Heparin and Heparan Sulfate Binding of the Antiparasitic Drug Imidocarb: Circular Dichroism Spectroscopy, Isothermal Titration Calorimetry, and Computational Studies.

Abstract

This study is aimed to assess the binding interaction between the antiparasitic cationic drug imidocarb (IMD) and sulfated glycosaminoglycans (GAGs), the ubiquitious nonprotein macromolecules of living organisms. These complex, heterogeneous polyanions are the integral constituents of cell membranes and the extracellular matrix and display affinity toward basic compounds, the binding of which may affect their biological functions. Exciton-type circular dichroism (CD) spectroscopic features measured at low salt concentration verify the heparin and heparan sulfate binding of IMD, which occurs in a cooperative manner by association of several drug molecules to a disaccharide unit. Isothermal titration calorimetry (ITC) measurements reassured the heparin interaction, resulting in a Kd value in the low micromolar range. In contrast, when considering high molar excess of the heparin-binding sites, closer resembling in vivo conditions, an entirely different CD signature was induced, suggesting a shift from the oligo- to monomeric binding mode. This observation was also supported by ITC measurements using an identical sample setup. To better mimic in vivo conditions, several measurements were performed in physiological salt concentration ranges. On the basis of these, the inter- and intramolecular origin of CD activity observed under low- and high-salt conditions refer to electrostatically held oligomeric and intermolecular H-bonded monomeric drug-GAG adducts, respectively. To complement the experimental data, quantum chemical calculations were performed to assess the photophysical and conformational properties of IMD, indicating the existence of nonlinear, nonplanar interconverting conformer populations. Such a structural flexibility may be important in the multiple, cooperative binding of IMD to sterically adjacent GAG sites.