CHARACTERIZATION OF THE NONLAMELLAR CUBIC AND H-II 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 bact erial lipopolysaccharide (LPS), from rough mutant Salmonella enterica sv. Minnesota R595 was analyzed at different water content, cation (Mg 2+) concentration, and temperature applying synchrotron radiation X-ra y diffraction and, in selected cases, freeze-fracture electron microsc opy. The X-ray diffraction spectra prove the existence of different la mellar, mixed lamellar/cubic, various cubic, and inverted hexagonal (H -II) structures depending on ambient conditions. The three mainly bico ntinuous cubic phases Q(224), Q(229), and Q(230) can be observed betwe en 30 and 50 degrees C in narrow water and cation concentration ranges : Above 50 degrees C, Q(212) an intermediate phase between bicontinuou s and micellar is adopted. In freeze-fracture electron microscopic exp eriments, cubic structures of these symmetries are not readily detecte d, which can be understood in the light of changes in hydration during freezing and the metastability of these phases. However,'lipidic part icles' closely related to cubic phases are observed. Above 65-70 degre es 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.