H. Maanaki, K. Lenz, T. Xu, J. Wang
University of North Carolina at Charlotte,
United States
Keywords: functional polymer nanorods, synthesis, characterization, boronic acid, glycoproteins, binding
Summary:
Herein, we present on the facile synthesis and characterization of poly(amino-phenylboronic acid) nanorods (PABA-NRs) and their high-affinity binding to glycoproteins. Boronic acid (BA) can form reversible covalent complexes with α-hydroxycarboxylate and cis-diol-containing biomolecules through the formation of cyclic diesters. These biomolecules include N-linked and O-linked glycoproteins, saccharides, nucleosides, etc. Furthermore, the unique pH-dependent reversibility of BA binding to diols has been highlighted in areas such as chromatography, protein separation, and drug delivery. However, many of these BA-containing monomeric compounds (e.g., phenylboronic acids) bind to glycoproteins at pH ranges higher than what is biologically relevant. To circumvent these issues, we aimed to synthesize PABA with nanofeatures similar to polyaniline nanofibers (PAnNFs), due to the similarities in their monomeric structures. As such, the nanofeatures of PABA-NRs with high surface area could provide enhanced binding-affinity to glycoproteins in comparison to its monomer. To achieve this aim, PABA-NRs were synthesized by chemical oxidative polymerization at 4 ℃ in the presence of sodium fluoride and 1-propanol under acidic conditions. Complexation of fluoride ions to boronic acid increases its electronegativity, resulting in a more reactive primary amine. 1-propanol serves as an inhibitor for reducing boronic acid interactions during polymerization. As-synthesized PABA-NRs were characterized using ultraviolet-visible spectroscopy (UV-Vis), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). UV-Vis demonstrated similar spectra characteristics of PABA-NRs to those of PAnNFs, confirming π-π* transition of benzenoid rings and polaron-π* transition of protonated imines. Morphological characterization via SEM and TEM revealed the presence of nanorod features with lengths and diameters of 0.5-2 µm and 50-80 nm, respectively. The binding of as-synthesized PABA-NRs to a model glycoprotein, horseradish peroxidase (HRP), was then evaluated at different conditions in comparison with that of PAnNFs, which served as a control. Under optimal conditions that minimize non-specific binding, it was found that PABA-NRs could successfully bind HRP at pH 7.4, with concentrations ranging from 0.078 to 2.5 ng/mL. On the other hand, no binding of HRP to PAnNFs was observed. Furthermore, the pH-dependent reversibility of binding was evaluated, and the results demonstrated strong binding of HRP to PABA-NRs between pH 6 and pH 8.5 and no binding at pH < 4. Under physiological pH 7.4, the binding dissociation constant Kd was calculated as < 0.078 ng/mL of HRP. Such unique characteristics of the novel PABA-NRs demonstrate its significant applicability in high-affinity capture of glycoproteins under biologically relevant conditions.