Two DimensionalCrystallization of Three Solid LipidA-Diphosphate Phases.

Surface-tension-induced liquid-crystalgrowth of monomeric lipidA-diphosphate in aqueous dispersions is reported as a function ofconcentration, (c), and temperature, (T), and at low ionic strength (10–3M). As the temperaturewas varied, a solid–liquid transition was revealed in the surfacelayer at a fixed lipid A-diphosphate bulk concentration. Here, thedevelopment of different two-dimensional (2-d) facetedcrystal morphologies was observed and, as growth proceeded, thesefaceted 2-dcrystals became unstable. Selected areaelectron microscopy diffraction (SAED) and X-ray diffraction (XRD)measurements of the faceted 2-dcrystalline lipidA-diphosphate layers exhibited a pseudohexagonal molecular arrangement.The crystalline layer was a smectic F, SF, phase below the critical temperature, TC, and a smectic I, SI, phase above TC(15 °C). Both phases could be describedin terms of the same C-centered monoclinic unit cell. The in-planeorder extended for a limited distance although the layers were coupled.The analysis of the SAED patterns revealed short-range order in the SFphase (5–15 °C), but long-rangeorder in the SIphase, for the temperaturerange 15–30 °C. The observed 2-dsolidhexatic phase and the 2-dliquid hexatic phase hadcorrelation lengths of 220 Å. This, the hexatic phase, displayedshort-range in-plane positional order and quasi long-range, sixfoldbond-orientational order. The SIphaseshowed long-range order characteristics of a hexatic 2-dcrystal. The two-, four-, or six-layer crystalline lipid A-diphosphatefilms exhibited 2-dhexatic order and 6n-fold bond-orientational order. These films did not evolve into the SFphase, demonstrating that the two phases werethermodynamically distinct. A finite tilt angle of φ = 15°was calculated for the lipid A-diphosphate molecule; the tilt wastoward the small side of the rectangular 2-dlattice.The constraint of six close-packed acyl chains in two distinct phaseswith the same symmetry suggests that the SF→ SItransition was first-order. [ABSTRACT FROM AUTHOR]