Deprotonation of synthetic polyadenyl-polyuridyl acid in aqueous solution

Ultraviolet and circular dichroism spectroscopies were used to examine the acid-base equilibrium of a double-helix synthetic polyadenyl-polyuridyl Poly(A)-Poly(U) acid in neutral and alkaline aqueous solutions. The deprotonation initially has a positive cooperativity which changes for negative at the deprotonation degree alpha > 1/2. A model is proposed, that explains the titration curves and the complex dependence of pK(a) on pH for Poly(A)-Poly(U) in going from the neutral to alkaline region. The model is based on the allowance for the effect of immediate environment on the internal protonation constant of ideal polymer.