Bond orientational order, molecular motion, and free energy of high-density DNA mesophases.

Abstract

By equilibrating condensed DNA arrays against reservoirs of known osmotic stress and examining them with several structural probes, it has been possible to achieve a detailed thermodynamic and structural characterization of the change between two distinct regions on the liquid-crystalline phase diagram: (i) a higher density hexagonally packed region with long-range bond orientational order in the plane perpendicular to the average molecular direction and (ii) a lower density cholesteric 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-orientational order. Transition to the lessdense cholesteric phase through osmotically controlled swelling shows the loss of this bond orientational order, which had been expected from the change in optical birefringence patterns and which is consistent with a rapid onset of molecular positional disorder. This (10,11). The range of osmotic pressures accessible through this method is substantially larger (especially at high stress) than by the equilibrium sedimentation approach (7).Because DNA is equilibrated against a vast excess of a polymer and water solution of known chemical potential, it is always in a single phase at thermodynamic equilibrium. This behavior should be contrasted with multiple-phase equilibria that usually emerge from stoichiometric mixtures.In this work, we combine both the structural and thermodynamic approaches to the condensed DNA phases so that structural and dynamical parameters of DNA packing and ordering (interhelical separation, bond orientational order parameter, 31P-NMR spectra) are all measured concurrently with the free energy and/or its derivatives. We report here the structural and dynamic changes that occur in the DNA concentration region from 120 to 600 mg/ml corresponding to interaxial separations of 25-55 A. We show that at lower densities (or higher spacings) DNA packing is characterized by short-range positional order, measured by x-ray diffraction, long-range cholesteric order, revealed by optical birefringence, and high mobility of the DNA backbone, inferred from 31P-NMR spectroscopy. At high densities (or small spacings) DNA packing is characterized by short-range positional order and long-range bond orientational order in the plane perpendicular to the average nematic director, revealed by the tOn leave from: J. Stefan Institute, Ljubljana, Slovenia. IITo whom reprint requests should be addressed.

Gravitational Symmetry Breaking Leads to a Polar Liquid Crystal Phase of Microtubules In Vitro

“…For DNA, if q eff goes below 10% of the unscreened value, then electrostatic repulsion between two chains switches to attraction [25]. Due to correlated fluctuations of counterions, the forces between MT cylinders should become attractive at high screening and are additive with respect to cylindercylinder attraction [26].…”

Assessing the Interaction of Urea and Protein-Stabilizing Osmolytes with the Nonpolar Surface of Hydroxypropylcellulose

“…For some time now we have been measuring intermolecular forces through the dependence of the distance between macromolecules in an ordered array measured by x-ray scattering on the osmotic pressure of a polymer that is excluded from the macromolecular phase and applies a force on it (13)(14)(15)(16)(17)(18)(19)(20)(21). The effect of solute exclusion on forces can be used to infer changes in solute concentration in the space between macromolecules as the distance between them changes.…”

Collective stringlike motion of semiflexible filamentous particles in columnar liquid crystalline phases

“…X-ray * m.s.naderi@tue.nl scattering patterns of monodomains turn out to show a sixfold symmetry with peaks that have substantial tails, indicative of a hexatic rather than hexagonal order. The hexatic structure is characterized by quasi-long-range sixfold orientational and a short-range translational order resulting from the presence of topological defects, presumably caused by the chiral nature of the particles [19,20].…”