Ordering of lipid A-monophosphate clusters in aqueous solutions

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 ⩽ ϕ ⩽ 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 ) × 10 5 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 ⩽ ϕ ⩽ 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 ⩽ ϕ ⩽ 40.0 × 10 − 4 revealed a weighted-average molecular weight of ( 10.15 ± 0.17 ) × 10 6 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 ϕ cr = 5.05 × 10 − 4 . In the intermediate volume-fraction range, the scattered intensity I ( Q ) vs Q curves (light and x rays) showed asymptotic behavior. From the asymptotic curves, the scattered intensity, the relationship between the average mass and radius, and the fractal dimension d f were determined. The d f value, which was evaluated from the expression I ( Q ) ∝ R G d f , was found to be 1.67 ± 0.03 , a value that was virtually independent of the ionic strength ( 0.1 - 5.0 mM NaCl) at 25 ° C . Even at a very low ionic strength ( I = 0.10 mM NaCl), lipid A monophosphate formed a number of differently shaped clusters. Electron microscope images showed that two types of self-assembled clusters existed at the lowest volume-fraction range studied and also dominated the ima