Disaccharides Contain a Glycosidic Bond

Disaccharides (such as maltose, lactose, and sucrose) consist of two monosaccharides joined covalently by an O-glycosidic bond, which is formed when a hydroxyl group of one sugar reacts with the anomeric carbon of the other (Fig. 7-11). This reaction represents the formation of an acetal from a hemiacetal (such as glu-copyranose) and an alcohol (a hydroxyl group of the second sugar molecule) (Fig. 7-5). Glycosidic bonds are readily hydrolyzed by acid but resist cleavage by base. Thus disaccharides can be hydrolyzed to yield their free monosaccharide components by boiling with dilute acid. ^-glycosyl bonds join the anomeric carbon of a sugar to a nitrogen atom in glycoproteins (see Fig. 7-31) and nucleotides (see Fig. 8-1).

The oxidation of a sugar's anomeric carbon by cupric or ferric ion (the reaction that defines a reducing sugar) occurs only with the linear form, which exists in equilibrium with the cyclic form(s). When the anomeric carbon is involved in a glycosidic bond, that sugar residue cannot take the linear form and therefore becomes a nonreducing sugar. In describing disaccha-rides or polysaccharides, the end of a chain with a free anomeric carbon (one not involved in a glycosidic bond) is commonly called the reducing end.

The disaccharide maltose (Fig. 7-11) contains two d-glucose residues joined by a glycosidic linkage between C-1 (the anomeric carbon) of one glucose residue and C-4 of the other. Because the disaccharide retains a free anomeric carbon (C-1 of the glucose residue on the right in Fig. 7-11), maltose is a reducing sugar. The configuration of the anomeric carbon atom in the gly-cosidic linkage is a. The glucose residue with the free anomeric carbon is capable of existing in a- and ^-pyra-nose forms.

To name reducing disaccharides such as maltose unambiguously, and especially to name more complex oligosaccharides, several rules are followed. By convention, the name describes the compound with its nonre-ducing end to the left, and we can "build up" the name in the following order. (1) Give the configuration (a or 33) at the anomeric carbon joining the first monosaccharide unit (on the left) to the second. (2) Name the

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