Structural analysis of linear mixed-linkage glucooligosaccharides by tandem mass spectrometry

Dextrans and glucooligosaccharides (GLOS) are produced by lactic acid bacteria (LAB) during sourdough fermentation. The dextrans can act as hydrocolloids in sourdough bread, while the GLOS may have antistaling and prebiotic properties, depending on their structure. Development of high-throughput methods for screening the structural properties of dextrans and GLOS produced by different LAB in varying fermentation conditions is therefore of interest. In this study we explored the possibility of using electrospray ionisation tandem mass spectroscopy (ESI-MS/MS) to unequivocally determine the structures of underivatised GLOS. The emphasis was on linear mixed linked model GLOS, especially those containing (1→3) linkages that are common in dextrans. After evaluation of the model GLOS, the ESI-MS/MS method was used to determine the linkage positions of two mixed-linked tetrasaccharides obtained by hydrolysis of Weissella confusa and Leuconostoc citreum dextrans. In positive mode, only the reducing end linkage could be determined because isomeric fragment ions, present in subsequent MS(n) cycles, hindered assignment of the remaining linkages. By contrast, it was possible to unambiguously assign all the linkages in each GLOS using the negative mode spectra. The present study thus shows that negative mode is the preferred method for ESI-MS/MS structural analysis of underivatised GLOS. In combination with liquid chromatography this method will enable rapid profiling of the structural variation of dextrans and prebiotic GLOS.