The model polynucleotide poly(dG-dC).poly(dG-dC) (polyGC) was titrated with a strong acid (HCl) in aqueous unbuffered solutions and in the quaternary w/o microemulsion CTAB/n-pentanol/n-hexane/water. The titrations, performed at several concentrations of NaCl in the range 0.005 to 0.600 M, were followed by recording the modifications of the electronic absorption and of the CD spectra (210≤ λ ≤350 nm) upon addition of the acid. In solution, the polynucleotide undergoes two acid-induced transitions, neither of which corresponds to denaturation of the duplex to single coil. The first transition leads to the Hoogsteen type ^synG.C⁺ duplex, while the second leads to the C⁺.C duplex. The initial B-form of polyGC was recovered by back-titration with NaOH. The apparent pK_a values were obtained for both steps of the titration, at all salt concentrations. A reasonably linear dependence of pK_a(1) and pK_a(2) from p[NaCl] was obtained, with both pK_a values decreasing with increasing ionic strength.

In microemulsion, at salt concentrations ≤ 0.300 M, an acid-induced transition was observed, matching the first conformational transition recorded also in solution. However, further addition of acid led to denaturation of the protonated duplex. Renaturation of polyGC was obtained by back-titration with NaOH.

At salt concentrations > 0.300 M, polyGC is present as a mixture of B-form and ψ(-) aggregates, that slowly separate from the microemulsion. The acid titration induces at first a conformational transition similar to the one observed at low salt or in solution, then denaturation occurs, which is however preceded by the appearance of a transient conformation, that has been tentatively classified as a left-handed Z double helix.