An NMR Spectroscopy and Molecular Mechanics Study of the Molecular Basis for the Supramolecular Structure of Lipopolysaccharides

Abstract

Lipopolysaccharides from Gram-negative bacteria interact with the mammalian immune system to trigger a cascade of physiological events leading to a shock syndrome which results in the death in over 70% of cases of severe shock. It is known that the supramolecular structures of lipopolysaccharide aggregates are critical contributors to their biological activities. Despite this, the molecular basis for the formation if the regular hexagonal plates and arrays observed in lipopolysaccharide films and suspensions is unknown. Since these structures are two dimensional, it is unlikely that X-ray crystallographic methods will shed much light on their detailed structure. Knowing this structure is important since it is becoming increasingly likely that the insertion of the lipopolysaccharide hydrocarbon chains in the target host cell membrane may be involved in triggering host responses. This work describes the three-dimensional structure of the lipopolysaccharide lipid A moiety. The structure was obtained by a combination of molecular mechanics calculations and nuclear magnetic resonance spectroscopy. This involved calculation of the dihedral angle between the two glucosamine residues of the lipid A molecule from coupling constants and measuring critical interresidue NOE values. The study also takes into account information from X-ray powder diffraction and electron microscopy studies.

Helical Disposition of Proteins and Lipopolysaccharide in the Outer Membrane ofEscherichia coli

“…A similar conclusion is supported by modeling that indicates that a single LPS molecule, viewed from above, is shaped like a triangular wedge or pie slice (64). Six such molecules can self-associate with their points facing inward to form a circular hexamer, and multiple hexamers can form a regular hexagonal lattice held together by divalent cations (27,64). Brandenburg and Wiese reemphasize the importance of these interactions, although they believe that the aqueous gaps between the LPS hexamers might not be favored in vivo (2).…”

“…The authors concluded that the variety of intermolecular contacts would allow the O polysaccharide to self-assemble as "a mechanically stable, feltlike network" (23). A similar conclusion is supported by modeling that indicates that a single LPS molecule, viewed from above, is shaped like a triangular wedge or pie slice (64). Six such molecules can self-associate with their points facing inward to form a circular hexamer, and multiple hexamers can form a regular hexagonal lattice held together by divalent cations (27,64).…”

“…For example, in artificial membrane vesicles stable islands of LPS persist for days in a "sea of phospholipid," indicating the existence of extremely durable lateral interactions between LPS molecules (59). Also, hexagonal arrays of tightly packed LPS subunits are observed in electron micrographs of crystallized LPS (24,64) and by X-ray crystallography and atomic force microscopy (25). Four forces generate this high degree of organization: van der Waals attractions between fatty acid chains, bridging of LPS molecules by divalent cations, hydrogen bonding, and interactions between the O polysaccharides of neighboring molecules (2,23,27,42).…”

The Molecular Structure and Conformational Dynamics of Chitosan Polymers: An Integrated Perspective from Experiments and Computational Simulations

“…On the other hand, NMR spectroscopy can provide information on the covalent structure and the complex conformational equilibria of oligosaccharides in solution [46,47,64,[77][78][79]. Moreover, it is the only available technique that can determine both the anomericities and linkages of a novel glycan.…”

Low-Temperature-Induced Changes in Composition and Fluidity of Lipopolysaccharides in the Antarctic Psychrotrophic BacteriumPseudomonas syringae

“…The LPS form macromolecular aggregates in vitro (2,3,33).The fluidity (order-disorder conformation) in the hydrocarbon chains of LPS aggregates was measured. The fluorescent dye pyrene, a probe for lipid fluidity (10,32), was used.…”