Molecular requirements for auxin action-I. : Halogenated indoles and indoleacetic acid

In order to obtain evidexe for or against the concept that the biological activity of the indole auxins depends upon the fractional positive charge of tlz -NH group and its distanw from the carboxyl group, two new indole compounds and their methyl and ethyl esters have been prepared. In these the intensity of the charge on the -NH would be predicted to be incxased, while the auxin activity according to other theories should be decreased. Of there compounds, 2chloro-indole-3-acetic acid has about 350 per cent of the activity of indoleacetic acid; the 2-bromo acid has 160 per cent activity, and the activities of the esters are even higher. Thus the activities of the 2halogenated derivatives contrast strongly with that of the 2-methyl derivative, which has only 12 per cent of the activity of indoleacetic acid In a larger series of indole compounds with substituents on the benzene ring, the fmquenq of the i.r. solvent-bonded -NH stretching band in acetonitrile has been found to vary linearly with the Hammett sigma vahre for the substituent. The 2halogenated derivatives agree perfectly with this behavior. Since o meaaurm the change in the tendency of an anion or proton to associate reversibly when a substituent is introduced, it is deduced that the -NI-I stretch frequency is a measure of the relative positive charge on the -NH group (other factors being equal). In standard auxin tests with the same compounds, using curvature of slit pea stems, it is shown that the log of the growth activity is also a roughly linear function of the solvent- bonded -NH stretch frequency and hence of u, providing that gross steric eifects and lipophilic properties of the compounds are not too diierent. The data are therefore all consistent with the theory stated above, and it is suggested that analogous considerations may hold for the biological activity of the indolealky~es of the tryptamine and serotonin series.