ISOTROPIC TO ANISOTROPIC PHASE-TRANSITION OF EXTREMELY LONG DNA IN ANAQUEOUS 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 pers istence lengths) from the isotropic to anisotropic phase was character ized by solid-state P-31 NMR spectroscopy and polarized light microsco py. The anisotropic phase first appeared in equilibrium with the isotr opic phase at a concentration Ci = 13 mg/mL and the isotropic phase d isappeared at a concentration C(a) = 67 mg/mL. The first critical con centration (C(i)) is much lower than that observed for a stiffer poly electrolyte (xanthan) and predicted by theory incorporating the influe nces of chain flexibility and charge. By contrast, C(a) is in reasona ble accord with expectations based on the static DNA persistance lengt h (almost-equal-to 50 nm) in dilute solution. Since the dominant lengt h scale for phase transitions of semiflexible polymers is the persiste nce length and not the contour length, this unexpected broadening of t he biphasic region may reflect local, sequence-dependent variations in DNA flexibility.