Spectrophotometric and fluorescence polarization studies of the binding of ethidium, daunomycin and mepacrine to DNA and to poly(I-C).

The changes in the visible absorption spectrum and the degree of fluorescence polarization of daunomycin and mepacrine on binding to DNA have been compared with those shown by the archetypical intercalating agent ethidium. The changes are qualitatively similar for the three drugs, namely a bathochromic shift and hypochromicity (giving an isosbestic point) and an increase in polarization of fluorescence when irradiated with polarized light. These effects are typical of intercalation. The effect of changing the nucleic acid conformation has been investigated by repeating the studies with poly(I-C) (a nucleic acid in the A conformation). Upon interaction with poly(I-C) only ethidium shows effects typical of intercalation, neither daunomycin nor mepacrine intercalates into poly(I-C) helix.