The thermal denaturation of the synthetic high molecular weight double stranded polynucleotide poly(dA-dT)·poly(dA-dT) has been studied in aqueous buffered solution (Tris 1.0mM; pH 7.8±0.2) in the presence of increasing concentrations of either Ni²⁺ (borderline cation) or Cd²⁺ (soft cation) at four different constant ionic strength values (NaCl), making use of UV and circular dichroism (CD) spectroscopies. The experimental results show that the B-type double helix of the polymer is stabilized against thermal denaturation in the presence of both cations at low concentrations, relative to the systems where only NaCl is present, in the same conditions of ionic strength and pH. The effect is more pronounced for Ni²⁺ than for Cd²⁺. At higher concentrations, both cations start to destabilize the double helix, with Cd cations inducing larger variations of T_m. In many cases, when denaturation starts, interstrand cross-linking occurs with formation of aggregates that precipitate.

Key words: Model polynucleotides; Thermal denaturation; Nickel (II) and Cadmium (II) ions; Circular Dichroism; UV-spectroscopy.