Complex-formation of Ethidium Bromide with poly[d(A-T)].poly[d(A-T)].

The interaction of Ethidium Bromide (EtBr) with double-stranded (ds-) and single-stranded (ss-) poly[d(A-T)] was studied in different ionic strengths solutions. Optical spectroscopy and Scatchard analysis results indicate that the ligand interacts to both helix and coiled structures of the polynucleotide by "strong" and "weak" binding modes. The association parameters (binding constant -K- and the number of nucleotides corresponding to a binding site -n) of the strong type of interaction were found to be independent of Na+ concentration.