxyacetic acid and ester. 



Dr. Bonner: Are these systems pH sensitive? 



Dr. Crosby: We carried these out actually only at two pH \alues, 

 4.5 and 6.0. 



Dr. Aberg: I should like to raise a question which turned out to 

 be a useful touchstone for various hypotheses on the relation between 

 structure and activity of the auxins, at the conferences in Ltuid and 

 at Wye. How does the present hypothesis explain the fairly strong 

 activity of d (-j-)-a-phenoxypropionic acid as contrasted to the very 

 weak activity of phenoxyacetic acid? 



Dr. Thimann: Dr. Aberg raised a very interesting question. I am 

 not prepared to explain it all, but this applies not only to phenoxy- 

 isopropionic acids. Frequently, alkyl substitution increases or modi- 

 fies activity. 1 think a complete explanation would depend on the 

 availablity of data on the effect of this on the charges of the atom. 

 As you know, in the case of indole, two alkyl groups do not destroy 

 activity, although in the case of phenoxy they do. I think all these 

 points depend on a much more careful evaluation of the properties 

 of the compound than I am now prepared to make. 



