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

Faunce, CA; Reichelt, H; Paradies, HH; Quitschau, P; Zimmermann, K

Abstract

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<=f<=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 5.0 nM, Mg2+ at 15.0 µM, and K+ at 100.0 µM (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±0.8 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 (~30%). It was also discovered that a given amount of Ca2+ (1.0-5.0 nM) or K+ (100 µM) influenced the structure much more than Na+ (0.1-10.0-mM NaCl) or Mg2+ (50 µM). 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 3.2×10-4<=f<=3.9×10-4. 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 (3.2×10-4<=f<=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.

Citation

Faunce, C., Reichelt, H., Paradies, H., Quitschau, P., & Zimmermann, K. (2005). 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. Journal of Chemical Physics, 122(21), 214727. https://doi.org/10.1063/1.1913477

• Journal Article Type: Article
• Publication Date: Jan 1, 2005
• Deposit Date: Sep 5, 2007
• Journal: Journal of Chemical Physics
• Print ISSN: 0021-9606
• Publisher: AIP Publishing
• Peer Reviewed: Peer Reviewed
• Volume: 122
• Issue: 21
• Pages: 214727
• DOI: https://doi.org/10.1063/1.1913477
• Keywords: Colloidal crystals, suspensions, lipid bilayers, cadmium, magnesium, sodium, potassium, positive ions, light scattering, X-ray scattering, transmission electron microscopy, scanning electron microscopy, particle size, integral equations, crystal structure
• Publisher URL: http://link.aip.org/link/?JCPSA6/122/214727/1