Thermodynamics of the induction of self-structure in polyadenylic acid by proflavine

Abstract

The energetics of self-structure induction in polyadenylic acid in the presence of proflavine has been studied using isothermal titration calorimetry. The self-structure induction process was exothermic and driven by large positive entropy change. The equilibrium constant at T=(298.15±0.01)K and [Na+]=(130±0.01)mM was calculated to be (1.01±0.08)·106M−1. Salt dependent calorimetric studies revealed that the equilibrium constant increased with increasing [Na+] in the 50–130mM range suggesting enhanced binding preference at higher salt concentrations, apparently due to easier self-structure formation at higher [Na+]. Dissection of the standard molar Gibbs energy change clearly established that the self-structure induction was driven by non-polyelectrolytic forces and the polyelectrolytic contribution was relatively small. The equilibrium constant decreased with increasing temperature indicating destabilization of the self-structure at higher temperatures. Negative standard molar heat capacity value obtained from the temperature dependence of the enthalpy change suggested that hydrophobic forces are important for the self-structure induction. Furthermore, a complete enthalpy–entropy compensation phenomenon was also observed.