Ordering of lipid A-monophosphate clusters in aqueous solutions

Abstract

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 degrees C. At volume fractions between 1.5x10(-4)<or=phi<or=5.4x10(-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)x10(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 5.4x10(-4)<or=phi<or=9.50x10(-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 10.0x10(-4)<or=phi<or=40.0x10(-4) revealed a weighted-average molecular weight of (10.15+/-0.17)x10(6) g mol(-1) and a hydrodynamic diameter of approximately d=70.6 nm. The crossover volume fraction between the small and the large clusters appeared at phicr=5.05x10(-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 df were determined. The df value, which was evaluated from the expression I(Q) proportional, RGdf, 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 degrees 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 images taken at the higher volume-fraction regimes. One type of cluster showed a cubic morphology and a size variation of 50-100 nm, while another type took on the appearance of a quadratic cylinder, with dimensions of 50x150 nm2. The other clusters appeared in various shapes: dimers, trimers, and distorted tetramers, which were quite different from the ones previously observed for lipid A diphosphate. Small-angle x-ray diffraction experiments on lipid A-monophosphate clusters suspended in water, containing 5 mM NaCl (25 degrees C), indicated the existence of long-range order of d=...

Mineralization of Lipid A-Phosphates in Three- and Two-Dimensional Colloidal Dispersions

“…At lower particle-number densities, as the length scale of the repulsive forces increased, the fluid-crystalline transition gave rise to BCC-type crystals. This implies that self-screening was much smaller than in previous studies and very different for Lipid A-monophosphate phases [38]. The experimental observations and the simulation support the existence of a transition from a fluid to a BCC structure rather than to an expected FCC structure for Lipid A-diphosphate clusters in e.g 5.0 mM NaCl.…”

“…8). The small cubic Pm 3 n (a = 6.35 nm) structure observed for the Lipid A-monophosphate clusters [38,48], but different form the large cubic unit cell with a = 49.2 nm materialized as a result of a space-filling combination of two polyhedra, a dodecahedra and a tetrakaidodecahedra. This is in contrast to the tetrakaidecahedra (Im 3 m) or rhombodo-decadecahedra (Fd3m) packings observed for the Lipid A-diphosphate assemblies.…”

“…The equilibrium separation distance between the interfaces of the "spheres" suggested that they repel each other as a result of electrostatic, steric, van der Waals forces as well as water layer surrounding the spheres. The small cubic Pm 3 n (a = 6.35 nm) structure observed for the Lipid A-monophosphate clusters [38,48], but different from the large cubic unit cell with a = 49.2 nm materialized as a result of a space-filling combination of two polyhedra, a dodecahedra and a tetrakaidodecahedra. This is in contrast to the tetrakaidecahedra (Im 3 m) or rhombodo-decadecahedra (Fd3m) packings observed for the Lipid A-diphosphate assemblies (Fig.…”

Two new colloidal crystal phases of lipid A-monophosphate: Order-to-order transition in colloidal crystals

“…Thus the interactions between colloidal particles like lipid A-monophosphate can be tuned by modifying either the particle surfaces or the properties of the ma- trix in which they are suspended. From recently obtained results, 35 it was found that the formation of nanocrystals of different shapes and sizes was a function of the volume fraction. The ordering of charged lipid A-monophosphate clusters in aqueous dispersions at low ionic strength was also indicated.…”

“…However, if the volume fraction was increased the dispersions became iridescent. 35 In fact, the iridescence resulted from the formation of lipid A-monophosphate colloidal crystals in the size range from ϳ10 nm to 1 m. Understanding the growth phenomenon is of major interest, not only because of a gain in insight into cluster-cluster interactions, 36 but also from a structural point of view. Thus, with an understanding of these features, there is a possibility of growing suitable single crystals, which may be used in detailed structural analyses.…”