Effect of cesium on the volume of the helix-coil transition of dA·dT polymers and their ligand complexes

The pressure dependence of the helix-coil transition of poly(dA)∙poly(dT) and poly[d(A-T)]·poly[d(A-T)] in aqueous solutions of NaCl and CsCl at concentrations between 10 and 200 mM is reported and used to calculate the accompanying volume change. We also investigated the binding parameters and volume change of ethidium bromide binding with poly(dA)∙poly(dT) and poly[d(A-T)]·poly[d(A-T)] in aqueous solutions of these two salts. The volume change of helix-coil transition of poly(dA)∙poly(dT) in Cs+-containing solutions differs by less than 1 cm3 mol− 1 from the value measured when Na+ is the counter-ion. We propose that this insensitivity towards salt type arises if the counter-ions are essentially fully hydrated around DNA and the DNA conformation is not significantly altered by salt types. Circular dichroism spectroscopy showed that the previously observed large volumetric disparity for the helix-coil transition of poly[d(A-T)]·poly[d(A-T)] in solutions containing Na+ and Cs+ is likely result of a Cs+-induced conformation change that is specific for poly[d(A-T)]·poly[d(A-T)]. This cation-specific conformation difference is mostly absent for poly(dA)∙poly(dT) and EB bound poly[d(A-T)]·poly[d(A-T)].