Precholesteric liquid crystalline states of DNA

Abstract

In aqueous concentrated solution, DNA forms cholesteric liquid crystalline phases whose textures were previously analysed. The transition between the isotropic and the cholesteric phase occurs generally with formation of cholesteric spherulites which grew inside of the isotropic phase and coalesce to give a homogeneous cholesteric phase (first order transition). Here is described with the same polymer, a higher order transition between the isotropic and the cholesteric phase. This evolution was arbitrarily divided into five steps that we called the precholesteric stages. We propose a geometrical interpretation of this evolution : three helical structures are introduced in the three directions of space. The proposed model does not present any discontinuity in the molecular orientations, the only possible singularities being + 1 and -1 disclination lines. The organization of these polymer structures present numerous similarities with the blue phases made of small liquid crystal molecules, namely double twist and defrustration