Effect of cations on the volume of the helix-coil transition of poly[d(A-T)

Abstract

The pressure dependence of the helix-coil transition temperature of poly[d(A-T)] has been measured in aqueous solutions of NaCl, KCl, and CsCl at concentrations between 0.02 and 1 M. In all cases the transition temperature increases with pressure. For solutions of NaCl, KCl and dilute CsCl the change in the transition temperature is linear with pressure up to 200 MPa. In more concentrated CsCl solutions the change in T m with pressure was hyperbolic. The molar volume change of the transition ( ΔV t) was calculated using the Clapeyron equation. At the lower salt concentrations, the derived values of ΔV t increase with the radius of the cation (Na + < K + < Cs +). At the higher salt concentrations ΔV t of poly[d(A-T)] in Na + and K + became equal; however, in CsCl solutions ΔV t was approximately twice as large as ΔV t in solutions containing the other two ions. In solutions of NaCl and KCl, ΔV t increased linearly with the logarithm of the salt concentration while in aqueous CsCl the concentration dependence of ΔV t was hyperbolic. The results are interpreted in terms of the role played by the radius of the cation in deciding the strength of the interactions formed with water.