A 31P‐Nuclear‐Magnetic‐Resonance Study of the Phosphate Groups in Lipopolysaccharide and Lipid A from Salmonella

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Abstract
Untreated and partially deacylated lipopolysaccharides from various P− and P+ strains of Salmonella were studied with 31P nuclear magnetic resonance spectroscopy and by conventional analytical methods. The spectral signals were assigned to various phosphate groups in the lipid A moiety and in the oligosaccharide part. A signal at + 2.3 ppm could be assigned to a phosphodiester linkage formed between 4-amino-4-deoxy-l-arabinose linked via the glycosidic hydroxyl group to the 4′-phosphate group of the glucosamine disaccharide in the lipid A moiety. A strong pyrophosphate signal at + 11 ppm in P− strains was identified as a pyrophosphoryl ethanolamine group at the glycosidic end of this glucosamine disaccharide unit. No evidence was found for phosphodiester or pyrophosphodiester bonds crosslinking lipopolysaccharide ‘subunits’. A revised version of the lipid A structure of Salmonella is presented. By a combination of 31P nuclear magnetic resonance spectroscopy data and conventional analytical methods the extent to which the lipopolysaccharides are substituted by various phosphate groups on the lipid A and the oligosaccharide moiety could be estimated. It was thus shown that substantial heterogeneity, leading to several molecular species of lipopolysaccharides is caused by addition or omission of certain groups. Since changes in substitution were found to be dependent on the growth conditions, it is thought possible that the overall negative surface charge of Salmonella can be modified by addition or omission of neutralising amino groups from ethanolamine and/or 4-amino-4-deoxy-l-arabinose, and can thus be adapted to the environment.