158 Fundamentals of Auxin Action 



methyl group for a hydrogen on the alpha carbon (XII) or a methyl- 

 ene group for both hydrogens (XIII) does not take away activity, but 

 the substitution of two methyl groups, as in the case of the wo-butyric 

 acid derivatives (XIV) removes activity entirely (Koepfli et al, 1938). 

 Such iso-butyric acids are anti-auxins (Burstrom, 1950). 



CHCOOH 



XII XIII 



This information suggests then that steric hindrance on the acid side- 

 chain can effectively prevent auxin activity. The presence of hydro- 

 gens on the alpha carbon is apparently not essential for activity. 



The substitution of a hydroxyl group on the side-chain has a re- 

 markable effect (Thimann, 1951). In two such auxin derivatives (XV 

 and XVI) auxin activity is eliminated by the hydroxyl substitution. 

 Other acids with hydroxyl groups on the side-chain have been found 

 to lack auxin activity. These include synthetic analogues of auxins a 

 and b (Kogl and de Bruin, 1950). This finding has cast considerable 

 doubt upon the existence of auxins a and b as growth hormones. 



OH CHzOH 



X'N CHCOOH f\ CHCOOH 



The degree of acidity of the acid side-chain plays a part in auxin 

 activity. Salts of the acid may be less effective in growth than the free 

 acid, and substitution of weak acid groups for the carboxyl reduces 

 auxin activity. For example, the substitution of — SO3H (XVII) or 

 — NO2 (XVIII) for the terminal carboxyl will reduce the activity, but 

 such compounds sometimes still show distinct auxin activity or some- 

 times none (Wain, 1949). Ester and amide derivatives of the carboxyl 

 appear to be less effective than the free acid, although they may have 

 distinct auxin activity (Kogl and Kostermans, 1935; Thimann, 1951). 

 Some evidence has suggested that indoleacetonitrile (XIX) may 

 possess auxin activity (Bentley and Housley, 1952), but such activity 

 may be owing to its conversion to the acid plus synergistic properties 

 of the nitrile described in the next section. 



