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Phase Transition from Cholesteric to High Density in Liquid Crystalline DNA Samples - As the concentration of liquid crystalline DNA solutions is increased, the local packing density also increases. Optical birefringence and microscopy experiments demonstrate that DNA solutions eventually form a high-density columnar hexatic phase. Photomicrographs in this section capture the phase transition between cholesteric and a higher density phase.

As the DNA concentration is raised above 300 milligrams per milliliter (mg/ml), specimens observed immediately after placement on a microscope slide usually exhibited a nearly uniform homeotropic texture. Batonnets initiated after a few hours, enlarged, and eventually merged to yield a classic focal conic fan texture. In controlled drying experiments, high density liquid crystalline DNA samples formed more complex phases at concentrations exceeding 500 mg/ml. Focal conic textures become highly striated and the solutions eventually reach a state close to that found in single crystals.

Creation of a higher density liquid crystal phase from a cholesteric phase requires unwinding of the twist axis. If there were no unwinding, columnar or smectic ordering would be accompanied by extensive formation of defects with a high cost in distortion free energy. For lyotropic liquid crystals, the concentration of rods is the appropriate thermodynamic variable to describe the critical properties of the transition. Untwisting of the cholesteric phase is an expected precursor for the formation of more ordered higher density phases to minimize defect formation (maximize the free volume) and may compete with the electrostatic twisting effect, thereby maintaining the cholesteric pitch constant over a relatively narrow concentration range.

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.


Batonnet Formation in Cholesteric Mesophases - Phase transitions are easily captured in the microscope using "controlled drying" experiments that rely on establishing an increasing concentration gradient of DNA (and salt) across a microscope slide. The photomicrograph in this section illustrates the cholesteric to high density liquid crystalline DNA phase transition that occurs between 300-450 milligrams per milliliter, depending upon the sodium counterion concentration.

Unwinding in Cholesteric Mesophases - Unwinding of the cholesteric mesophase occurs during controlled drying experiments with liquid crystalline DNA. The photomicrograph in this section illustrates progressive unwinding as the DNA concentration is increased.

Texture of the Cholesteric Unwinding Region - The transition region between cholesteric and high density phases in solutions of rod-like DNA molecules is the subject of debate. The structure of the mesophase in this region is largely unknown, but is believed to involve an unwinding of the cholesteric phase in a transition to a more highly ordered hexatic phase.

Batonnets in the Cholesteric Unwinding Region - Batonnet formation is often observed in the cholesteric unwinding region at the boundary between the cholesteric and high density phases, as evidenced by the image presented in this section. Phase transitions in the high density region are also apparent in this photomicrograph.

Cholesteric Mesophase Unwinding at the High Density Phase Transition - The cholesteric mesophase begins to unwind prior to the phase transition from this phase to a higher density hexatic phase. The unstructured birefringent cholesteric areas in the photomicrograph presented in this section are represented by several shades of blue color.

Birefringence in the Cholesteric Unwinding Region - Birefringent areas exhibit a wide spectrum of higher order colors in the unwinding region separating the true cholesteric from the high density phase. The image presented in this section illustrates the colors and textures observed in this region.

Organization of the Transition State - Structure of the transition region between cholesteric and high density liquid crystalline DNA phases has not yet been fully elucidated. The photomicrograph presented in this section illustrates this birefringent region, but yields few clues to the exact nature of the organization.

Organization of the Transition State - A second view of the structure of the transition region between cholesteric and high density liquid crystalline DNA phases has not yet been fully elucidated. The photomicrograph presented in this section illustrates this birefringent region, but like the link above, yields few clues to the exact nature of the organization.

Spontaneous Batonnet Formation - In controlled drying experiments with dilute isotropic DNA solutions, a number of liquid crystalline states are observed as the DNA and salt concentrations are increased by evaporation of the solvent (water). In areas where the DNA concentration range falls between 250 and 350 milligrams per milliliter, batonnets spontaneously emerge as the first step in the transition to a higher density liquid crystalline phase.

Cholesteric to High Density Transition Region - Another view of the cholesteric to high density transition region showing the birefringent nature of this seemingly disorganized mesophase. Higher order colors exhibited by the phase adjacent to the high density phase suggest the beginning of an ordered structure.

Birefringence of the Transition Region - Examination of the cholesteric to high density phase transition region with the aid of a full-wave retardation plate was conducted to observe the sign of birefringence for this mesophase.

Long Range Birefringence of the Transition Region - Examination of the long range cholesteric to high density phase transition region with the aid of a full-wave retardation plate was conducted to observe the sign of birefringence for this mesophase.

Birefringence of Unwinding Mesophases - Unwinding of the cholesteric liquid crystalline phase is apparent in the image presented in this section. Areas devoid of apparent structure have increasingly deeper blue birefringence traveling from left to right in the photomicrograph.

Sharp Boundaries in Transition Regions - Employing short DNA fragments of defined molecular length increases the speed and sharpens the boundaries in liquid crystalline phase transitions. The image presented in this section illustrates the transition from cholesteric to high density mesophases in DNA controlled drying experiments.


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