Stabilization of nucleic acid triplexes by high concentrations of sodium and ammonium salts follows the Hofmeister series.

The thermal stability of the triplexes d(C(+)-T)(6):d(A-G)(6);d(C-T)(6) and d(T)(21):d(A)(21);d(T)(21) was studied in the presence of high concentrations of the anions Cl(-), HPO(4)(2-), CH(3)COO(-), SO(4)(2-) and ClO(4)(-). Thermally-induced triplex and duplex transitions were identified by UV- and CD-spectroscopy and T(m) values were determined from melting profiles. A thermodynamic analysis of triplex transitions shows the limitations of commonly used treatments for determining the associated release or uptake of salt, solute or water. Enhancement of the stability of these triplexes follows the rank order of the Hofmeister series for anions of sodium and ammonium salts, whereas water structure-breaking solutes have the opposite effect. The rank order for the Hofmeister series ClO(4)(-)<I(-)<Br(-)<Cl(-)<HPO(4)(2-)<SO(4)(2-)is shown to follow their effective surface charge densities.