Magnetic Field Effects on Cholesteric Helical Pitch

Magnetic Field Effects on Cholesteric Helical Pitch

Monodisperse DNA fragments with a contour length (500 Angstroms) near the persistence length in simple aqueous solutions of a 1:1 electrolyte undergo a complex series of phase changes and form at least three distinct liquid-crystal phases, which appear strongly analogous to phases formed by certain thermotropic and lyotropic small molecules. The image above illustrates the effects of a high magnetic field (7 Tesla) on the cholesteric helical pitch at the cholesteric/high density transition boundary. Orientation of the cholesteric phase by the magnetic field has resulted in an increase in the helical pitch of this phase to approximately 10 microns. The DNA concentration in this experiment is approximately 300 milligrams per milliliter, and the photomicrograph magnification is approximately 600x. Originally recorded on Fujichrome 64T transparency film using a Nikon Optiphot-Pol microscope with crossed polarized illumination, the image above was digitized using a Nikon CoolScan transparency film scanner. Exposures were recorded about 2 f-steps under the recommended value given by an in-camera photomultiplier and were push-processed approximately 1.3 f-steps in the first E-6 developer.

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