Effect of the Alkaline Cations on the Stability of the Model Polynucleotide Poly(dG-dC)∙Poly(dG-dC)

When the model polynucleotide poly(dG-dC)∙poly(dG-dC) [polyGC] is titrated with a strong acid (HCl) in unbuffered aqueous solutions containing the chlorides of the alkali metals in the concentration range 0.010 M-0.600 M, two transitions in the absorbance vs. pH plots are evidenced, characterized by the constants pKa(1) and pKa(2). The limiting values at infinite saline concentrations of these two constants, namely pKa1∞ and pKa2∞ obtained making use of the “one site saturation constant” equation or, in turn, of the double logarithmic plot: pKa vs. log([salt]21), exhibit a clear dependence on the nature of the cations. The effects of the different alkali cations on the pKa∞ values follow the Hofmeister series. In fact, the pKa1∞ and the pKa2∞ values are smaller for Li1 and Na1 than for Rb1 and Cs1, with K1 at the border between the two, showing that the transitions require higher concentrations of protons to occur in the presence of high concentrations of the cosmotropic ions.