[Confirmation of the partial insertion of a steroidal diamine between the base pairs of deoxyribonucleic acid : study by 270 MHz proton nuclear magnetic resonance of the complexes of delta-5-dehydromalouetine with DNA of calf thymus gland and with poly(dA-dT)].

Delta 5- dehydromalouetine , a bis-quaternary steroidal diamine, has been selected for a 270 MHz proton NMR study of its binding to nucleic acids. Due to the presence of five well-identified probes located all around the molecule a precise description of the interaction could be achieved. The chemical shifts of the protons in the complex with Calf thymus DNA were followed at constant temperature whereas the complex with poly (dA-dT) was studied by varying the temperature. A good agreement between the data of the two kinds of experiments was obtained: shielding of the diamine protons and unshielding of the base protons on complex formation. This demonstrates partial insertion of the steroid backbone between unstacked base pairs.