Two new colloidal crystal phases of lipid A-monophosphate: Order-to-order transition in colloidal crystals
Authors: Faunce Chester A. , Paradies Henrich H. 
Published: December 2009

A study of the structure of stable regular-shaped nanocrystals of hexa-acylated (C14) lipid A-monophosphate from Escherichia coli was carried out using dilute electrostatically stabilized aqueous dispersions at low ionic strength (I=1.0×10−5M 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 Pm3¯n and Ia3¯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 Pm3¯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 Pm3¯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, et al., J. Chem. Phys. 122, 214727 (2005); C. A. Faunce, H. Reichelt, P. Quitschau, et al., J. Chem. Phys. 127, 115103 (2007)]. However, the cubic Ia3¯d phase consists of two three-dimensional networks of rods, mutually intertwined but not connected. For this cubic Ia3¯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 diameter is approximately 2.2 nm, which is similar in diameter to the disk-shaped aliphatic chiral core of lipid A-monophosphate (2.14 nm) with an ellipticity of 0.62 seen for the “c” position of the tetrakaidecahedra in the Pm3¯n cubic unit cell. An epitaxial relationship appears to exist between the {211} planes of the cubic Ia3¯d phase and the (001) planes of the lamellar phase as well as with the {10} planes of the hexagonal phase. The transformation of the cubic into the hexagonal phase can be reconciled by the growth of a cylinderlike assembly of lipid A-monophosphate molecules of the hexagonal phase parallel to the ⟨111⟩ directions of the cubic Ia3¯d phase. Upon cooling from 35 to 20 °C the cubic Ia3¯d lipid A-monophosphate phase unexpectedly transforms and gives rise to an intermediate R3¯m structure (a=3.90±0.12 nm, c=7.82±0.05 nm, and γ=120°). Both cubic Ia3¯d and hexagonal R3¯m phases originate from similar rodlike units of lipid A-monophosphate clusters. However, the overall shapes of the assemblies are different because of their spatial distribution. Both assemblies morphologically bridge the lipid A-monophosphate hexagonal and lamellar phases. The structural path followed during the phase transitions is governed by topological similarities between the phase which forms and the one from which it originates. Although the two phases, Ia3¯d and R3¯m, have similar curvature energies on cooling, the topology is more than likely to be the initial factor determining the overall phase transition path.