Extraction, isolation and MALDI-QTOF MS/MS analysis of β-d-Glucan from the fruiting bodies of Daedalea quercina

We report for the first time the extraction, isolation, and the proposed structure of a polysaccharide from the fruiting bodies of Daedalea quercina. The monosaccharide composition of D. quercina isolate (DQW1Pa1) was mainly glucose as identified using GC–MS. FTIR-ATR spectroscopy and absolute configuration studies showed that this polysaccharide is a β-d-glucan. Its average molecular weight obtained using size exclusion chromatography was 1.6 × 104 Da, consistent with glucans derived from 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, the linear backbone by 1-6 linkage, and the 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 MALDI-QTOF MS/MS, 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 connected to backbone by 1-6 linkage.