CHOLESTERIC HELICAL PITCH OF NEAR PERSISTENCE LENGTH DNA

Simple aqueous solutions of DNA, a semirigid, strong polyelectrolyte, at high concentration in a 1:1 supporting electrolyte medium, undergo a series of transitions between anisotropic phases including a cholesteric liquid-crystalline phase with a pitch of ≈2 μm.19 Measurements were made, by polarized light microscopy and laser light diffraction, of the cholesteric helical pitch in buffered aqueous solutions of DNA molecules with a contour length near the persistence length (500 Å) as a function of increasing DNA concentration, 1:1 supporting electrolyte concentration, and ionic species. The lowest density anisotropic DNA phase, termed precholesteric, exhibited weakly birefringent periodicities with a spacing strongly dependent on local DNA concentration, ranging from 10 to 50 μm. As the DNA concentration was increased, a highly regular cholesteric phase with a pitch of ≈2 μm came into coexistence with the precholesteric phase, and ultimately the solutions became fully cholesteric. No change in pitch of this cholesteric was noted with increasing DNA concentration or ionic strength over a DNA concentration range of ≈130-300 mg/mL and a supporting electrolyte concentration of 0.01-1.0 M NaCl. Transition to a high-density phase at DNA concentration > 300 mg/mL was preceded by unwinding of the pitch from 2.2 ± 0.2 to >10 μm. The constant ≈2-μm -pitch cholesteric phase was stable to higher DNA concentrations when sodium rather than ammonium was the principal counterion. These results are discussed in tropic liquid crystals of chiral rods. © 1990, American Chemical Society. All rights reserved.