Spectrophotometric study on the supramolecular interactions of nile blue sulphate with nucleic acids

The supramolecular interaction of nile blue sulphate (NBS) with nucleic acids was studied by investigating the characteristics of the interaction absorption spectra on the basis of the drug binding process in organic system in which small amount of drug interacting with large amount of biological macromolecules involves, and an accordingly binding model for organic dyes with large amount of macromolecules was established. At pH 7.40 and ionic strength 0.004, the H-aggregation of NBS occurs with increasing NBS concentration. The NBS aggregates can be bound to both calf thymus DNA and fish sperm DNA by the ratio of each nucleotide residue with a molecule of NBS if the concentration of DNAs is more than 15-fold excessive. The corresponding binding constant for the interaction of NBS with DNAs is about 10(3) order, with which thermodynamic parameters for the interactions, such as the change of free energy, enthalpy and entropy at 25 degrees C, were calculated. It was found that the binding of NBS with thermally denatured DNA is similar to that with native yeast RNA, which indicates H-aggregation of NBS can be encouraged by single stranded nucleic acids.