Two dimensional crystallization of three solid lipid A-diphosphate phases

Surface-tension-induced liquid-crystal growth of monomeric lipid A-diphosphate in aqueous dispersions is reported as a function of concentration, (c), and temperature, (T), 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 (2-d) faceted crystal morphologies was observed and, as growth proceeded, these faceted 2-d crystals became unstable. Selected area electron microscopy diffraction (SAED) and X-ray diffraction (XRD) measurements of the faceted 2-d crystalline lipid A-diphosphate layers exhibited a pseudohexagonal molecular arrangement. The crystalline layer was a smectic F, S(F), phase below the critical temperature, T(C), and a smectic I, S(I), phase above T(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 S(F) phase (5-15 °C), but long-range order in the S(I) phase, for the temperature range 15-30 °C. The observed 2-d solid hexatic phase and the 2-d 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 S(I) phase showed long-range order characteristics of a hexatic 2-d crystal. The two-, four-, or six-layer crystalline lipid A-diphosphate films exhibited 2-d hexatic order and 6n-fold bond-orientational order. These films did not evolve into the S(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 2-d lattice. The constraint of six close-packed acyl chains in two distinct phases with the same symmetry suggests that the S(F) → S(I) transition was first-order.