Intercalative binding to DNA of antitumour drugs derived from 3-nitro-1,8-naphthalic acid

Two new antitumour drugs, imide derivatives of 3-nitro-1, 8-naphthalic acid having different basic side chains linked to the imide nitrogen, have been shown to bind to double-helical DNA by intercalation. At ionic strength 0.01 mol/litre, pH 7, their intrinsic association constants are about 1.45 × 105 M−1 and each bound ligand molecule occludes about 3.4 nucleotides of the DNA lattice. They remove and reverse the supercoiling of closed circular duplex PM2 DNA with apparent unwinding angles of 11–12° per bound drug molecule, referred to an assumed unwinding angle of 26° for ethidium. They increase the viscosity of sonicated rod-like DNA fragments, each bound drug molecule producing a calculated increment in length of 2.2 – 2.5 Å. No important differences between the DNA-binding characteristics of the two drugs were detected, though one appears marginally more active than the other in certain biological tests.