Correlation of Biological Activity of Phenoxyacetic Acids with Hammett Substituent Constants and Partition Coefficients

IN the ‘two-point attachment’ theory1 on the mechanism of action for growth regulators of the auxin type we have assumed as a working hypothesis that the reaction between auxin and substrate is more chemical than physical in nature and that covalent-bond formation is a possibility. The need for an aromatic ring with at least one unsubstituted position for auxin activity2 confirms our original belief that an ortho position in the phenoxyacetic acids is most suitable stereoelectronically to serve as one of the two points which attach the molecule to the plant substrate. The carboxyl serves as the second point, permitting the auxin to form a ring when joined to a plant protein. This reaction mechanism is supported by the work of Bonner et al.3,4.