Development and optimization of a MALDI-QTOF MS Method for the Analysis of Glucans from Daedalea quercina

This study demonstrated the applicability of Maltooligosaccharides (MOS) from a commercial beer as cheap and readily-available calibrant and model compounds for the optimization of MALDI-MS and MS/MS analysis of glycans in a Quadrupole Time-of-Flight (Q-TOF) MS platform. Important parameters such as homogeneity of matrix, in-source fragmentation and collision energy were investigated. The optimized methodology was used to characterize the glycan from the bioactive fraction of Daedalea quercina. In this study, we report for the first time the extraction, isolation, and the proposed structure of a polysaccharide from the fruiting bodies of D. quercina. The monosaccharide composition was mainly glucose as identified using GC-MS of the isolate. FTIR-ATR spectroscopy showed that DQW1Pa1 has a β configuration. The average molecular weight of this β-glucan obtained using size exclusion chromatography was 1.6 x 104 Da, consistent with glucans derived from mushrooms of the order Polyporaceae. MALDI-QTOF MS/MS was carried out to identify the linkage and connectivity of the glucose units. Collision Induced Dissociation (CID) of selected parent ions of different oligosaccharide lengths showed the presence of characteristic glycosidic bond cleavages Bn/Cn and the linear backbone by 1-6 linkage, cross-ring fragment, 0,3An. Presence of branching unit was identified from high intensity 0,3A4 fragment and verified from diagnostic ion of [D] and [D-H2O] types. To confirm the linkage assignment obtained using the developed and optimized MALDI-QTOF MS/MS method, DQW1Pa1 was subjected to methylation analysis. Results showed the presence of 1-3, 1-6, 1- and 1-3-6 linked glucose in the order of decreasing abundance, respectively. The repeating unit of isolate DQW1Pa1 was deduced as 1-3 linked linear glucose backbone with branches composed of three 1-3 linked glucose units linked to backbone by 1-6 linkage of approximately 7%