Studies of the interaction between poly(diallyldimethyl ammonium chloride) and DNA by spectroscopic methods

The interaction of poly(diallyldimethyl ammonium chloride) (PDDA) with DNA has been studied by spectroscopic analysis of different kinds of nucleic acid probes, microscopic IR- and CD-spectroscopy. The two spectroscopic changes of ethidium bromide (EB) on its binding to DNA, namely red-shift of its maximum absorption wavelength (νmax) and enhancement of fluorescence intensity are utilized to study the interaction of PDDA with DNA. Reversion of νmax and decrease in fluorescence of DNA–EB system on addition of PDDA show the dissociation of DNA/EB complex and the formation of the DNA/PDDA complex. At the same time, the binding constant of EB and the number of binding sites per nucleotide decrease with increase in the concentrations of PDDA, indicating non-competitive inhibition of EB binding to DNA in the presence of PDDA. From the Scatchard analysis, we obtain the association constant of PDDA to DNA is 4.7×104 M−1. Contrary to EB, the binding of PDDA with DNA can greatly enhance the fluorescence intensity of groove-binding probes, Hoechst 33258 and DAPI, which can also be used to investigate the formation of the DNA/PDDA complex. The fluorescence analysis shows that the interaction of DNA with PDDA may have effect on the conformation of secondary structure of DNA. IR-spectra show that PDDA interacts with DNA through both the phosphate groups and the bases of DNA and the formation of DNA/PDDA complex causes the change of the conformation of the DNA secondary structure, which is further proved by CD-spectra.