Molecular Imprinting of 3-Hydroxybenzoic Acid: Special and General Binding Sites
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Molecular Imprinting of 3-Hydroxybenzoic Acid: Special and General Binding Sites Yue Hu and Robert A. Orwoll Departments of Applied Science and Chemistry College of William and Mary Williamsburg, VA 23187-8795, U.S.A
ABSTRACT A resin, imprinted with 3-hydroxybenzoic acid (3HBA), was synthesized from acrylamide (AA, the functional monomer) and ethylene glycol dimethacrylate (EGDMA, the crosslinking agent). Batch analyses showed that the imprinted polymer has a special affinity for the meta-substituted 3HBA, but not for its para-substituted isomer (4HBA) nor for benzoic acid (BA). These results are consistent with the principle that an imprinted resin’s ability to recognize is dependent on the target’s size, shape, and functionality. Another resin, prepared from AA and EGDMA but in the absence of a template, had similar affinities for 3HBA, 4HBA, and BA; and thus it could not differentiate among the three. The results can be interpreted with a simple two-binding-site model with one site special for 3HBA and the other being more general with similar affinities for 3HBA, 4HBA and BA. The binding of 3HBA to the imprinted resin is characterized by an association constant and the density of each kind of site using a two-site Scatchard equation. The binding sites common to both the imprinted resin and the non-imprinted reference resin were found to have greater affinity but are less numerous than the sites unique to the imprinted resin.
INTRODUCTION Molecularly imprinted polymer (MIP) can be synthesized by polymerizing appropriate functional monomers and crosslinking agents in the presence of templates. After the template has been extracted from the resulting resin, the tailor-made MIP possesses recognition sites of predetermined specificity with chemical, mechanical and thermal stabilities. The recognition sites are generally believed to form from prearranged template-monomer complexes in the mixture before polymerization followed by fixation during the polymerization [1-5]. The affinity for the template species and selectivity of the sites can be measured by chromatography or batch analyses [6]. The effectiveness of the imprinting can be evaluated by comparing the activity of MIP with the corresponding activity for a non-imprinted polymer (NIP), which has been prepared under the same condition as MIP except for the absence of the template. In this paper, batch analyses were used to identify the recognition sites in a resin imprinted with 3-hydroxybenzoic acid (3HBA). The binding isotherms showed specific affinity by the MIP for the meta-substituted 3HBA, but not for its para-substituted isomer (4HBA) nor for benzoic acid (BA). These results are consistent with the principle that an imprinted resin’s ability to recognize is dependent on the target’s size, shape, and functionality. A simple two-binding-site model with one kind of sites special for 3HBA and the other kind being more general with similar affinities for 3HBA, 4HBA and BA is proposed here.
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EXPERIMENTAL Materials 3-hydroxybenzoic acid (3HBA),
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