Comparative X‐ray and Fourier‐transform‐infrared investigations of conformational properties of bacterial and synthetic lipid A of Escherichia coli and Salmonella minnesota as well as partial structures and analogues thereof

Abstract

By using the X-ray and infrared spectroscopic characteristics of various synthetic analogues and partial structures of lipid A in the dried state, a comparison of these compounds with their natural counterparts was undertaken. As judged by their X-ray diffraction and infrared spectroscopic features, the compounds tested could be divided into two main groups. The first group covered those samples synthesized in accordance with a previously assumed structure, while those synthesized in accordance with present knowledge on the lipid-A primary structure formed the second group. Members of the first group were characterized by a liquid-like, atype arrangement of their fatty acyl chains in a non-lamellar supramolecular structure, while all members of the second group formed bilayered phases with a much more ordered, p-type conformation of their fatty acyl chains. Synthetic Escherichia coli-type lipid A (compound 506), proved to be essentially identical to its natural counterpart with respect to those conformational properties accessible by our methods. The synthetic hepta-acyl species of Salmonella minnesota lipid A (compound 516) revealed an unexpected conformational behaviour, whereby a fattyacyl-chain packing could be detected which was different from the hexagonal arrangement found for all other compounds of the second group.The pathophysiological activities of lipopolysaccharides (endotoxins) of gram-negative bacteria in mammals, including lethal toxicity, pyrogenicity, immunomodulating and antitumor activities, are dependent on their lipid-A component [I]. The conformational rigidity of lipopolysaccharide and lipid A also seems to play an important role for the well known permeation-barrier properties of the outer membrane of gram-negative organisms [2 -51.Compared to other membrane-forming lipids, lipid A exhibits a considerable chemical complexity in consisting of a (p 1 -6)-linked D-glucosamine disaccharide substituted by phosphoryl groups, ester-and amide-linked (R)-3-hydroxy fatty acids, which may, in turn, carry other fatty acids at their 3-hydroxyl groups. Furthermore, although the primary structure of the lipid-A component in many bacteria appears to be quite similar, several minor variations exist [6], and even in samples isolated from a single strain of bacteria an intrinsic chemical heterogeneity has been observed [7, 81. It was, therefore, found to be extremely difficult to unequivocally elucidate its detailed chemical structure and those entities of the molecule responsible and/or necessary for the expression of the various biological properties of lipid A.In order to overcome these problems lipid A, lipid-A partial structures and analogues of lipid A have been chemically synthesized and biologically analysed [9-141. It was shown

Molecular Basis of Bacterial Outer Membrane Permeability Revisited

“…This is precisely the conformation found for LPS bound to FhuA (described in the next section). The quasi-crystalline, ordered arrangement of hydrocarbon chains was also confirmed by using synthetic lipid A with the correct structure, whereas the synthetic compounds representing the earlier, incorrect structure showed a liquid crystalline, disordered structure (364). The modeling was extended to hexa-acylated lipid A from E. coli, and the prediction was similar in that the six hydrocarbon chains were optimally located in the nodes of a hexagonal lattice (326,439) (Fig.…”

Biochemistry Of Endotoxins

“…Acyloxyacyl residues may modulate the physical state of lipid A (61,62). Mutants thought to be unable to add laurate and myristate are temperature sensitive and apparently accumulate something resembling KDOz-IV A (158).…”

“…Low angle X-ray studies (61,62) and model building (3,61,62) Lipid A Struct ures in Non-ente ric Ba cte ria…”

Crystallization of R-form lipopolysaccharides from Salmonella minnesota and Escherichia coli

“…Recently, studies on the relationship between chemical structure and biological activity of lipid A components have progressed greatly, since synthetic preparations of lipid A components and related compounds have been available (16,18,20,22,23,26,32,33,46). Although several studies have been done on the three-dimensional structure of LPS (4,10,11,34,35,39,40,56) and a schematic model of LPS has been proposed (34), the conformation of LPS has not been determined conclusively. We have found (29)(30)(31) that certain of the R-form LPSs form ordered two-dimensional hexagonal structures in the presence of MgCl2.…”

“…In previous X-ray diffraction studies, it has been suggested that S. minnesota S-form and Re LPSs (9, 34, 56), S. minnesota free lipid A (34, 35), E. coli R-form LPS (8), and E. coli free lipid A (35) in a dried and a hydrated state form bilayered arrangements and that the fatty acid chains of lipid A are tightly packed in a hexagonal lattice with a lattice constant of 0.47 nm (9, 56), 0.49 nm (34,35), or 0.50 nm (8). X-ray diffraction patterns obtained from LPS foils, which were obtained by slowly drying a suspension drop on a glass plate and irradiated perpendicularly to the foil membrane normal, suggesting that fatty acid chains of lipid A are oriented perpendicularly to the membrane surface (34).…”