Browsing by Author "Lawrence, MJ"

Now showing 1 - 2 of 2
  • No Thumbnail Available
    Item
    Calcium mediated interaction of calf-thymus DNA with monolayers of distearoylphosphatidylcholine: a neutron and X-ray reflectivity study
    (Royal Society of Chemistry, 2013-01-01) Dabkowska, AP; Talbot, JP; Cavalcanti, L; Webster, JRP; Nelson, A; Barlow, DJ; Fragneto, G; Lawrence, MJ
    X-ray and neutron reflection studies, the latter in conjunction with contrast variation, have been combined to study the interaction of calf thymus DNA (ctDNA) with monolayers of distearoylphosphatidylcholine (DSPC) in the presence of 20 mM Ca2+ ions, at the air-liquid interface as a function of surface pressure (10, 20, 30 and 40 mN m-1). Analysis of the X-ray and neutron reflection data showed that, regardless of the surface pressure of the monolayer, a layer of ctDNA was present below the DSPC lipid head groups and that this ctDNA-containing layer (thickness [similar]12.5 to 15 A) was separated from the DSPC head groups by a layer of water of [similar]9 A thickness. The thickness of the ctDNA-containing layer was thinner than that reported for monolayers of cationic lipid at the air-water interface (18-25 A) although in these monolayers no water layer separating the lipid head groups from the layer containing ctDNA has been reported. At all surface pressures the amount of ctDNA present in the layer was in the range 30-40% by volume. As no significant re-arrangement of the DSPC film was required to accommodate the presence of the ctDNA, this suggests that the distribution of charges in the lipid film matches well the charge spacing of ctDNA. Brewster angle microscopy measurements of DSPC on water in the absence of Ca2+ showed the presence of a continuous film containing small, regular shaped domains at all four surface pressures examined. When Ca2+ ions were present in the sub-phase, although the film was still continuous, the domains comprising the film were more irregular in appearance while the presence of Ca2+ ions and ctDNA resulted in the domains becoming smaller and more regularly packed on the surface. © 2013, Royal Society of Chemistry.
  • No Thumbnail Available
    Item
    Nonequilibrium effects in self-assembled mesophase materials: unexpected supercooling effects for cubosomes and hexosomes
    (American Chemical Society, 2010-06-01) Dong, YD; Tilley, AJ; Larson, I; Lawrence, MJ; Amenitsch, H; Rappolt, M; Hanley, TL; Boyd, BJ
    Polar lipids often exhibit equilibrium liquid crystalline structures in excess water, such as the bicontinuous cubic phases (QII) at low temperatures and inverse hexagonal phase (HII) at higher temperatures. In this study, the equilibrium and nonequilibrium phase behavior of glyceryl monooleate (GMO) and phytantriol (PHYT) systems in excess water were investigated using both continuous heating and cooling cycles, and rapid temperature changes. Evolution of the phase structure was followed using small-angle X-ray scattering (SAXS). During cooling, not only was supercooling of the liquid crystalline systems by up to 25 °C observed, but evidence for nonequilibrium phase structures (not present on heating; such as the gyroid cubic phase only present at low water content in equilibrium) was also apparent. The nonequilibrium phases were surprisingly stable, with return to equilibrium structure for dispersed submicrometer sized particle systems taking more than 13 h in some cases. Inhibition of phase nucleation was the key to greater supercooling effects observed for the dispersed particles compared to the bulk systems. These findings highlight the need for continued study into the nonequilibrium phase structures for these types of systems, as this may influence performance in applications such as drug delivery. © 2010, American Chemical Society