Spectroscopic studies on the binding of methylene blue with DNA by means of cyclodextrin supramolecular systems

The interaction of methylene blue (MB) with calf thymus DNA (ctDNA) was comparatively investigated in detail by spectrometric methods in different kinds of cyclodextrin (CD) systems. Absorption spectra, fluorescence spectra and fluorescence polarization measurements indicated that the interaction mechanism of MB with DNA depended on the ratio (R) of the concentration of MB versus the concentration of ctDNA: intercalation into ctDNA base pairs at lower R value (R < 0.125); while electrostatic interaction at higher R values (R > 0.125). It was shown that the binding of MB with DNA at higher R value was inhibited by the anion CD derivatives, e.g., carboxymethyl β-CD (CMCD) and sulfated β-CD (SCD), and whereas this inhibition effect from the neutral CD was negligible. The inhibition effect from anion CDs on the MB/DNA binding is accounted for the competition between the negatively charged group of CMCD or SCD and the phosphate backbone (also negatively charged) of the DNA molecules, when these CDs and DNA molecules coexist with MB molecules. As a contrast, at lower R values, neutral as well as anion CDs were found to have similar effects on the absorption and fluorescence spectra of MB–ctDNA system, implying that different binding mechanism will be applied at low concentration ratio of MB/ctDNA. The corresponding intrinsic binding constants for these complexes were obtained by UV–vis and fluorescence spectroscopic methods, respectively.