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
Authors: Faunce C. A. , Reichelt H. , Paradies H. H. , Quitschau P. , Zimmermann K. 
Published: June 2005

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°C) over a range of volume fraction of 1.0×10−4⩽ϕ⩽4.95×10−4. 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 5.0nM, Mg2+ at 15.0μM, and K+ at 100.0μM (25°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-μM NaCl. Significant changes in the rms radii of gyration (RG2¯)1∕2 of the lipid A-diphosphate particles were observed in the presence of Ca2+ (24.8±0.8nm), Mg2+ (28.5±0.7nm), and K+ (25.2±0.6nm), whereas the Na+ salt (29.1±0.8nm) has a value similar to the one found for the de-ionized lipid A-diphosphate suspensions (29.2±0.8nm). 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 (∼30%). It was also discovered that a given amount of Ca2+ (1.0–5.0nM) or K+ (100μM) influenced the structure much more than Na+ (0.1–10.0-mM NaCl) or Mg2+ (50μM). 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=56.3nm) at a volume fraction of 3.2×10−4⩽ϕ⩽3.9×10−4. The K+ salt also exhibited a fcc lattice (a=55.92nm) 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=45.50nm and a=41.50nm, respectively (3.2×10−4⩽ϕ⩽3.9×10−4), displaying the same intensity distribution with the exception of the (220) diffraction peaks, which differ in intensity for both salts of lipid A-diphosphate.