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 (conto ur 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 mode rate magnetic field strength dependence of the anisotropy has been obs erved, but at the higher field strength the liquid crystal is nearly c ompletely aligned. A decrease in peak intensity as well as anisotropy is observed with increasing ionic strength, but fixed DNA concentratio n. This indicates a decrease in position and orientation order with in creased screening of electrostatic interactions. From a comparison of the anisotropic data to the scattering contribution of a single fragme nt, the standard deviation of the presumed Gaussian orientation distri bution could be derived. The results compare favorably with the second virial theory of lyotropic liquid crystals, provided that electrostat ic interactions in terms of an ionic strength dependent effective diam eter and DNA flexibility are taken into account.