Henrich H. Paradies

The liquidlike ordering of lipid A-diphosphate colloidal crystals: the influence of Ca2+, Mg2+, Na+, and K+ on the ordering of colloidal suspensions of lipid A-diphosphate in aqueous solutions

A comprehensive study was performed on electrostatically stabilized aqueous dispersion of lipid A-diphosphate in the presence of bound Ca2+, Mg2+, K+, and Na+ ions at low ionic strength (0.10-10.0-mM NaCl, 25 degrees C) over a range of volume fraction of 1.0 x 10(-4)< or =phi< or =4.95 x 10(-4). These suspensions were characterized by light scattering (LS), quasielastic light scattering, small-angle x-ray scattering, transmission electron microscopy, scanning electron microscopy, conductivity measurements, and acid-base titrations. LS and electron microscopy yielded similar values for particle sizes, particle size distributions, and polydispersity. The measured static structure factor, S(Q), of lipid A-diphosphate was seen to be heavily dependent on the nature and concentration of the counterions, e.g., Ca2+ at 5.0 nM, Mg2+ at 15.0 microM, and K+ at 100.0 microM (25 degrees C). The magnitude and position of the S(Q) peaks depend not only on the divalent ion concentration (Ca2+ and Mg2+) but also on the order of addition of the counterions to the lipid A-diphosphate suspension in the presence of 0.1-microM NaCl. Significant changes in the rms radii of gyration (R2G) 1/2 of the lipid A-diphosphate particles were observed in the presence of Ca2+ (24.8+/-0.8 nm), Mg2+ (28.5+/-0.7 nm), and K+ (25.2+/-0.6 nm), whereas the Na+ salt (29.1+/-0.8 nm) has a value similar to the one found for the de-ionized lipid A-diphosphate suspensions (29.2+/-0.8 nm). Effective particle charges were determined by fits of the integral equation calculations of the polydisperse static structure factor, S(Q), to the light-scattering data and they were found to be in the range of Z*=700-750 for the lipid A-diphosphate salts under investigation. The light-scattering data indicated that only a small fraction of the ionizable surface sites (phosphate) of the lipid A-diphosphate was partly dissociated (approximately 30%). It was also discovered that a given amount of Ca2+ (1.0-5.0 nM) or K+ (100 microM) influenced the structure much more than Na+ (0.1-10.0-mM NaCl) or Mg2+ (50 microM). By comparing the heights and positions of the structure factor peaks S(Q) for lipid A-diphosphate-Na+ and lipid A-diphosphate-Ca2+, it was concluded that the structure factor does not depend simply on ionic strength but more importantly on the internal structural arrangements of the lipid A-diphosphate assembly in the presence of the bound cations. The liquidlike interactions revealed a considerable degree of ordering in solution accounting for the primary S(Q) peak and also the secondary minimum at large particle separation. The ordering of lipid A-diphosphate-Ca2+ colloidal crystals in suspension showed six to seven discrete diffraction peaks and revealed a face-centered-cubic (fcc) lattice type (a=56.3 nm) at a volume fraction of 3.2 x 10(-4)< or =phi< or =3.9 x 10(-4). The K+ salt also exhibited a fcc lattice (a=55.92 nm) at the same volume fractions, but reveals a different peak intensity distribution, as seen for the lipid A-diphosphate-Ca2+ salt. However, the Mg2+ and the Na+ salts of lipid A-diphosphate showed body-centered-cubic (bcc) lattices with a=45.50 nm and a=41.50 nm, respectively (3.2 x 10(-4)< or =phi< or =3.9 x 10(-4)), displaying the same intensity distribution with the exception of the (220) diffraction peaks, which differ in intensity for both salts of lipid A-diphosphate.

