CHEMISTRY OF THE AUXINS 115 



them. The decreasing activities may be ascribed to de- 

 creasing rates of enzymic hydrolysis. The double bond 

 appears to be essential for activity, for the 2-3-dihydro- 

 derivative,^ hke the dihydi'o-derivatives of auxin a and b, 

 is inactive. Introduction of an alkyl group into the 1- or 

 5-position of the nucleus always reduced the activity, some- 

 times completely. Methylation of the N atom reduced 

 the activity more than methylation elsewhere,^ and the 

 ethyl group reduces the activity more than methyl. Intro- 

 duction of methoxj^l in any position completely removes 

 all activity (u). The activity of all active substances is 

 listed in Table XII, pp. 137ff. 



The length of the acid side chain also greatly affects 

 the activity; indole-3-carboxylic acid is completely inactive, 

 as is also indole-2-carboxyUc acid. Indole-S-propionic acid 

 has only very slight activity, but indole-3-isopropionic acid 

 was almost comparable with indole-acetic acid. Of other 

 derivatives indole-lactic acid was found inactive, while 

 indole-pyru\dc acid had low acti\dty. 



Commercial tryptophane (indole-3-alanine) has slight 

 auxin activity, but this is lost on recrystallization and is 

 therefore due to impurities. However, the behavior of 

 tryptophane is interesting from another standpoint. If 

 applied to Avena, curvatures are produced after a lapse of 

 two hours or so. It also accelerates the growth of coleoptiles 

 when applied to the base (u). Tryptamine, indole-3-ethyl- 

 amine, behaves in the same way (Skoog, 1937). The latter 

 substance, while it does not contain an acid group, could 

 doubtless be oxidized to indole-acetic acid, \'ia indole-3- 

 acetaldehyde. Tryptophane, as we have seen, may be con- 



1 Numbering of the ring-atoms of indole-compounds is as follows: 



^ This does not hold for activity on straight growth of Avena. 



