Abstract

The interaction of two anti-trypanosomal agents, berenil and pentamidine, with the A+T-rich dodecamer d(CGCAAATTTGCG)2 has been examined by high-resolution 1H-NMR, optical spectroscopy, and molecular modeling. Proton assignments for the free DNA and each DNA-ligand complex were obtained using nuclear Overhauser enhancement spectroscopy and total correlation spectroscopy. Complexation induces large changes in chemical shift for protons in the DNA minor groove for the A5-T9 segment, and intermolecular NOEs reveal contacts between the DNA bases and each ligand. The asymmetric binding site for berenil indicated by the NMR data suggests that at least two overlapping sites are involved. Rapid exchange between symmetrically-equivalent binding sites, via dissociative rearrangement, is consistent with retention of twofold degeneracy for both the ligand and the DNA host. Calculations of binding energy confirm that this DNA duplex contains overlapping sites of similar binding affinity. In contrast, the larger pentamidine molecule occupies a site that spans four or five bp, with asymmetric binding to the minor-groove 5'-ATTT sequence. The B-type conformation of the DNA is not altered substantially by either ligand.