Mono-, di- and trimeric binding of a bis-netropsin to DNA

An unusual 3:1 stoichiometry for complex formation between an elongated bis-netropsin compound and its binding site on DNA has been observed. Circular dichroism measurements distinguish two types of complexes formed between this bisnetropsin and poly[d(A-T)] · poly[d(A-T)]. The first type is characterized by a 1:1 saturating ratio of bound molecules per ten base pairs. Formation of the second type results from the cooperative binding of two additional bis-netropsin molecules to the first type of complex. In contrast to these results observed for binding to the alternating polynucleotide, only the 1:1 type of complex is formed when this ligand binds to the homopolymer poly(dA) · poly(dT).