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The β-D-glucopyranosyl group which is obtained by the removal of the hemiacetal hydroxyl
group from β-D-glucopyranose
A glycosyl group is a univalent free radical or substituent structure obtained by removing the hemiacetal hydroxyl group from the cyclic form of a monosaccharide and, by extension, of a lower oligosaccharide. Glycosyl also reacts with inorganic acids, such as phosphoric acid, forming an ester such as glucose 1-phosphate.
In cellulose, glycosyl groups link together 1,4-beta-D-glucosyl units to form chains of (1,4-beta-D-glucosyl)n. Another example is ribityl in 6,7-Dimethyl-8-ribityllumazine.
Substituent groups other than glycosyl groups 
-yl group which is obtained by the removal of a hydrogen from the C3 hydroxyl of β-D-glucopyranose
Instead of the hemiacetal hydroxyl group, a hydrogen atom can be removed to form a substituent, for example the hydrogen off the hydroxyl on C3 of a glucose molecule. Then the substituent is called D-glucopyranos-3-O-yl as it appears in the name of the drug Mifamurtide.
See also