Small ligands that neither bind to nor alter the structure of d(GA·TC)n sequences in DNA

Three minor-groove binding ligands have been used to study the characteristics of two d(GA·CT)_n DNAs embedded in longer DNA fragments. The binding of mithramycin, netropsin or Thia-Net to these sequences has been studied using DNAse I footprinting. None of these ligands appeared to bind to d(GA·CT)₅ nor to d(GA·CT)₂₂ extensively, although with mithramycin some protected bonds were detected at the very edge of these sequences. In general, these small ligands did not enhance the DNAse I cleavage patterns at the alternating d(GA·CT)_n flanking sequences located near DNA regions where the drug was bound. The d(GA·CT)_n sequences could act as a rigid block in which it is not easy to propagate structural changes, whereas other sequences flanking the binding sites showed cleavage enhancements.