3-WAY CURVE RESOLUTION APPLIED TO THE STUDY OF SOLVENT EFFECT ON THE THERMODYNAMIC AND CONFORMATIONAL TRANSITIONS RELATED TO THE PROTONATION OF POLYCYTIDYLIC ACID

Solvent effect on the acid-base behavior of polycytidylic acid [poly(C )] is studied by means of potentiometric and spectrometric (UV and CD) procedures. Low-polarity biological environments are mimicked by usin g a 30% (v/v) dioxane-water mixture. Experiments performed in this med ium are compared with previously reported results in aqueous solution in order to determine changes in both thermodynamic and structural asp ects associated with modifications of the biomolecular surroundings. P otentiometric and spectrometric studies reveal the presence of a nonli near polyelectrolytic effect associated with the protonation of poly(C ) in the hydroorganic mixture, different from the analogous effect in aqueous solution. The curve resolution method alternating least square s is applied to the poly(C) spectrometric data. Concentration profiles and spectra of both deprotonated poly(C) and half-protonated [poly(C) .poly(CH+)] are thus obtained. The fully protonated species poly(CH+) precipitates in the hydroorganic medium at pH values lower than 4. Evi dence for the ordered structure of both poly(C) and [poly(C).poly(CH+) ] species is seen through the comparison of the macromolecule spectra with those of the cytidine-3',5'-cyclic monophosphate monomer. Polarit y decreases around the macromolecule produce significant hypochromicit y in the CD spectra and hyperchromicity in the UV spectra, both signs of a disordering effect in the macromolecular structure due to the wea kening of base stacking interactions. (C) 1997 Academic Press.