Liquid crystal formation in supercoiled DNA solutions.

Area of Science:

• Biophysics
• Materials Science
• Physical Chemistry

Background:

• Understanding the phase behavior of DNA in solution is crucial for various applications.
• Plasmid DNA, like pUC18, exhibits complex phase transitions under specific conditions.
• Liquid crystal formation in biological macromolecules is an active area of research.

Purpose of the Study:

• To determine the critical concentrations for liquid crystal formation of pUC18 plasmid in saline solutions.
• To investigate the influence of temperature and ionic strength on the phase transition.
• To characterize the liquid crystalline phase and compare experimental findings with theoretical models.

Main Methods:

• (31)P nuclear magnetic resonance spectroscopy
• Polarized light microscopy (including magnetically oriented samples)
• Phase equilibrium experiments

Main Results:

• A strongly first-order transition was observed between isotropic and anisotropic phases with a broad gap.
• Critical boundaries showed strong, reversible temperature dependence and weak ionic strength dependence.
• The liquid crystalline phase was identified as cholesteric with a pitch of approximately 4 micrometers.
• Preliminary data indicated true crystal formation at higher concentrations.

Conclusions:

• The isotropic-cholesteric transition is influenced by charge, orientation entropy, and excluded volume effects.
• Molecular free energy related to superhelix topology significantly controls the phase gap width.
• Theoretical models accurately predict the disappearance of the isotropic phase but not the onset of the anisotropic phase.