Search

A new structural form of charged lipid A diphosphate, with a molecular weight of 5.9×106 Da and a rodlike shape (L=800 nm), was found in aqueous solutions at pH 8.5. The experimental techniques used in the investigation were light scattering, small-angle x-ray scattering (SAXS), and electron microscopy. Measurements of the static-structure factor S(Q) as a function of the ionic strength are presented over the concentration regimes C≥C* and C≤C*, with C*=1 particle/length3. The position of the first maximum of the structure factor S(Q) was found to scale with C1/3 below and with C1/2 above, the critical concentration C* (2.5 μg/mL). SAXS results in the semidilute concentration range C>=C* show that strong interparticle correlations exist even at concentrations as high as 15C*, in contrast with results for hard-rod systems. The magnitude of the correlations depends on both the lipid A diphosphate concentration at pH 8.5 and the Debye screening length k-1. For a constant lipid A diphosphate concentration at pH 8.5, as the amount of salt was increased a decrease in structure was observed. There was also a shift in the peak of the first maximum position Qmax to larger scattering wave vectors. The observed phase behavior (C=15C*) exhibited an isotropic I-Sm transition and an I-N-Sm transition, which were recorded on electron microscope images. [ABSTRACT FROM AUTHOR]

In this investigation, a study of the self-assembly of electrostatically stabilized aqueous dispersions of nanometric lipid A-monophosphate clusters from Escherichia coli was carried out in three different volume-fraction regimes. The experimental techniques used in the investigation were osmotic pressure, static and quasielastic light scattering, scanning electron microscopy and transmission electron microscopy, and small-angle x-ray scattering. Experiments were carried out at low ionic strength (I=0.1–5.0 mM NaCl) at 25 °C. At volume fractions between 1.5×10-4<=[lowercase_phi_synonym]<=5.4×10-4, the lipid A-monophosphate clusters had an average rms hydrodynamic diameter of d=7.5 nm, and a weighted-average molecular weight of (1.78±0.23)×105 g mol-1. Quasielastic light scattering (LS) experiments yield similar values for the particle size and particle size distribution compared to electron microscopy, small-angle x-ray scattering, and LS experiments. When the volume fraction was increased to a higher regime 5.4×10-4≤=[lowercase_phi_synonym]≤=9.50×10-4, much larger clusters of lipid A monophosphate formed. The clusters detected in this volume-fraction range were assembled from between 8 and 52 of the d=7.5 nm clusters and the assemblies are densely packed in such a way that colloidal crystals composed of the monodisperse microspheres are in physical contact with their nearest neighbors. Clusters that formed in volume fractions between 10.0×10-4≤=[lowercase_phi_synonym]<=40.0×10-4 revealed a weighted-average molecular weight of (10.15±0.17)×106 g mol-1 and a hydrodynamic diameter of ∼d=70.6 nm. The crossover volume fraction between the small and the large clusters appeared at [lowercase_phi&#95... [ABSTRACT FROM AUTHOR]