Studies on structures of lipid A-monophosphate clusters

Abstract

Single crystalline clusters of lipid A-monophosphate were grown from organic dispersions containing 5-15% (v/v) water at various volume fractions, φ, and temperatures. The morphology of the single lipid A-monophosphate crystals was either rhombohedral or hexagonal. The hexagonal crystals were needlelike or cylindrical in shape, with the long dimension parallel to the c axis of the unit cell. The crystalline clusters were studied using electron microscopy and x-ray powder diffraction. Employing molecular location methods following a Rietveld refinement and whole-pattern refinement revealed two monoclinic crystal structures in the space groups P2 1 and C2, both converged with R F = 0.179. The two monoclinic crystal structures were packing (hydrocarbon chains) and conformational (sugar) polymorphs. Neither of these two structures had been encountered previously. Only intramolecular hydrogen bonding was observed for the polymorphs, which were located between the amide and the carboxyl groups. Another crystalline structure was found in the volume-fraction range 2.00 × 10 −3 ≤ φ ≤ 2.50 × 10 −3 , which displayed hexagonal symmetry. The hexagonal symmetry of the self-assembled lipid A-monophosphate crystalline phase might be reconciled with the monoclinic symmetry found at low-volume-fractions. Therefore, lowering the symmetry from cubic, i.e., Ia 3 d, to rhombohedral R 3 m, and finally to the monoclinic space group C2 was acceptable if the lipid A-monophosphate anion was completely orientationally ordered.

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

“…As a result the density of the Lipid A-diphosphate nuclei is increasing with  (n), which is contrary of the classical nucleation theory (CNT) [60]. According to this theory the density of the crystal is the same as the bulk density, or the density decreases with increasing of the Young-Laplace pressure, c = /Lc = g   , where Lc is the capillary length and depends on the curvatures inside and outside of the nuclei, due to capillary forces [74,75], which takes also the surface tension and the chemical potential () into account. Thus the assumption of CNT is independent on  is no longer valid.…”

Two Dimensional Crystallization of Three Solid Lipid A-Diphosphate Phases

“…As a result of the dimensional restrictions placed on the lipid A-diphosphate molecules, they were compressed into an ordered and condensed phase. The crystalline surface area (S cry ) incorporated the polar disaccharide-phosphate moiety and the hydrogen bonding scheme, 42 which included the N-and O-acyl esterified residues. The result was an ordered and repeated arrangement of the headgroup thickness of 4.3 Å.…”

“…The lipid A-diphosphate crystals were grown from electrostatically stabilized solutions of 1.0 mM NaCl using a lipid A-diphosphate concentration range of 10 ng to 40 ng at the desired temperature, T , well below the CMC = 63 ng/L (3.50 × 10 –8 M) at 7.5 °C and 61 ng/L (3.38 × 10 –8 M) at 20 °C. The crystallization procedure was as follows: 50 μL of the lipid A-diphosphate dispersion at the aforementioned concentrations was placed in the inside chamber of a 5-mm-diameter modified Conway dish . The outer chamber of the dish (diameter 15 mm) contained a solution comprising 250 μL of the 1 mM NaCl solution at the required temperature T .…”

“…The crystallization procedure was as follows: 50 μL of the lipid A-diphosphate dispersion at the aforementioned concentrations was placed in the inside chamber of a 5-mm-diameter modified Conway dish. 42 The outer chamber of the dish (diameter 15 mm) contained a solution comprising 250 μL of the 1 mM NaCl solution at the required temperature T. The growth of the lipid A-diphosphate crystals was carried out at constant c, with both controlled T and vapor pressure. The samples were held in their chambers for at least a week before the onset of crystallization, at temperatures of between 5 and 30 °C.…”

Protonation-Driven Aqueous Lyotropic Self-Assembly of Synthetic Six-Tail Lipidoids

“…One key function of lipid A is to anchor lipopolysaccharides (LPS), also known as endotoxins, to the exterior surface of the outermost bilayer in Gram-negative bacteria. Although structural variations exist across species, lipid A typically bears at least six aliphatic tails and one or more phosphate-containing headgroups. , Studies of bacterial lipid A isolated from different species indicate that its homologues form various LLC phases, which depend on both their chemical structures and the surrounding aqueous environment. − ,− The number and length of the lipid tails, in conjunction with the number and protonation state of the anionic phosphate headgroups, dictate the formation of I II , inverse hexagonal (H II ), and L α LLCs as functions of pH, temperature, and aqueous ionic strength. ,,,,, The ability of lipid A to switch between morphologies with different transport properties in response to external stimuli could be a useful property for functional materials (e.g., from the connected water channels of a H II phase to isolated water pools of I II as a molecular “valve”). However, its inherent toxicity, hydrolytic instability, species-to-species variability, and low yields of isolated lipid necessitate the design of new synthetic analogues.…”