Observations of liquidlike order of charged rodlike lipid A diphosphate assemblies at pH 8.5.

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 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.