Precholesteric Phase Helical Pitch

Precholesteric Phase Helical Pitch

The extremely large twist of the precholesteric liquid crystalline DNA phase is evident in this photomicrograph, which illustrates the transition to a cholesteric phase. Domains with a light blue background (on the left side of the image) represent those areas of the specimen that have undergone the transition from precholesteric to cholesteric. Areas with a darker background and having a lesser degree of birefringence are precholesteric. Note the large pitch (30 microns) in the precholesteric areas on the right side of the image. The DNA concentration for this specimen was initially 100 milligrams per millimeter, but this changes as the solvent evaporates. The magnification is approximately 150x. 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 2.5 f-steps under the recommended value given by an in-camera photomultiplier and were push-processed approximately 1.5 f-steps in the first E-6 developer.

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