Re-entrant cholesteric phase in DNA liquid-crystalline dispersion particles

Area of Science:

• Biophysics
• Materials Science
• Polymer Science

Background:

• Double-stranded DNA (ds DNA) can form liquid-crystalline phases.
• Poly(ethylene glycol) (PEG) is used to induce phase separation and DNA packing.
• Understanding DNA phase behavior is crucial for nanotechnology and biomaterials.

Purpose of the Study:

• To investigate the transition between hexagonal and cholesteric packing of ds DNA in dispersion particles.
• To theoretically rationalize the observed packing transitions.
• To explore the influence of poly(ethylene glycol) concentration and temperature on DNA phase behavior.

Main Methods:

• Phase exclusion of ds DNA from water-salt solutions with varying PEG concentrations (120–300 mg/ml).
• Characterization using circular dichroism spectra, X-ray scattering curves, and liquid-crystalline phase textures.
• Theoretical modeling based on competition between osmotic pressure and Frank orientation energy.

Main Results:

• At room temperature, PEG concentrations of 120–220 mg/ml yielded cholesteric ds DNA packing.
• Higher PEG concentrations (>220 mg/ml) resulted in hexagonal ds DNA packing.
• Heating hexagonal packing dispersions (>220 mg/ml PEG) induced a re-entrant cholesteric phase.

Conclusions:

• A novel re-entrant cholesteric phase of ds DNA was observed upon heating hexagonal packing.
• The transition is explained by the partial melting of quasinematic layers, driven by osmotic pressure and twist energy.
• This study provides insights into DNA self-assembly and phase transitions in complex fluids.