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 ofNaCl (from 0.00 to 0.60M) either in water solution or with the polynucleotide solubilizedin the aqueous core of reverse micelles, i.e., the cationic quaternary water-in-oil microemulsionCTAB/n-hexane/n-pentanol/water. The results for matched samples in the twomedia were compared. CD and UV spectroscopies and, for the solution experiments, pHmeasurements were used to follow the course of deprotonation. In both media the primaryeffect of the addition of base was denaturation of the polynucleotide, reversible byback-titration with a strong acid.In solution, the apparent pKa of the transition decreases with increasing the salt concentrationand a roughly linear dependence of pKa on p[NaCl] has been found. A parallel monotonic decaywith ionic strength has been found in solution for ROH, defined as the number of hydroxylions 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 50times lower than in solution). This result is proposed as an indirect evidence of the independenceof 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-ionson the ionic strength could well explain this discrepancy of behavior in the two media.