In the laboratory, scientists usually investigate the physical and chemical properties of DNA in very dilute buffered aqueous solutions. However, in vivo, DNA exists in domains where the localized concentrations are very high, often exceeding 400-600 milligrams per milliliter (mg/ml) or 70 percent weight/volume. As the aqueous concentration of DNA is slowly increased, through evaporation of the aqueous solvent or by dialysis, the macromolecular solution undergoes spontaneous phase transitions to form at least three distinct liquid crystalline phases which are termed lyotropic phase transitions. This is due to the natural tendency of semi-rigid polymers to form liquid crystalline phases in concentrated solutions.
The three distinct liquid crystalline phases formed by DNA in aqueous solution occur at concentrations that are comparable to those in vivo, with phase transitions occurring over relatively narrow ranges of DNA concentration. At the lowest DNA concentrations necessary for phase separation (100-200 mg/ml), a weakly birefringent, dynamic, "precholesteric" mesophase occurs with microscopic textures intermediate between those of a nematic liquid crystal and a true cholesteric phase first. At slightly higher DNA concentrations (200-300 mg/ml), a second mesophase forms that is a strongly birefringent, well-ordered cholesteric liquid crystalline phase with a concentration-dependent pitch varying from 2 to 10 micrometers. At DNA concentrations higher than 350 mg/ml, a two-dimensionally ordered columnar hexatic phase is formed, which exhibits a characteristic focal conic texture similar to those seen in both lyotropic and thermotropic liquid crystalline phases with small molecules.
Monodisperse DNA fragments, obtained from calf thymus nucleosomes, with a contour length (500 Angstroms) near the DNA persistence length were used in these studies. In simple aqueous solutions of a 1:1 electrolyte, nucleosome core-length DNA undergoes a complex series of phase changes and forms at least three distinct liquid-crystalline phases, as described above. These phase transitions are sharp and distinct, allowing them to be easily captured using an optical microscope with polarized illumination. Photomicrographs of various liquid crystalline DNA textures and phase transitions are presented in the Molecular Expressions DNA Collection gallery.
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.