title: Observations of liquidlike order of charged rodlike lipid A diphosphate assemblies at pH 8.5.
abstract: A new structural form of charged lipid A diphosphate, with a molecular weight of 5.9x10(6) Da and a rodlike shape (L=800 nm), was found in aqueous solutions at pH 8.5. The experimental techniques used in the investigation were light scattering, small-angle x-ray scattering (SAXS), and electron microscopy. Measurements of the static-structure factor S(Q) as a function of the ionic strength are presented over the concentration regimes C>C(*) and C<C(*), with C(*)=1 particle/length(3). The position of the first maximum of the structure factor S(Q) was found to scale with C(13) below and with C(12) above, the critical concentration C(*) (2.5 microg mL). SAXS results in the semidilute concentration range C> or =C(*) show that strong interparticle correlations exist even at concentrations as high as 15C(*), in contrast with results for hard-rod systems. The magnitude of the correlations depends on both the lipid A diphosphate concentration at pH 8.5 and the Debye screening length k(-1). For a constant lipid A diphosphate concentration at pH 8.5, as the amount of salt was increased a decrease in structure was observed. There was also a shift in the peak of the first maximum position Q(max) to larger scattering wave vectors. The observed phase behavior (C=15C(*)) exhibited an isotropic I-Sm transition and an I-N-Sm transition, which were recorded on electron microscope images.
authors.affiliation: Joule Physics Laboratory, Materials Research Institute, The University of Salford, Manchester M5 4WT, United Kingdom.
authors.forename: Chester A
authors.initials: CA
authors.lastname: Faunce
authors.forename: Henrich H
authors.initials: HH
authors.lastname: Paradies
mesh_terms: D011756:Diphosphates / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D008027:Light; D008050:Lipid A / Q000737:chemistry*; D008854:Microscopy, Electron; D015394:Molecular Structure; D008970:Molecular Weight; D009994:Osmolar Concentration; D010316:Particle Size; D053838:Scattering, Small Angle; D014961:X-Ray Diffraction
chemical_list: D011756:Diphosphates; D008050:Lipid A