Density Fluctuation in Coulombic Colloid Dispersion: Self-Assembly of Lipid A-Phosphates
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0947-A03-11
Density Fluctuation in Coulombic Colloid Dispersion: Self-Assembly of Lipid APhosphates Henrich H. Paradies and Chester A. Faunce Joule Physics Laboratory, Materials Research Institute, The University of Salford, The Crescent, Salford, M5 4WT, United Kingdom
ABSTRACT Dilute electrostatically–stabilized aqueous solutions of hexa-acylated (C14) lipid A-diphosphate from Escherichia coli were prepared with low polydispersities in shape, size and charge. A high degree of ordering was exhibited for volume fractions between φ ≅1.5 x 10-4 and 3.5 x 10-4. The structure factor S(Q) was strongly dependent on the particle number density, the nature of ions, e.g. Ca2+, Mg2+, K+, Na+ and H+, the effective colloidal charge (Z*), and the Debye screening length, k. The magnitude and position of the S(Q) peaks vary not only with counterions, e.g. Ca2+ or Mg2+, and concentration (nM to µM), but also with the order of their addition to the lipid Adispersions. Different types of colloidal-crystal structures were obtained for φ ≅ 3.5 x10-4. The Ca2+ and K+ salts exhibited FCC type-lattices with a = 56.3 nm and 55.9 nm, whereas the Na + and Mg2+ salts of lipid A-diphosphate formed BCC type-lattices with a = 41.5 nm and 45.5 nm, respectively. INTRODUCTION The arrangement of particles in colloidal dispersions is one of the fundamental physical properties of, and the key to, understanding virtually all physical properties of dispersions, including rheological phenomena. All properties depend on the way in which colloidal particles self assemble or are organized. Only recently has it been possible to prepare stable aqueous colloidal dispersions of lipid A-diphosphate from E. coli with low polydispersities in shape, size, and charge over the above-mentioned volume fraction range [1,2]. Consequently, it was not previously possible to examine the thermodynamics and structures of lipid A-diphosphate from Escherichia coli in solution. Neither was it possible to study the onset of the formation of colloidal crystalline arrays at low volume fractions (φ ≅ 10-4), low ionic strength (10-4 M) and in concentrations with ranges as low as 5-15 µg/mL, or 50-100 ng/mL in the case of Ca2+. This was mainly due to the severe size and mass heterogeneities of lipid A-diphosphate (polydispersity), solubility problems and the use of high ionic concentrations. The presence of organic solvents, e.g. methanol or n-hexane, as well as differences or heterogeneities in chemical composition, also presented problems. The colloidal particles of lipid A-diphosphate investigated in this study were prepared by self-assembly at very low ionic strength (I ≅ 10-4M) in aqueous solutions. Preparation methods used in the past were either emulsion polymerization or condensation, e.g. coated silica with derivatives of methacrylate or silica [3,4]. When these fabrication techniques were used, the lipid A-diphosphate and the antagonistic monophosphorylated derivatives of lipid A were not covalently linked
Lipopolysaccharides (LPS) or Gram-negative endotoxins from the su
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