Structural reinvestigation of the core oligosaccharide of a mutant form of Aeromonas salmonicida lipopolysaccharide containing an O-4 phosphorylated and O-5 substituted Kdo reducing end group using electrospray quadrupole time of flight tandem mass spectrometry

The molecular structure of the mutant form of the lipopolysaccharide (LPS) of Aeromonas salmonicida was determined to contain an O-4 phosphorylated and O-5 substituted Kdo reducing group, and is proposed as the following: PO ₄ ↓ 4 L-α-D-Hepp-(1→2)-L-α-D-Hepp-(1→3)-L- α-D-Hepp-(1→5)-αKdop-(2→LipidA) It was established that during the cleavage of this LPS with 1% acetic acid, to release the core oligosaccharide from the Lipid A portion, we obtained a degraded core oligosaccharide which eliminated its phosphate group with extreme ease. The precise molecular structure of this dephosphorylated core was deduced by electrospray mass spectrometry and is proposed as the following: L-α-D-Hepp-(1→2)-L-α-D-Hepp-(1→3)-L-α-D-Hepp- (1→5)-α-D-Kdop-(4,7-anhydro)-α-keto acid. Low-energy collision electrospray ionization quadrupole quadrupole time-of-flight tandem mass spectrometry (ESI-QqToF-MS/ MS) analysis of the dephosphorylated core oligosaccharide confirmed the presence of the O-5 glycosylated 4,8- and 4,7-anhydro derivatives of the enolizable α-keto-acids. The collision-induced dissociation (CID) tandem mass spectrometric analysis of the heterogeneous mixture of the permethylated core oligosaccharide established the unreported methylation reaction on the diastereomeric 4,8- and 4,7-anhydro α-keto-acids and the complete permethylation and addition reaction of the O-5 glycosylated open chain reducing end terminal D-arabino-3-en-2-ulonic acid. The stereo-specific fragmentation routes obtained during the tandem mass spectrometric analysis permitted the precise sequencing of this dephosphorylated rough core oligosaccharide of the mutant LPS of A. salmonicida.