Application of polyelectrolyte theories for analysis of DNA melting in thepresence of Na+ and Mg2+ ions

Numerical calculations, using Poisson-Boltzmann (PB) and counterion condens ation (CC) polyelectrolyte theories, of the electrostatic free energy diffe rence, Delta G(el), between single-stranded (coil) and double-helical DNA h ave been performed for solutions of NaDNA + NaCl with and without added MgC 2. Calculations have been made for conditions relevant to systems where exp erimental values of helix coil transition temperature (T-m) and other therm odynamic quantities have been measured. Comparison with experimental data h as been possible by invoking values of T-m for solutions containing NaCl sa lt only. Resulting theoretical values of enthalpy, entropy, and heat capaci ty (for NaCl salt-containing solutions) and of T-m as a function of NaCl co ncentration in NaCl + MgCl2 solutions have thus been obtained. Qualitative and, to a large extent, quantitative reproduction of the experimental T-m, Delta H-m, and S-m, and Delta C-p values have been found from the results o f polyelectrolyte theories. However, the quantitative resemblance of experi mental data is considerably better for PB theory as compared to the CC mode l. Furthermore, some rather implausible qualitative conclusions are obtaine d within the CC results for DNA melting in NaCl + MgCl2 solutions. Our resu lts argue in favor of the Poisson-Boltzmann theory, as compared to the coun terion condensation theory.