Characterization of the nonlamellar cubic and HII structures of lipid A from Salmonella enterica serovar Minnesota by X-ray diffraction and freeze-fracture electron microscopy

The aggregate structures of lipid A, the 'endotoxic principle' of bacterial lipopolysaccharide (LPS), from rough mutant Salmonella enterica sv. Minnesota R595 was analyzed at different water content, cation (Mg2+) concentration, and temperature applying synchrotron radiation X-ray diffraction and, in selected cases, freeze-fracture electron microscopy. The X-ray diffraction spectra prove the existence of different lamellar, mixed lamellar/cubic, various cubic, and inverted hexagonal (H-II) structures depending on ambient conditions. The three mainly bicontinuous cubic phases Q(224), Q(229), and Q(230) can be observed between 30 and 50 degrees C in narrow water and cation concentration ranges: Above 50 degrees C, Q(212) an intermediate phase between bicontinuous and micellar is adopted. In freeze-fracture electron microscopic experiments, cubic structures of these symmetries are not readily detected, which can be understood in the light of changes in hydration during freezing and the metastability of these phases. However,'lipidic particles' closely related to cubic phases are observed. Above 65-70 degrees C, the existence of the H-II phase with hexagonal periodicities d(H) between 4.0 and 6.0 nm for different hydration states is shown using both techniques. Possible biological implications for the preference of lipid A for nonlamellar structures are discussed. (C) 1998 Elsevier Science Ireland Ltd.