Two Dimensional Crystallization of Three Solid Lipid A‑Diphosphate Phases

Abstract

Surface-tension-induced liquid-crystal growth of monomeric lipid A-diphosphate in aqueous dispersions is reported as a function of concentration, (<i>c</i>), and temperature, (<i>T</i>), and at low ionic strength (10^–3 M). As the temperature was varied, a solid–liquid transition was revealed in the surface layer at a fixed lipid A-diphosphate bulk concentration. Here, the development of different two-dimensional (<i>2-d</i>) faceted crystal morphologies was observed and, as growth proceeded, these faceted <i>2-d</i> crystals became unstable. Selected area electron microscopy diffraction (SAED) and X-ray diffraction (XRD) measurements of the faceted <i>2-d</i> crystalline lipid A-diphosphate layers exhibited a pseudohexagonal molecular arrangement. The crystalline layer was a smectic F, <i>S</i>_F, phase below the critical temperature, <i>T</i>_C, and a smectic I, <i>S</i>_I, phase above <i>T</i>_C (15 °C). Both phases could be described in terms of the same C-centered monoclinic unit cell. The in-plane order extended for a limited distance although the layers were coupled. The analysis of the SAED patterns revealed short-range order in the <i>S</i>_F phase (5–15 °C), but long-range order in the <i>S</i>_I phase, for the temperature range 15–30 °C. The observed <i>2-d</i> solid hexatic phase and the <i>2-d</i> liquid hexatic phase had correlation lengths of 220 Å. This, the hexatic phase, displayed short-range in-plane positional order and quasi long-range, sixfold bond-orientational order. The <i>S</i>_I phase showed long-range order characteristics of a hexatic <i>2-d</i> crystal. The two-, four-, or six-layer crystalline lipid A-diphosphate films exhibited <i>2-d</i> hexatic order and 6<i>n</i>-fold bond-orientational order. These films did not evolve into the <i>S</i>_F phase, demonstrating that the two phases were thermodynamically distinct. A finite tilt angle of φ = 15° was calculated for the lipid A-diphosphate molecule; the tilt was toward the small side of the rectangular <i>2-d</i> lattice. The constraint of six close-packed acyl chains in two distinct phases with the same symmetry suggests that the <i>S</i>_F → <i>S</i>_I transition was first-order