Application of a self-modeling curve resolution approach to the study of solvent effects on the acid-base and copper(II)-complexing behavior of polyuridylic acid

The solvent effect on the acid-base and complexation behavior of the homopolynucleotide polyuridylic acid (poly(U)) has been studied by means of potentiometric and spectrometric titrations (circular dichroism and UV-VIS) in water and in 30 and 50% (v/v) dioxane-water media. The potentiometric studies revealed the absence of polyelectrolytic effects in the acid-base equilibrium, and the spectrometric experiments detected only a random coil conformation associated with both the protonated and deprotonated species. The common behavior observed in the three media seems to indicate the weakness of both intramolecular interactions, i.e., base stacking, and solute/solvent interactions, i.e., hydrogen-bonding, and consequently their small effect during the protonation process. Differences regarding the solubility of the deprotonated species in the solvents used are due to the difficult stabilization of such a charged species in the low polar environment of the dioxane-water mixtures. Complexation has been exhaustively studied in aqueous media, and no conformational changes have been noticed in the only copper(II)-poly(U) complex detected. The inclusion of the copper(II) ion in the macromolecular skeleton of the polynucleotide does not contribute to an ordination of the structure, which remains as a random coil. No comparison between this equilibrium in aqueous solution and in the hydroorganic mixtures could be carried out since the limited pH range of the soluble complex in those solvent mixtures prevented a rigorous quantitative monitoring of such a chemical process.