Interaction of indole-papaverine with DNA in solutions of various ionic strength

Abstract

Abstract.Interaction of synthetic alkaloid of isoquinoline series, which is an analogue of the biologically active compound papaverine, was studied by spectral, microcalorimetric, optical and hydrodynamic methods at different ionic strengths of medium. It was found that the investigated compound may interact with DNA in various ways depending on the ratio of ligand -DNA concentrations and ionic strength of solution (µ). When μ = 0.001, indole-papaverine intercalates into the double helix of DNA. The increase of μ resulted in a decrease of the affinity of the compound to DNA and a change its binding method. IntroductionThe study of the formation mechanism of non-covalent reversible complexes of biologically active substances with DNA is an actual task, since the understanding of these processes plays an important role in the improvement of existing drugs, and may serve as the first step in the design of new ones.Intercalation and groove binding are mainly caused by van der Waals interactions and hydrogen bonds that are non-covalent interactions. Forasmuch as DNA in solution at neutral pH is a polyanion with negatively charged phosphate groups of the sugar phosphate backbone, electrostatic interactions also play a significant role in the formation of such complexes [1]. At intercalation there is embedding of planar aromatic molecules of ligand between DNA base pairs [2]. At external interactions there is a fitting of ligand molecules into the grooves or a formation of aggregates on the surface [3]. This paper presents the results of a study of interaction of DNA with the indole derivatives of isoquinoline -indole-papaverine, which has a positive charge on the isoquinoline chromophore, at various ionic strengths of medium. Materials and methodsThere were used calf thymus DNA from the company «Sigma» (USA) with molecular weight M = 10 7 Da. DNA concentration was determined by the spectrophotomical method described in [4]. Indole-papaverine was synthesized at the Research Institute of Hygiene, Occupational Pathology and Human Ecology.[5] Aqueous solutions of NaCl of different ionic strength, μ, and pH 6.0 -6.5 were used as a solvent. Complexes were prepared by direct mixing of DNA and the substance at a certain concentrations of the latter.The stoichiometry of complexes, defined as the amount of bound ligand per pair of nitrogenous bases of DNA (r), was obtained from the spectrophotometric titration data (SFT). The absorption spectra of the solutions were recorded using «Specord UV-Vis» and «Shimadzu UV-1800" spectrophotometers. Circular dichroism spectra were recorded at dichrographs MarkV.The enthalpy of interaction and thermodynamic parameters of the indole-papaverine binding were determined by isothermal titration calorimetry (ICT) using microcalorimeter TA Instruments Nano ITC 2G at the Resource Center of St. Petersburg State University "Thermogravimetric and calorimetric methods of investigation".[6] The calculation of the thermodynamic binding parameters in this case was carried out on the bases of a model wit...