The Formation of Colloidal Crystals of Lipid A Diphosphate:  Evidence for the Formation of Nanocrystals at Low Ionic Strength

Dilute electrostatically stabilized aqueous solutions of hexa-acylated (C14) lipid A diphosphate from Escherichia coli form stable and regularly shaped colloidal crystals in a size range of approximately 50−1000 nm in width and 50−100 nm in thickness. The formation of these nanocrystals occurs over a range of volume fractions between 3.5 × 10-3 and 1.2 × 10-2 and at a low ionic strength, ∼10-5. The shape of these crystals appears to be cubic or rhombohedral, and when exposed to the electron beam, these fragile nanocrystals are easily damaged. Electron diffraction patterns obtained from single particles reveal that they are orientated (001) crystals that conform to a trigonal or hexagonal unit cell (a = 3.65 ± 0.07 nm and c = 1.97 ± 0.04 nm), revealing crystal-like pore walls that exhibit structural periodicity with a spacing of 0.65 nm and are at least four times the size of the unit cell adopted by lipid A diphosphate.