NUCLEIC ACID-SMALL MOLECULE INTERACTIONS .7. FURTHER CHARACTERIZATION OF DEOXYRIBONUCLEIC ACID-DIAMINO STEROID COMPLEXES

The strong, stereospecific interactions, occurring at low ionic strength between deoxyribonucleic acid (DNA) and steroidal diamines such as irehdiamine A (pregn-5-ene-3β,20α-diamine) and malouetine (5α-pregnan-3β,20α-ylenebis(trimethyliodide)) have been investigated by means of five types of optical measurements: absorbance vs. temperature profiles, high- and room temperature absorption spectroscopy, as well as optical rotatory dispersion and circular dichroism. The results obtained suggest that in the presence of these ligands two different types of complexes are formed. The first requires a stoichiometry of [steroid]/[DNA-P] ≈ 0.20. It is characterized by enhanced thermal stability, increased hyperchromicity at 260 mμ, and a shift of the long-wavelength circular dichroic band toward the red with a considerable increase in rotational strength. It may involve a ligand-induced alteration in the bihelix with a resultant disorientation of the bases away from the B form. The second requires the presence of appropriate diprimary, primary-secondary, or disecondary amines at concentrations of [steroid]/[DNA-P] > 0.3. It is less stable thermally, and at room temperature its optical parameters are those characteristic of partially disoriented DNA. At somewhat higher temperatures this disorientation becomes complete and is retained on recooling. Yet the native degree of order is restored completely on increasing [Na+] to 0.2 M. This behavior may result from the stacking of steroid micelles, largely on the outside of the helix. The two forms are thought to be related to, but cannot be identical with, those assumed by DNA in the presence of planar, aromatic rings, and Cu2+ ions, respectively. © 1968, American Chemical Society. All rights reserved.