26 



PLANT GROWTH SUBSTANCES 



membered ring confers activity, however, as shown by the acetic deriva- 

 tives of indole, benzofurane, and thionaphthene. The activity of auxin-a 

 and -b seems exceptional, but if we consider formula I it might be 

 suggested that the secondary butyl group has almost the configuration 

 of a ring. This phenomenon, of course, is well known in the case of the 

 estrogens, where the substituted derivatives of stilbene reach a maximum 

 activity with the diethyl derivative (XIV), which most nearly resembles 

 estradiol (XV). Further studies with alkyl-substituted 5-membered rings 

 are desirable to clear up this point. 



The need of unsaturation in the ring is characteristic and cannot be 

 replaced by unsaturation in the side chain. Thus A^-cyclohexeneacetic 



-CH 



CH, 



^<> 



HO 



'^ 



OM 



CH3 



OH 



xy 



(XVI) is active, but cyclohexylideneacetic (XVII) is not (3) ; indene-3- 

 acetic (III) is highly active, but benzofulvenecarboxylic (XVIII) is very 

 weak (11), although, of course, it retains unsaturation in the benzene 

 ring. It is not that unsaturation in the side chain destroys activity, for 

 aV-cinnamic acid(XII) and atropic acid (X) are active. Thus, irrespective 

 of the degree of saturation of the side chain, there must be unsaturation 

 in the ring itself. Veldstra in 1944 (13) brought forward evidence that 

 the unsaturation of the ring is needed to confer interfacial or surface 

 activity upon it. This evidence depends on the behavior of a series of 

 substances when their current-voltage curves are determined with the 

 dropping mercury electrode in acid methanol solution. Under these 

 conditions the control curve passes through a sharp maximum, ascribed 

 to oxygen adsorbed on the cathode, while active auxins strongly depress 

 this maximum. Seven active compounds behaved in this way while 

 seven inactive ones had no such effect. Eight other inactive compounds 

 did depress the maximum, however. Veldstra concludes that the ring 

 must have strong surface activity, that is, it must be capable of ad- 



CHj,COOH 



a 



CHCOOH 



^ 



f 



CHCOOH 



^/CH, 



a 



CHCOOH 

 2 



yvT 



"yvn 



will 



TTTT 



