Two new colloidal crystal phases of lipid A-monophosphate:Order-to-order transition in colloidal crystals

A study of the structure of stable regular-shaped nanocrystals of hexa-acylated ( C 14 ) lipid A-monophosphate from Escherichia coli was carried out using dilute electrostatically stabilized aqueous dispersions at low ionic strength ( I = 1.0 × 10 − 5 M NaCl). An order-to-order transition of colloidal clusters of lipid A-monophosphate was found at two volume fractions: ϕ = 5.9 × 10 − 4 and ϕ = 11.5 × 10 − 4 . The clusters belonged to the cubic space groups P m 3 ¯ n and I a 3 ¯ d with unit-cell dimensions of a = 4.55 nm and a = 6.35 nm , respectively, as revealed by small-angle x-ray diffraction and electron-diffraction results of thin nanocrystals of lipid A-monophosphate. When viewed in the scanning electron microscope these fragile clusters displayed a number of shapes: cubic, cylindrical, and sometimes-rounded hexagons, which were extremely sensitive when exposed to an electron beam. The smallest and most numerous of the clusters appeared as ∼ 7 nm cubes. Crystalline cluster formation occurred over a wide volume-fraction range, between 1.5 × 10 − 4 and 40.0 × 10 − 4 , and at temperatures of 20 and 35 ° C . The crystalline networks of the lipid A-monophosphate clusters may be represented by space-filling models of two pentagonal dodecahedra with six tetrakaidecahedra arrangements of lipid A-"micelles" in the cubic space group P m 3 ¯ n . The simulated electron density profiles are in accord with spherical clusters of lipid A-monophosphate at the corners and at the body centers of the cubic P m 3 ¯ n unit cell. The profiles are rounded tetrahedrally at distances of 1/4 and 3/4 along one of the bisectors of each face of the cubic unit cell. These nanocrystalline systems provide examples of "cellular" crystalline networks, which rearrange themselves spontaneously into three-dimensional polyhedral structures. It appears that a closely related analogy exists between the tetrahedrally close-packed networks as revealed for the lipid A-mono- and diphosphates [ C. A. Faunce , H. Reichelt , H. H. Paradies , , J. Chem. Phys. 122 , 214727 ( 2005 ) ; C. A. Faunce , H. Reichelt , P. Quitschau , , J. Chem. Phys. 127 , 115103 ( 2007 ) ]. However, the cubic I a 3 ¯ d phase consists of two three-dimensional networks of rods, mutually intertwined but not connected. For this cubic I a 3 ¯ d phase each junction involves three coplanar rods at an angle of 120°, showing an interwoven labyrinth of lipid A-monophosphate rods which are connected three by three. The rod dia