I will give a broad overview of recent work on the equation of state of DNA in aqueous monovalent salt solutions. The picture that I will develop shows that at non-negligeable monovalent salt concentrations the direct electrostatic interactions between DNA molecules are almost always masked by the thermal conformational fluctuations of the DNA chains. These thermal fluctuations act to boost the magnitude as well as the spatial range of the electrostatic interactions. This renormalization of the bare electrostatic interactions is a salient feature of dense systems composed of flexible polyelectrolyte moleclues.
INTRODUCTIONIn the phase diagram of DNA in aqueous solutions there is a range extending from the DNA crystal at densities corresponding to interaxial separations below ≈ 24 all the way to the nematic -isotropic transition at densities corresponding to interaxial separations ≈ 120 (for a bathing solution of 0.5 M NaCl) where DNA is orientationally ordered or even shows long range hexatic order perpendicular to the long axes of the molecules but is positionally still a liquid with only short range positional order [1]. This is in particular valid for long ∼ µm fragments of DNA. The local structure of these DNA arrays is presented on Fig. 2. In this part of the phase diagram, the equation of state, i.e. the dependence of the osmotic pressure on the macromolecular (DNA) concentration, has been