Liquid-crystalline phases in concentrated DNA solutions

Liquid-crystalline phase formation is concentrated DNA solutions with the ionic strengths of 0.01, 0.3 and 1 M Na{dollar}sp{lcub}+{rcub}{dollar} was investigated using phosphorus-31 NMR spectroscopy and optical microscopy. The phase diagrams for isotropic to liquid-crystalline transitions were determined for all three ionic strengths from {dollar}sp{lcub}31{rcub}{dollar}P NMR data and were found to be in good agreement with theoretical predictions of P. J. Flory (Proc. Roy. Soc. A 234, 73; 1956) and Stroobants et al. (Macromolecules 19, 2232; 1986). The critical DNA concentration required for the anisotropic phase formation was found to be weakly dependent on ionic strength, indicating that the effective DNA radius is not strongly dependent on ionic strength.; Two types of mesophases were formed in solutions of all ionic strengths investigated: a weakly birefringent, precholesteric phase and a cholesteric phase. In addition, concentrated solutions with DNA concentrations exceeding 250 mg/mL in 0.01 M and 0.3 M Na{dollar}sp{lcub}+{rcub}{dollar} buffer exhibited microscopic textures similar to the textures observed in smectic phases formed in solutions of small molecules, indicating a possible two-dimensional ordering of DNA helices.; The sodium-DNA interactions in solutions with DNA concentrations in the range of 10-300 mg/mL and ionic strengths of 0.01 and 1 M Na{dollar}sp{lcub}+{rcub}{dollar} were investigated using sodium-23 NMR. The longitudinal relaxation rate of bound ions in the range of DNA concentrations of 10-200 mg/mL was found to be {dollar}sim{dollar}200 Hz in 0.01 M Na{dollar}sp{lcub}+{rcub}{dollar} buffer and {dollar}sim{dollar}380 Hz in 1 M Na{dollar}sp{lcub}+{rcub}{dollar} buffer. The relaxation rate was larger in samples which exhibited cholesteric ordering of DNA molecules.; Quadrupole splitting was observed in samples in which the cholesteric phase first appeared: at 190 mg/mL in 0.01 M Na{dollar}sp{lcub}+{rcub}{dollar} and 250 mg/mL in 1 M Na{dollar}sp{lcub}+{rcub}{dollar} buffer. The magnitude of quadrupole splitting decreased with increasing DNA concentration in 0.01 M Na{dollar}sp{lcub}+{rcub}{dollar} buffer and remained relatively constant in 1 M Na{dollar}sp{lcub}+{rcub}{dollar} buffer. In addition, the quadrupole splitting changed sign when the temperature was increased from 20 to 60 {dollar}spcirc{dollar}C.