Protonated polynucleotide structures : 14. Poly(dG). poly(dC) and its metastable form

The circular dichroism spectra of poly(dG). poly(dC) have been studied as a function of pH in 0.15 M NaCl solution. Acid titration to pH 2.5 showed two transitions, one around pH 5, the second below pH 3. These transitions are disproportionation reactions and are not due to the dissociation of the complexes. Alkaline back titration to neutrality showed only one step above pH 7, without appearance of the intermediate form. The hysteresis loop observed gave rise to a metastable (probably protonated) form at pH 7 which reverted to the neutral form upon heating or alkali treatment. In order to obtain again the metastable hysteretic form, a whole titration cycle to pH 2.5 had to be performed.