DNA length and concentration dependencies of anisotropic phase transitions of DNA solutions

Critical concentrations for the isotropic to cholesteric phase transitions of double-stranded DNA fragments in simple buffered saline (0.1 M NaCl) solutions were determined as a function of DNA contour length ranging from similar to 50 nm to 2700 nm, by solid-state P-31 NMR spectroscopy and polarized light microscopy. As expected for semirigid chains, the critical concentrations decrease sharply with increasing DNA length near the persistence length in the range from 50 to 110 nm, and approach a plateau when the contour length exceeds 190 nm. The biphasic region is substantially wider than observed for xanthan, another semirigid polyelectrolyte approximately twice as stiff as DNA, primarily because of low critical concentrations for first appearance of the anisotropic phase, C-i* in DNA samples greater than or equal to 110 nm (320 base pairs) long. The limiting C-i* for DNA greater than or equal to 490 nm long is exceptionally low (only 13 mg/ml) and is substantially lower than the C-i* of similar to 40 mg/ml reported for the stiffer xanthan polyelectrolyte. The much higher values of the critical concentrations, C-a*, for the disappearance of the isotropic DNA phase (greater than or equal to 67 mg/ml) are modestly higher than those observed for xanthan and are predicted reasonably well by a theory that has been applied to other semirigid polymers, if a DNA persistence length in the consensus range of 50-100 nm is assumed. By contrast, the broad biphasic region and low C-i* values of DNA fragments greater than or equal to 190 nm long could only be reconciled with theory by assuming persistence lengths of 200-400 nm. The latter discrepancies are presumed to reflect some combination of deficiencies in current theory as applied to chiral, strong polyelectrolytes such as DNA, and sequence-dependent variations in DNA properties such as flexibility, curvature, or interaction potential. The propensity of DNA to spontaneously self-order at low concentrations well in the physiological range may have biological significance.