The 13C CP/MAS NMR solid-state NMR technique was used to detect the presence of β-D-glucan and trace compounds in samples prepared from dried, naturally grown oyster mushroom (Pleurotus ostreatus) and commercially available products of dried, specially cultivated oyster mushroom and β-D-glucan isolated from this mushroom. The NMR spectra of all samples displayed signals typical for (1→3, 1→6)-β-D-glucan; however, signals which could be assigned to other trace compounds—(1→3)-α-glucan, chitin, and proteins—were also observed in the spectra. The amount of trace compounds was negligible in the commercially available products. 1. Introduction Oyster mushroom (Pleurotus ostreatus) is highly fancied due to its content of biologically active substances. Among the most notable metabolites extracted from this edible mushroom are polysaccharides belonging mainly to β- and α-D-glucans, which are important regarding their immunomodulating, antitumor, and tissue-healing abilities [1–7]. The content of cholesterol-lowering agents—chitin and chitosan—increases the nutritional value of this medical mushroom [8, 9]. β-D-Glucans are polysaccharides composed of D-glucose monomers linked by β-glycosidic bonds, and they are intensively investigated owing to their anticancer efficacy [1–3, 5, 8, 10–19]. Research has shown that insoluble (1→3, 1→6)-β-D-glucan (Figure 1) has greater biological activity than its soluble (1→3, 1→4)-β-D-glucan counterparts. The differences between β-D-glucan linkages and chemical structure are significant in terms of solubility, mode of action, and overall biological activity [1]. The most active forms of (1→3)-β-D-glucans are those containing side chains in positions 1 and 6. Activity of β-D-glucans depends on molecular weight, degree of branching, and structure of the β-D-glucans molecule. The highest anti-cancer effect is achieved by β-D-glucans with the degree of branching between 0.20 and 0.33 and high molecular weight [2]. β-D-Glucan isolated from Pleurotus ostreatus—pleuran—belongs to this group of β-D-glucans. The content of β-D-glucans in dried Pleurotus ostreatus ranges between 24 and 38% wt. [9]. Pleuran has a (1→3)-β-D-glucan backbone with (1→6)-linked residues. It may contain a small proportion of interior (1→6)- and (1→4)-linked residues [7, 20]. Figure 1: Structure of (1→3, 1→6)- β-D-glucan. NMR spectroscopy is considered to be a unique method for the study of structure and molecular dynamics in solids. Modern solid-state NMR techniques such as magic angle spinning (MAS) and cross-polarisation (CP) enable conformation study of
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