Structural analysis of gellans produced by Sphingomonas elodea strains by electrospray tandem mass spectrometry

A commercial gellan sample (Gelrite) and a gellan-like polymer (JB3) obtained by exposure of the producing strain to chemical mutagenesis were subjected to partial acid hydrolysis and the resultant oligosaccharides were identified by Electrospray Mass Spectrometry (ESI-MS) and Tandem Mass Spectrometry (ESI-MS/MS and MSⁿ). In both gellans, the main fragments were in accordance with the tetrasaccharide repeating unit, d-Glucose (Glc)-d-Glucuronic acid (GlcA)-d-Glucose (Glc)-l-Rhamose (Rha), described for the wild-type gellan, showing the higher acid lability of the Rha-(1 → 3)-Glc linkage when compared to the (1 → 4) of the other residues. Under the experimental conditions used in the study, as expected, no acyl substituents were observed in the commercial gellan but in JB3 oligosaccharides a glyceryl moiety was identified, substituted in the 3-linked Glc residue. Furthermore, the analysis of the MS/MS and MSⁿ spectra of both gellans allowed the identification of structural details, some of them not yet reported for these exopolysaccharides. The presence of oligosaccharides with single Glc and Rha residues substituent of the tetrasaccharide unit of gellan may represent novel side chains of the backbone unit that to our knowledge have never been reported previously for the gellan exopolysaccharide.