Melting of columnar hexagonal DNA liquid crystals

The persistence length DNA hexagonal-cholesteric phase transition upon dilution and/or increase in solvent ionic strength is investigated with polarized light microscopy. The ionic strength dependence of the transition follows Lindemann criterion C-L = 0.098 +/- 0.003, i.e., the hexagonal lattice melts when the root-mean-square fluctuations in transverse order exceed 10% of the interaxial spacing. The spacings are derived from density and the fluctuations are estimated with a theory of undulation enhanced electrostatic interactions. Additional support for this theory is given by the DNA equation of state and anisotropic neutron radiation scattering from magnetically aligned cholesteric samples just below the phase transition.