Binding of Mg<SUP>2+</SUP>, Cd<SUP>2+</SUP>, and Ni<SUP>2+</SUP> 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 ≤0.050) was investigated by polarized light microscopy and multinuclear ³¹P, ²H, and ²³Na NMR. Our findings show that the state of the cholesteric NaDNA phase at equal MgCl<INF>2</INF>, CdCl<INF>2</INF>, or NiCl<INF>2</INF> 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⁺. ²³Na 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.