Thermodynamics and Kinetics of Propidium Binding to Poly(A)·Poly(U) in Aqueous Solution

The propidium binding to poly(A)·poly(U) has been investigated thermodynamically and kinetically in buffers containing different concentrations of NaCl at various temperatures. Free propidium had an absorption maximum at 493 nm, and this shifted to about 530 nm for propidium fully bound to poly(A)·poly(U) in all solutions. The association constant for the reaction was 1.42×106 mol−1 dm3 at 2.02×10−2mol dm−3 of Na+concentration, and about 26 times larger than that at 1.02 mol dm−3 Na+. The dependence of the association constants indicated the enthalpies for the reaction at the low and high Na+ concentrations were 39.8 and −8.3 kJ mol−1, respectively. The entropies were 251 and 63 J K−1 mol−1, respectively. These thermodynamical results suggested that the association reaction of propidium to poly(A)·poly(U) may play different behaviors at the low and high salt concentrations. Kinetic results by a micro stopped-flow method indicated that the propidium binding to poly(A)·poly(U) was a single step at the low Na+ concentration, while at the high Na+concentration it consisted of two steps which contained the slow unimolecular process after the binding step.