Anthracycline antitumor antibiotic nucleic-acid interactions. Structural aspects of the daunomycin poly(dA-dT) complex in solution.

The helix-to-coil transition of the synthetic DNA poly(dA-dT) in the presence of the anthracycline antitumor antibiotic daunomycin has been investigated by high-resolution proton nuclear magnetic resonance (NMR) spectrocopy in 1 M salt solution. The dissociation of the complex, containing molar ratios of phosphate to daunomycin (Pi/drug) of 50, 25, 9 and 5, with increasing temperature can be monitored independently at the nucleic acid and the antibiotic resonances under conditions of fast exchange. The antibiotic complex formation shifts suggest that either ring B and/or C of the intercalated anthracycline chromophore of daunomycin overlaps with adjacent nucleic acid base pairs. Ultraviolet/visible melting studies of daunomycin complexes with a series of synthetic DNAs substituted with halogen atoms (Br, I) at position 5 of the pyrimidine ring suggest that intercalation of the antibiotic into poly(dA-dU) is not perturbed by bulky substituents at this position. A comparison of the melting curves for the daunomycin . poly(dA-dT) complex with an analog of the antibiotic where the NH3 + group is replaced by dimethylglycine demonstrates the important contributions of electrostatic interactions between the amino sugar and backbone phosphates to the stability of the complex in low salt solution. The ultraviolet/visible and NMR studies monitor biphasic melting transitions at the nucleic acid markers in the daunomycin . poly(dA-dT) complexes, Pi/drug = 50--9, so that antibiotic-free base-pair regions and those centered about bound daunomycin can be independently studied at the synthetic DNA level in solution.