| Abstract
| - Abstract. The salt dependence of the binding constant (K) and enthalpic (ΔH) and entropic (ΔS) components for magnesium binding to poly-RNA was determined as a function of the concentration and identity of monovalent counter ions (M+). Both ΔH and ΔS were found to vary linearly with ln [M+]. A theoretical analysis of the experimental data revealed that the temperature dependence of the product of the density of bound counter ions and the electrostatic interaction parameter, δ(m′ψ)/δT, is non-negligible, although it has previously been ignored. The sign of δ(m′ψ)/δT was negative for poly(A) and positive for poly(U), indicating that the charge density of poly(A) decreased with temperature, while that of poly(U) increased. These results are related to the distinct solution structures of the RNA homopolymers. Considerable support was lent to this calorimetric approach by the excellent agreement obtained in a test comparison between experimental and calculated parameters. From the intercept of energy term versus ln [M+] plots, the non-electrostatic contributions, ΔH° and ΔS°, were determined. For each polynucleotide, the similarity in ΔG° over the series of monovalent ions used in each study suggests a compensatory relationship between ΔH° and ΔS°, each of which shows significant variation. The non-electrostatic contribution to binding of divalent magnesium is generally entropically favorable and enthalpically unfavorable for both poly(A) and poly(U).
|
