MAGNETIC-FIELD ALIGNMENT OF CHOLESTERIC LIQUID-CRYSTALLINE DNA

In moderately high fields (3.5 and 6.4 T) the cholesteric phase of liq uid-crystalline DNA aligns with its twist axis parallel to the applied field. Thus when the field is applied in the plane of a thin sample, the well-known ''fingerprint'' texture is observed with striations of width similar to 1.1 mu m orienting perpendicular to the field. In a 9 .4 T field, the cholesteric aligns, still with the twist axis on avera ge parallel to the field bur locally distorted into longer wavelength stripes. The orientation of these larger stripes is parallel to the ma gnetic field, thus perpendicular to the fingerprint striations. The fi eld stabilizes the stripes, and upon removal from the magnet, the stri pes anneal away. The time scale for relaxation of the stripes to an al igned fingerprint texture is several days. Defect sites present in the held-stabilized stripe structure persist and become quite mobile (in the presence of a concentration gradient) after the stripes have annea led away.