Binding of Mg2+, Cd2+, and Ni2+ to liquid crystalline NaDNA: Polarized light microscopy and NMR investigations

The interaction of the divalent metal ions Mg2+, Cd2+, and Ni2+ with liquid crystalline NaDNA solutions (molar ratios Me2+/DNA-phosphate less than or equal to0.050) was investigated by polarized light microscopy and multinuclear P-31, H-2, and Na-23 NMR. Our findings show that the state of the cholesteric NaDNA phase at equal MgCl2, CdCl2, or NiCl2 concentration is affected in a different way and to a different extent by the nature of the divalent cation. Indeed, we found that the occurrence of an isotropic phase is markedly favored by Mg2+, and to a lower extent by Cd2+, and that the pitch of the cholesteric phase decreases in the presence of Cd2+ or Ni2+. Na-23 NMR spectroscopy also shows differences between the binding behavior of Mg2+ and the transition metal ions in the counterion atmosphere around DNA. The results are discussed in terms of different binding modes of the metal ions.