Liquid-like ordered colloidal suspensions of lipid A: The influence of lipid A particle concentration

Electrostatically stabilized aqueous dispersions of nm-sized free lipid A particles at low volume fractions (1.0×10−4⩽∅⩽3.5×10−4) in the presence of 1.0–10.0 mM NaCl (25 °C) have been characterized by static and quasielastic light scattering (QELS) techniques, electron microscopy (SEM and TEM), conductivity measurements, and acid–base titrations. QELS and electron microscopy (ρTEM=8.0±0.6%) yield similar values for the particle size and particle size distribution (ρQELS=10.9±0.75 %), whereas conductivity and acid–base titrations estimate surface chemical parameters (dissociation constant, ionizable sites, and Stern capacitance). Effective particle charges were determined by fits of the integral equation calculations of the polydisperse static structure factor, S(Q), to the light scattering data. Using the particle properties as determined from these experiments, the polydisperse structure factor, S(Q), was calculated as a function of volume fraction, ∅, which was found to be consistent with a S(Q) de...

Observations of liquidlike order of charged rodlike lipid A diphosphate assemblies at pH 8.5

A new structural form of charged lipid A diphosphate, with a molecular weight of 5.9x10(6) 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/length(3). The position of the first maximum of the structure factor S(Q) was found to scale with C(13) below and with C(12) above, the critical concentration C(*) (2.5 microg mL). SAXS results in the semidilute concentration range C> or =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 Q(max) 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.

The Phase Diagram of Charged Colloidal Lipid A-Diphosphate Dispersions

Small-angle X-ray-scattering, light-scattering, and electron microscope experiments were used to determine the phase transitions of colloidal lipid A-diphosphate aqueous dispersions. The phases detected were a correlated liquid phase, a face-centered cubic (Fd3m) and a body-centered cubic (Im3m) colloidal crystal phase and a new glass phase. These experimentally determined phases were shown to be in accord with theoretically predicted equilibrium phases.

Studies on structures of lipid A-monophosphate clusters

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 3d, 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.

Phase transformations in lipid A-diphosphate initiated by sodium hydroxide.

The nature of the fluid phase transitions of charged-stabilized spherical lipid A-diphosphate clusters in aqueous dispersions was explored using a combination of small-angle X-ray scattering (SAXS) and electron microscopy. In contrast to previous studies, rather than removing NaCl, NaOH was added to the dispersions to promote crystallization. The fluid phase experiments were carried out employing titrations with mM·L(-1) NaOH (c(S)), along with variations in the particle-number density, n. When c(S) was increased, a new fluid disordered phase of self-assembled lipid A-diphosphate was encountered, followed by a crystalline bcc phase and then a new fluid phase containing 70 nm lipid A-diphosphate particles. The bcc crystal structure found in this regime had a lattice constant of 35.6 nm. By varying c(S) (mM L(-1)), it was possible to determine the effective charge, z(eff), for various n values and the screening parameter, k, for the excess electrolyte. For sufficiently large values of n, lipid A-diphosphate crystallized because of an increase in z(eff) at a constant c(S). When the c(S) was increased, the crystals melted with little change in z(eff). The existence of a bcc-fluid phase transition for different values of c(S) was supported by applying the Debye correlation function to the obtained data. An increase in c(S) enhanced interparticle interaction and attraction. The effective charge and k accounted for counterion condensation and many-body effects. If the effective charge determined from scattering measurements was used in the simulations, the equilibrium phase boundaries were consistent with predicted universal melting-line simulations. At any particle-number densities, n, when the melting line was reached, 70 nm clusters were formed.

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 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=7.5 nm. At low polydispersity, two distinct types of lipid A-monophosphate colloidal clusters were able to form at low polydispersity and were subsequently identified using light scattering, small-angle x-ray scattering, and selected-area electron diffraction. From an analysis of experimental results obtained from these clusters, distinct peaks could be assigned to a body-centered cubic (bcc) lattice, with a=49.5+/-1.8 nm. The solution structure found for lipid A diphosphate at volume fractions of 3.75x10(-4)<or=phi<or=4.15x10(-4) also exhibited a (bcc)-type lattice; however, a=36.1 nm [C. A. Faunceet al. J. Phys. Chem. 107, 2214 (2003)]. Using the particle and cluster properties determined from small-angle x-ray scattering, light scattering, and osmotic-pressure measurements as a function of volume fraction, good agreement was found between the directly measured osmotic-pressure values and those calculated from scattering experiments.

