Study of the acid–base equilibria and conformational changes of double stranded polyadenylic–polyuridylic acid in aqueous solution

UV-absorption and circular dichroism (CD) spectroscopy have been applied to the study of the acid–base properties of the heteropolynucleotide ribopolyadenylic–polyuridylic acid (poly(A)–poly(U)) and to the study of the possible interaction of this polynucleotide with 2,2′-bipyridyl (bipy) in aqueous solutions as a function of pH. Using soft modelling chemometric procedures based on factor analysis and multivariate curve resolution, the individual absorption spectra and pH–concentration profiles of all the species detected under equilibrium conditions are estimated for the systems studied. From these results, chemical models are proposed, which include the protonation of the adenine base of poly(A)–poly(U) on the transition from neutral to acidic solutions and the deprotonation of the uridine base of poly(A)–poly(U) on the transition from neutral to basic solutions. The conformational changes accompanying acid–base equilibria of poly(A)–poly(U) are deduced from the UV and CD spectral data. Under the experimental conditions of this work, no interaction was detected between the heteropolynucleotide poly(A)–poly(U) and bipy. Similarly, the interaction between the monomers cUMP, cAMP and bipy was proved not to be significant.