Thermal melting of poly(dA-dT).poly(dA-dT) in methanol-water solutions

Thermal melting of a synthetic DNA poly(dA-dT).poly(dA-dT) was investigated in methanol-water solutions. Methanol decreased melting temperature of the polynucleotide but the decrease was qualitatively different in the presence of milimolar concentrations of sodium and cesium cations. The difference is a consequence of the fact that poly(dA-dT).poly(dA-dT) undergoes a helix-helix conformational isomerization in methanol-water solutions which is cesium cation specific. The arising conformation is much more stable than the conformation which poly(dA-dT).poly(dA-dT) adopts in solutions with sodium cations at high methanol concentrations. The (A+T) rich DNA of Bacillus cereus displays a similar behaviour.