Nucleation of Calcium Carbonate as Polymorphic Crystals in the Presence of Lipid A-Diphosphate

The well-defined structure of lipid A-diphosphate in aqueous solutions provides a way of observing the formation of calcium carbonate crystals. The crystals are either tetrahedral or rhombohedral calcite at a volume fraction of phi = 5.4 x 10 (-4) at pH 5.8 or the vaterite polymorph of CaCO(3) at a volume fraction of phi = 7.8 x 10 (-4) at pH 5.8. In both cases, nucleation, adsorption pH, and the shape-dependent template of lipid A-diphosphate control the formation of the calcite and vaterite.

Influence of the anions on the N-cationic benzethonium salts in the solid state and solution: Chloride, bromide, hydroxide and citrate hydrates

The crystal structures of the hydrated cationic surfactant benzethonium (Bzth) chloride, bromide, hydroxide, and citrate have been determined by X-ray diffraction analysis and compared with their structures in solution well above their critical micelle concentration. The differences in the nature of the various anions of the four Bzth-X materials lead to unique anion environments and 3-D molecular arrangements. The water molecule in the monoclinic Bzth-Cl or Bzth-Br forms is hydrogen bonded to the halides and particularly to the hydrogens of the methoxy groups of the Bzth moiety notwithstanding the weak Bronsted acidity of the methoxy hydrogens. The citrate strongly interacts with the hydrogens of the methoxy group forming an embedded anionic spherical cluster of a radius of 2.6 A. The Bzth-OH crystallizes in a hexagonal lattice with two water molecules and reveals free water molecules forming hydrogen bonded channels through the Bzth-OH crystal along the c-axis. The distances between the cationic nitroge...

Structures of the 2-nitrophenol alkali complexes in solution and the solid state

The materials studied in this investigation were aqueous solutions (0.02-25.0 mM) of the salts of alkali metal ion (Me(+)) and 2-nitrophenol (2-NP). In the investigation, small-angle X-ray scattering, wide-angle X-ray scattering, and membrane-pressure osmometry were used to study the 2-NP-Me(+) molecular salt structures and the onset of crystallization as a function of concentration and temperature. The experimental methods used to examine the 2-NP-Me(+) molecular salt complexes provided corroborative evidence for the existence of spherical clusters with hydrodynamic diameters between ∼12 Å (Li) and 14 Å (Cs). Guinier plots of the zero-angle scattering peak were characteristic of the scattering from lamellae-like shapes with thicknesses of ∼290 Å. Tetramer and pentamer 2-NP-Me(+) molecular clusters for Me(+) = Li, Na, K, and Rb were assembled from four or five 2-NP molecules bound to a central alkali metal ion. The coordination symmetry around the six coordinated Li(+), Na(+), and K(+) ions was that of a trigonal prism (D3h), with an octahedral arrangement (D2h). The Rb(+) also revealed six-coordinate geometry and the central Rb(+) ion adopted an octahedral arrangement (D2h). The eight-coordinated Cs(+) ions with six 2-NP ligands were characteristic of a square antiprism (D4d). The square antiprism was the outcome of leaving two o-nitro groups and two phenolic oxygens being left intermolecularly uncoordinated to the Cs(+) ion. The 2-NP residues were strictly planar and contained short non-bonded intramolecular distances. van der Waals forces were present between the adjacently stacked phenyl rings. No water molecules were involved as ligands for any of the 2-nitrophenol-Me(+) complexes.