Liquid crystalline phases in concentrated aqueous solutions of Na+ DNA.

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RESUMO

Concentrated aqueous saline solutions of short (146-base-pair) DNA fragments suddenly become turbid and iridescent when the DNA concentration is slightly increased or the temperature is decreased. Microscopic examination through crossed polarizing filters shows that turbidity and iridescence is due to formation of a liquid crystalline DNA phase similar to cholesteric liquid crystals formed by other semirigid, but nonelectrolyte, chiral polymers. Several distinct textures of the liquid crystalline phase or phases are observed depending on DNA concentration, temperature, and method of sample preparation. Textures observed include spherulites with Maltese crosses, striated and highly colored ribbons, whorls of periodic interference fringes, and colored flakes. The liquid crystalline DNA phase coexists in metastable equilibrium with the isotropic phase over a relatively narrow temperature/concentration range--approximately 175-250 mg/ml and 25-62 degrees C (limit of measurements). At higher concentrations and temperatures above approximately equal to 25 degrees C, the solutions appear fully liquid crystalline. When concentrated solutions are cooled below room temperature, crystals form due to precipitation of supporting electrolyte. A partial phase diagram is reported for the isotropic----liquid crystal----crystal transitions of solutions of DNA in buffered saline (2 M Na+). The general features of this phase diagram and the critical DNA volume fraction for formation of the anisotropic phase are consistent with the observed and theoretically predicted phase behavior of rodlike or semirigid nonelectrolyte polymers.

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