A novel branching pattern in the lipopolysaccharide expressed by non-typeable Haemophilus influenzae strain 1232

•We determined the LPS structures of non-typeable H. influenzae strain 1232.•The strain expresses d,d-Hep in the outer core of LPS in a novel manner.•The branching at d,d-Hep was indicated by MS4 experiments and confirmed by NMR.•d,d-Hep is substituted at both O-4 and O-7.

We report the novel branching pattern in lipopolysaccharide (LPS) expressed by non-typeable Haemophilus influenzae (NTHi) strain 1232. The strain expressed the β-d-Glcp-(1→4)-[α-d-Galp-(1→4)-β-d-Galp-(1→7)]-d-α-d-Hepp-(1→6)-β-d-Glcp chain linked to the proximal heptose (HepI) of the conserved triheptosyl inner-core moiety of NTHi LPS: l-α-d-HepIIIp-(1→2)-[PEtn→6]-l-α-d-HepIIp-(1→3)-l-α-d-HepIp-(1→5)-[PPEtn→4]-α-Kdop-(2→6)-lipid A. The structure has been elucidated using NMR spectroscopy, electrospray ionization mass spectrometry (ESI-MS) and capillary electrophoresis coupled to electrospray ionization tandem mass spectrometry (CE-ESI-MSn) on O-deacylated LPS and core oligosaccharide (OS) materials, as well as HPLC-ESI-MSn on permethylated, dephosphorylated OS. It was also found that a tetrasaccharide unit bearing sialic acid [α-Neu5Ac-(2→3)-β-d-Galp-(1→4)-β-d-GlcNAcp-(1→3)-β-d-Galp-(1→] could substitute O-4 of the β-d-Glcp linked to HepI. In addition, the distal heptose (HepIII) was substituted by PCho→6-β-d-Galp-(1→ at the O-2 position.

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