Structural Characterization of a Model Gram-NegativeBacterial Surface Using Lipopolysaccharides from Rough Strains of Escherichia coli.

Lipopolysaccharides (LPS) make upapproximately 75% of the Gram-negativebacterial outer membrane (OM) surface, but because of the complexityof the molecule, there are very few model OMs that include LPS. TheLPS molecule consists of lipid A, which anchors the LPS within theOM, a core polysaccharide region, and a variable O-antigen polysaccharidechain. In this work we used RcLPS (consisting of lipid A plus thefirst seven sugars of the core polysaccharide) from a rough strainof Escherichia colito form stable monolayers ofLPS at the air–liquid interface. The vertical structure RcLPSmonolayers were characterized using neutron and X-ray reflectometry,while the lateral structure was investigated using grazing incidenceX-ray diffraction and Brewster angle microscopy. It was found thatRcLPS monolayers at surface pressures of 20 mN m–1and above are resolved as hydrocarbon tails, an inner headgroup,and an outer headgroup of polysaccharide with increasing solvationfrom tails to outer headgroups. The lateral organization of the hydrocarbonlipid chains displays an oblique hexagonal unit cell at all surfacepressures, with only the chain tilt angle changing with surface pressure.This is in contrast to lipid A, which displays hexagonal or, above20 mN m–1, distorted hexagonal packing. This workprovides the first complete structural analysis of a realistic E. coliOM surface model. [ABSTRACT FROM AUTHOR]