Interaction of phenosafranine with nucleic acids and model polyphosphates. III. Heterogeneity in phenosafranine interactions with DNA base pairs.

Fluorescence and circular dichroism spectral measurements, thermal denaturation studies and binding competition experiments with netropsin and actinomycin D were carried out in systems containing phenosafranine bound to DNA's differing in base composition. The investigated properties exhibit a heterogeneity related to the content of A.T and G.C pairs in DNA and to the nature of phenosafranine binding modes. At low level of saturation of binding sites (r less than 0.1) phenosafranine does not show strong preference for any of the DNA base pairs in the overall binding. However, the strong monomer non-cooperative binding outside the helix (mode I1) occurs predominantly, even though not exclusively in G.C rich regions. The strong binding modes involving intercalated dye molecules (mode I2 and eventually mode II1) prevail in A.T rich regions. These binding modes become the principal types of strong phenosafranine interaction with DNA when the level of saturation of binding sites increases, i.e. at r greater than 0.1.20