434 



C. HanscJi and R. i\I. Muir 



reaction at a specific point seems to. Such a reaction could vary all 

 the way from weak pi complex formation through Sigfna complex to 

 "normal" covalent bond formation at the particular ring carbon 

 atom. For a working hypothesis we have visualized the following: 



0-CH2COOH 



HoN 



O-CHoCONH 



CI 



m 



CI 



CI 



enzyme 



V 



CI 



+ 

 H-enzyme 



This scheme is drawn showing nucleophilic attack, and thus inter- 

 mediate IV shows delocalization of a negative charge. Of course 

 the same type of intermediate would result from electrophilic or 

 radical attack and delocalization of a positive charge or single elec- 

 tron would be equally important for the stabilization of the inter- 

 mediate. Intermediate III is shown as an amide, although most cer- 

 tainly simple salt formation could function to hold the auxin to the 

 site for the much slower subsequent reaction, be it complex or covalent 

 bond formation. Substance II is assumed to be protein in nature. A 

 consideration of the available information has led us to believe that 

 reaction must occur on one molecule (14). 



There are three possible ways in which X in the above series 

 of reactions could attack the aromatic ring: X-|-, X- or X:. In each 

 case stabilization of an intermediate through delocalization of a posi- 

 tive charge or electrons will be very important in promoting the reac- 

 tion. Such stabilization would not be possible in an isolated double 

 bond, and therefore it is not surprising that such compounds have 

 invariably been found to be inactive. In still another attempt to find 

 an active aliphatic acid, we tested 4-pentynoic acid and foimd it to 

 be inactive. In view of the relatively electron-rich character of most 

 of the rings in the known auxins, one might expect that X-(- would 

 be the most likely group for two-point attachment. However, if this 

 were true, then one would expect general activation by groups such 

 as methyl and methoxy. That this is not so is readily seen from the 



