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 pK(a(1)) and pK(a(2)). The limiting values at infinite saline concentrations of these two constants, namely pK(a(1))(infinity) and pK(a(2))(infinity) obtained making use of the "one site saturation constant" equation or, in turn, of the double logarithmic plot: pK(a) vs. log([salt](-1)), exhibit a clear dependence on the nature of the cations. The effects of the different alkali cations on the pK(a)(infinity): values follow the Hofmeister series. In fact, the pK(a(1))(infinity) and the pK(a(2))(infinity) values are smaller for Li(+) and Na(+) than for Rb(+) and Cs(+), with K(+) 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.