Chemistry · Ch 14 — Biomolecules
Disaccharides
Disaccharides
Disaccharides are sugars that yield two molecules of monosaccharides on hydrolysis, a reaction usually catalysed by dilute acid or by a specific enzyme. Their general formula, Cₙ(H₂O)ₙ₋₁, has one fewer water than two free monosaccharides combined, because in forming a disaccharide the two monosaccharide units are joined by an oxide bridge called a glycosidic linkage -- formed when the anomeric (carbonyl-derived) carbon of one monosaccharide reacts with a hydroxyl group of the other, releasing one molecule of water. Sucrose, lactose and maltose are the three examples worked through in this unit.
Sucrose (table sugar) is the most abundant disaccharide, obtained mainly from the juice of sugar cane and sugar beet. Insects such as honey bees carry the enzyme invertase, which hydrolyses sucrose into a glucose-fructose mixture; honey is, in fact, primarily a mixture of glucose, fructose and sucrose. Hydrolysis of sucrose gives equal amounts of glucose and fructose. Sucrose itself is strongly dextrorotatory (+66.6°), as is the glucose it releases (+52.5°), but fructose is strongly levorotatory (-92.4°); because fructose's large negative rotation outweighs the combined positive rotation of sucrose and glucose, the optical rotation of the reaction mixture flips from dextro to levo as hydrolysis proceeds, which is why sucrose is also called invert sugar and the resulting mixture invert sugar. Structurally, in sucrose the C1 of α-D-glucose is joined to the C2 of β-D-fructose, a linkage called the α-1,2 glycosidic bond; because this bond ties up both monosaccharides' carbonyl (reducing) carbons at once, neither is free to reduce Tollens'/Fehling's reagent, so sucrose is a non-reducing sugar.
Lactose (milk sugar) is a disaccharide found in the milk of mammals. On hydrolysis it yields galactose and glucose, joined as β-D-galactose linked to β-D-glucose through a β-1,4 glycosidic bond. Because the aldehyde carbon of the glucose unit is left free (not tied up in the glycosidic bond), lactose retains its reducing property and is a reducing sugar. …
What this figure shows. The full structure of sucrose, α-D-glucopyranosyl-β-D-fructofuranoside, drawn as the pyranose ring of α-D-glucose joined through its C1 to the C2 of the furanose ring of β-D-fructose via the α-1,2 glycosidic oxygen bridge, showing both anomeric (carbonyl-derived) carbons c …
What this figure shows. The full structure of lactose, β-D-galactopyranosyl-(1→4)-β-D-glucopyranose, drawn as the pyranose ring of β-D-galactose joined through its C1 to the C4 of the pyranose ring of β-D-glucose via the β-1,4 glycosidic oxygen bridge, with the glucose unit's own anomeric carbon left free to show why la …
What this figure shows. The full structure of maltose, α-D-glucopyranosyl-(1→4)-α-D-glucopyranose, drawn as two α-D-glucopyranose rings joined by an α-1,4 glycosidic oxygen bridge between the anomeric carbon of the first ring and C4 of the second, with the second ring's own anomeric carbon left free to show why mal …