Overview
β-Glucans are polysaccharides that contain only glucose as structural components, and are linked with β-glycosidic bonds.
Glycosidic bonds are etheric oxygen bridges which link the monosaccharide units in a polysaccharide, and they are designated by a pair of numbers to indicate which carbons in each of the monosaccharide units are linked (according to the standard numbering for simple monosaccharides). When a glycosidic bond involves the number 1 carbon of an aldose monosaccharide (or the number 2 carbon of a ketose monosaccharide), a second designation is needed to indicate the spatial orientation of the etheric oxygen linkage at that carbon (the anomeric carbon). An "α-" (alpha) indicates the etheric oxygen linkage attaches to the anomeric carbon below the ring, and a "β-" (beta) indicates the etheric oxygen linkage attaches to the anomeric carbon above the ring (in the standard Haworth projection). The orientation of the etheric linkage at any of the other carbons is predetermined and fixed by the identity of the monosaccharide and the position, and no further descriptor is required for the glycosidic bond.
Thus, the designation of β(1-3) for a glycosidic linkage indicates that the etheric oxygen bridge between two consecutive monosaccharide units of the polysaccharide connects the number 1 carbon of the first unit to the number 3 carbon of the second unit, and that etheric oxygen bridge attaches to carbon 1 of the first unit from above the ring.
Likewise, the designation of β(1-6) for a glycosidic linkage indicates that the etheric oxygen bridge between two consecutive monosaccharide units of the polysaccharide connects the number 1 carbon of the first unit to the number 6 carbon of the second unit, and that etheric oxygen bridge attaches to carbon 1 of the first unit from above the ring.
Read more about this topic: Beta-glucan