ISOTROPIC TO ANISOTROPIC PHASE-TRANSITION OF EXTREMELY LONG DNA IN AN AQUEOUS SALINE SOLUTION

The transition of simple aqueous saline solutions (buffered 0.1 M NaCl) of extremely long DNA (8 kilobases - 2.7 mum almost-equal-to 54 persistence lengths) from the isotropic to anisotropic phase was characterized by solid-state P-31 NMR spectroscopy and polarized light microscopy. The anisotropic phase first appeared in equilibrium with the isotropic phase at a concentration Ci* = 13 mg/mL and the isotropic phase disappeared at a concentration C(a)* = 67 mg/mL. The first critical concentration (C(i)*) is much lower than that observed for a stiffer polyelectrolyte (xanthan) and predicted by theory incorporating the influences of chain flexibility and charge. By contrast, C(a)* is in reasonable accord with expectations based on the static DNA persistance length (almost-equal-to 50 nm) in dilute solution. Since the dominant length scale for phase transitions of semiflexible polymers is the persistence length and not the contour length, this unexpected broadening of the biphasic region may reflect local, sequence-dependent variations in DNA flexibility.