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 wi th polarized light microscopy. The ionic strength dependence of the tr ansition follows Lindemann criterion C-L = 0.098 +/- 0.003, i.e., the hexagonal lattice melts when the root-mean-square fluctuations in tran sverse order exceed 10% of the interaxial spacing. The spacings are de rived from density and the fluctuations are estimated with a theory of undulation enhanced electrostatic interactions. Additional support fo r this theory is given by the DNA equation of state and anisotropic ne utron radiation scattering from magnetically aligned cholesteric sampl es just below the phase transition.