Absorption and fluorescence spectra of polynucleotide complexes with ethidium bromide

We study the ethidium bromide (EtBr) interaction with the poly(dA)-poly(dT) and poly[d(A-T)]-poly[d(A-T)] polynucleotides at 0.02 M ionic strength of solution and pH = 7.2. It was revealed that the EtBr binds to the poly[d(A-T)]-poly[d(A-T)] by three modes: the intercalation, semiintercalation and weak (electrostatic), while with the poly(dA)-poly(dT) by two modes: the intercalation and semi-intercalation. Moreover, EtBr binds to the poly(dA)-poly(dT) cooperatively and to the poly[d(A-T)]-poly[d(A-T)] non-cooperatively.