Two dimensional crystallization of three solid Lipid A-Diphosphate Phases

Faunce, C; Paradies, HH

Abstract

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, SF, phase below the critical temperature, TC, and a smectic I, SI, phase above TC (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 SF phase (5–15 °C), but long-range order in the SI 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 SI 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 SF 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 SF → SI transition was first-order.

Citation

Faunce, C., & Paradies, H. Two dimensional crystallization of three solid Lipid A-Diphosphate Phases. Journal of Physical Chemistry B, 118(3), 800-811. https://doi.org/10.1021/jp408282x

• Journal Article Type: Article
• Deposit Date: May 1, 2014
• Journal: The Journal of Physical Chemistry B
• Print ISSN: 1520-6106
• Publisher: American Chemical Society
• Peer Reviewed: Peer Reviewed
• Volume: 118
• Issue: 3
• Pages: 800-811
• DOI: https://doi.org/10.1021/jp408282x
• Publisher URL: http://dx.doi.org/10.1021/jp408282x
• Related Public URLs: http://pubs.acs.org/journal/jpcbfk