Sodium and manganese salt DNA thin films: An infrared spectroscopy study

In this work we have investigated influence of divalent Mn ions on the structure of dsDNA utilizing Fourier transform infrared spectroscopy on DNA thin films obtained from sodium and manganese salt DNA (Na-DNA and Mn-DNA) in manganese chloride solutions and manganese salt DNA in pure water. In the range of low Mn content, 0.0067 ≤ r = [manganese]/[phosphate] ≤ 0.5, the difference between vibrational spectrum of thin films Na-DNA and Mn-DNA is revealed. Former one is more influenced by an increase of Mn content and shows stabilization of B form dsDNA, while in thin films Mn-DNA in MnCl₂ and Mn-DNA in pure water, B form is stable even at the lowest Mn content. An increase of Mn content over r > 0.5 induces spectral changes in both base and phosphate region that fully actualize once intrinsic Na⁺ ions are completely suppressed by divalent Mn²⁺ ions. Finally, the difference in vibrational spectrum of Na-DNA and Mn-DNA at high Mn concentrations almost completely disappears. The observed results consistently demonstrate that Mn²⁺ ions interact with both base sites of DNA (primarily C8N7 sites of guanine and adenine) and phosphate groups; both asymmetric and symmetric PO₂ vibrations show prominent blue shift in the presence of high Mn content, while B conformation remains stable. Nature of the Mn cation-DNA interaction seems to be electrostatic and water mediated, as demonstrated by almost complete reversal of perturbations in base and sugar-phosphate region in thin films Mn-DNA in pure water.