Chemical structure of the lipopolysaccharide of Haemophilus influenzae strain I‐69 Rd−/b+

The chemical structure of the lipopolysaccharide of a deep‐rough mutant (strain I‐69 Rd − /b + ) of Haemophilus influenzae was investigated. The hydrophilic backbone of lipid A was shown to consist of a β‐(1′,6)‐linked D‐glucosamine disaccharide with phosphate groups at C‐1 of the reducing D‐glucosamine and at C‐4′ of the nonreducing one. Four molecules of ( R )‐3‐hydroxytetradecanoic acid were found directly linked to the lipid A backbone, two by amide and two by ester linkage (positions 2, 2′ and 3, 3′, respectively). Laser‐desorption mass spectrometry showed that both 3‐hydroxytetradecanoic acids linked to the non‐reducing glucosamine carry tetradecanoic acid at their 3‐hydroxyl group, so that altogether six molecules of fatty acid are present in lipid A. The lipopolysaccharide was the first described to contain only one sugar unit linked to lipid A. This, sugar in accordance with a previous report Zamze et al. (1987) Biochem. J. 245 , 583–587, was shown to be a dOclA phosphate. The phosphate group was found at position 4, but the analytical procedures employed (permethylation and methanolysis followed by gas‐liquid chromatography/mass spectrometry) also revealed dOclA 5‐phosphate. Since a cyclic 4,5‐phosphate could be ruled out by 31 P‐NMR, we conclude that, in this lipopolysaccharide, a mixture of dOclA 4‐ and 5‐phosphate is present. By methylation analysis of the dephosphorylated, deacylated and reduced lipopolysaccharide the attachment site of the dOclA was assigned to position C‐6′ of the nonreducing glucosamine of lipid A. The anomeric linkages present in the lipopolysaccharide were assessed by 1 H‐NMR and 13 C‐NMR of deacylated lipopolysaccharide. The saccharide backbone of this Haemophilus influenzae lipopolysaccharide possesses the following structure: image