Alkaline titrations of poly(dG-dC).poly(dG-dC): microemulsion versus solution behaviour

PolyGC was titrated with a strong base in the presence of increasing concentrations of NaCl (from 0.00 to 0.60M) either in water solution or with the polynucleotide solubilized in the aqueous core of reverse micelles, i.e., the cationic quaternary water-in-oil microemulsion CTAB/n-hexane/n-pentanol/water. The results for matched samples in the two media were compared. CD and UV spectroscopies and, for the solution experiments, pH measurements were used to follow the course of deprotonation. In both media the primary effect of the addition of base was denaturation of the polynucleotide, reversible by back-titration with a strong acid. In solution, the apparent pKa of the transition decreases with increasing the salt concentration and a roughly linear dependence of pKa on p[NaCl] has been found. A parallel monotonic decay with ionic strength has been found in solution for ROH, defined as the number of hydroxyl ions required per monomeric unit of polyGC to reach half-transition. By contrast, in microemulsion, ROH has been found to be independent of the NaCl concentration (and 10 to 50 times lower than in solution). This result is proposed as an indirect evidence of the independence of pKa on the salt concentration in microemulsion, where the pH cannot be measured. A sort of buffering effect of the positive charges on the micellar wall and of their counter-ions on the ionic strength could well explain this discrepancy of behavior in the two media.