Molecular recognition of indole derivatives by polymers imprinted with indole-3-acetic acid: A QSPR study

Three molecularly imprinted polymers (MIPs) were prepared using the phytohormone indole-3-acetic acid (IAA) as a template molecule, 4-vinylpyridine (MIP-1 and MIP-2) or N,N-dimethylaminoethyl methacrylate (MIP-3) as functional monomers, ethylenglycol dimethacrylate as a cross linker and acetonitrile (MIP-1), a methanol–water mixture (MIP-2) or chloroform (MIP-3) as porogens. Retention factors for IAA and 29 indole derivatives were determined by high-performance liquid chromatography, using the molecularly imprinted polymers as stationary phases and acetonitrile as an eluent. High correlations between selectivity factors of above mentioned polymers indicate that their retention mechanisms are basically the same. A quantitative structure–property relationships analysis revealed that the presence of the terminal carboxyl group on the 3-side chain plays an essential role in the binding of the indole derivatives to the polymers. The derivatives without the carboxyl group exhibit a drastically lower affinity toward the polymers. Another factor which favors the binding is electronic density of indole nucleus. Substituents with electro-withdrawing properties enhance the binding, while electro-donating substituents have the opposite effect. The length of the 3-side chain also affects the binding. Indole-3-carboxylic acid having the carboxyl group directly attached to the ring as well as the derivatives whose side chain is longer than that of IAA bind to the polymers with a lower affinity.