BOND-ORIENTATIONAL ORDER, MOLECULAR-MOTION, AND FREE-ENERGY OF HIGH-DENSITY DNA MESOPHASES

By equilibrating condensed DNA arrays against reservoirs of known osmo tic stress and examining them with several structural probes, it has b een possible to achieve a detailed thermodynamic and structural charac terization of the change between two distinct regions on the liquid-cr ystalline phase diagram: (i) a higher density hexagonally packed regio n with long-range bond orientational order in the plane perpendicular to the average molecular direction and (ii) a lower density cholesteri c region with fluid-like positional order. X-ray scattering on highly ordered DNA arrays at high density and with the helical axis oriented parallel to the incoming beam showed a sixfold azimuthal modulation of the first-order diffraction peak that reflects the macroscopic bond-o rientational order. Transition to the less-dense cholesteric phase thr ough osmotically controlled swelling shows the loss of this bond orien tational order, which had been expected from the change in optical bir efringence patterns and which is consistent with a rapid onset of mole cular positional disorder. This change in order was previously inferre d from intermolecular force measurements and is now confirmed by P-31 NMR. Controlled reversible swelling and compaction under osmotic stres s, spanning a range of densities between approximate to 120 mg/ml to a pproximate to 600 mg/ml, allow measurement of the free-energy changes throughout each phase and at the phase transition, essential informati on for theories of liquid-crystalline states.