EFFECTS OF IONIC-STRENGTH ON THE SUPRAMOLECULAR STRUCTURE IN LIQUID-CRYSTALLINE SOLUTIONS OF PERSISTENT LENGTH DNA FRAGMENTS

Cholesteric liquid crystals of persistence length DNA fragments (contour length 55 nm) were studied with small angle neutron scattering. The cholesteric axis was oriented either perpendicular or parallel to the incoming neutron beam with the help of a magnetic field in the range 0.8-1.65 T. A single diffraction peak is observed, which shows in the perpendicular configuration an angular anisotropy in intensity. A moderate magnetic field strength dependence of the anisotropy has been observed, but at the higher field strength the liquid crystal is nearly completely aligned. A decrease in peak intensity as well as anisotropy is observed with increasing ionic strength, but fixed DNA concentration. This indicates a decrease in position and orientation order with increased screening of electrostatic interactions. From a comparison of the anisotropic data to the scattering contribution of a single fragment, the standard deviation of the presumed Gaussian orientation distribution could be derived. The results compare favorably with the second virial theory of lyotropic liquid crystals, provided that electrostatic interactions in terms of an ionic strength dependent effective diameter and DNA flexibility are taken into account.