Effect of the alkaline cations on the stability of the model polynucleotide poly(dG-dC)·poly(dG-dC).

Abstract 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 0.010M-0.600M, 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 PK∞ a(i) and PK∞ a(2) obtained making use of the “one site saturation constant” equation or, in turn, of the double logarithmic plot: pKa 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 values follow the Hofmeister series. In fact, the PK∞ a(1)) and the pK∞ a(2) 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.