Correlation of structural and physico-chemical parameters with the bioactivity of alkylated derivatives of indole-3-acetic acid, a phytohormone (auxin)

As part of molecular recognition studies on the phytohormone indole-3-aceti c acid (IAA) a series of alkylated IAAs has been examined. Phenyl-ring subs titution (all;yl = methyl and ethyl) at positions 4-, 6- or 7- as well as p yrrole substitution at the 2-site resulted in the six compounds which are a nalyzed: 2-Me-IAA, 4-Me-IAA, 6-Me-IAA, 7-Me-IAA, -1-Et-IAA and 6-Et-IAA. Th e structure-activity relationships investigated include those between the g eometrical parameters of the molecular structures determined by X-ray analy sis, the growth-promoting activities in the Avena coleoptile straight-growt h bioassay and relative lipophilicities calculated from retention times on a reversed-phase HPLC column and from R-F values in reversed-phase TLC. Lip ophilicities are correlated with the moments of inertia, average polarizabi lity, molecular mass, and the van der Waals radii of the ring substituents, The influence of substitution on the electronic properties of the indole r ing and its geometry is discussed on the basis of the UV and H-1 NMR spectr a.