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Cholesteric Liquid Crystalline DNA - At relatively high sodium concentrations (over 0.2 Molar), DNA spontaneously forms cholesteric liquid crystalline phases in aqueous solution at DNA concentrations exceeding 100 milligrams per milliliter. Liquid crystalline phases are traditionally characterized by their microscopic textures as observed through crossed polarizers.

Cholesteric liquid crystals, in which the long axes of the molecules lie in pseudoplanes that are slightly twisted with respect to each other, have periodic variations in refractive index and fringe patterns with spacings of P/2 where P is the cholesteric pitch. This type of behavior is typical of rod-like or semi-rigid polymers in solution, which spontaneously form ordered phases above some critical concentration to minimize the polymer excluded volume. These ordered phases can be formally defined as lyotropic liquid crystals.

Lyotropic liquid crystalline phases display different behavior depending on the concentration of molecules in solution. This is in contrast to the thermotropic liquid crystalline phase behavior commonly displayed by anisotropic small molecules in the molten state. The simplest liquid crystalline phase is termed nematic and possesses orientational order, with the long molecular axis preferentially aligned in one direction (the director). Chiral molecules (such as DNA) commonly form cholesteric phases, also known as twisted nematics, in which the rod-like molecules can be viewed as lying in pseudoplanes rotated at some constant angle with respect to one another.

Photomicrographs of cholesteric liquid crystalline DNA solutions are presented in this gallery. These images were obtained using crossed polarized illumination with specimens prepared on acid-cleaned microscope slides. In many instances, the samples were part of controlled drying experiments in which the DNA solution undergoes a series of transitions from dilute solution through multiple liquid crystalline phases.

This research was conducted in collaboration with Dr. Randolph L. Rill of the Department of Chemistry and Institute of Molecular Biophysics and Dr. David H. Van Winkle of the Center for Materials Research and Technology at the Florida State University.


Precholesteric to Cholesteric Phase Transition - Transition of the dilute precholesteric to cholesteric phase is occurring in this photomicrograph of liquid crystalline calf thymus DNA. In this specimen, domains with a light blue background 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.

Precholesteric Phase Helical Pitch - The large twist of the precholesteric liquid crystalline DNA phase is evident in the photomicrograph presented in this section, 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.

Spherulitic Domains in Cholesteric DNA Mesophases - Spherulitic cholesteric domains appear in this image as birefringent spheres with blue and yellow interference colors generated by a full-wave retardation plate.

Cholesteric Liquid Crystalline DNA Textures - Scattered oily streak cholesteric DNA textures have merged into a large domain in the right hand side of the photomicrograph presented in this secdtion. The periodicity of the cholesteric pitch remains constant at 2-3 microns in this mesophase.

The Precholesteric Mesophase - Weak birefringence of the precholesteric liquid crystalline DNA mesophase is illustrated in the photomicrograph presented in this section. The large pitch and relative disorganization of this low-density anisotropic phase is evident in the image.

Defects in the Cholesteric Texture - At intermediate concentrations, liquid crystalline DNA exists in a cholesteric lyotropic mesophase. This phase exhibits textures consistent with periodic variations in refractive index and fringe patterns. The photomicrograph illustrated in this section exhibits refractive index variations due to the cholesteric pitch and liquid crystalline defects at the edges of the mesophase.


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