Concept 02 / 05Biomolecules
Fructose, Disaccharides and Polysaccharides
Structure of fructose (furanose ring), the glycosidic linkage, sucrose and invert sugar, maltose, lactose, reducing versus non-reducing sugars, and the polysaccharides starch (amylose and amylopectin), glycogen and cellulose.
Fructose, , is a ketohexose (carbonyl at C2) and is laevorotatory: D-(−)-fructose, . It belongs to the D-series because its C5-OH is on the right. Its cyclic form is a five-membered hemiketal ring formed between C2 and the C5-OH (a furanose, by analogy with furan), existing as α and β anomers. Though a ketose, fructose reduces Tollens' and Fehling's reagents: in alkaline medium it isomerises to glucose and mannose through an enediol, and α-hydroxy ketones are easily oxidised.
Glycosidic linkage. Two monosaccharide units join by an oxide (C-O-C) bond formed with loss of water between the anomeric OH of one unit and an OH of the other. If the anomeric carbons of both units are used, no free hemiacetal remains and the sugar is non-reducing.
Sucrose (cane sugar), : α-D-glucose C1 linked to β-D-fructose C2. Both anomeric carbons are bonded, so sucrose is a non-reducing sugar and shows no mutarotation. It is dextrorotatory (). Hydrolysis by dilute acid or the enzyme invertase gives an equimolar mixture of D-(+)-glucose () and D-(−)-fructose (). Fructose's larger rotation makes the mixture laevorotatory. The sign of rotation changes from + to −, so the process is called inversion, and the product is invert sugar.
Maltose: two α-D-glucose units linked C1-C4. The C1 of the second unit is free, so maltose is reducing. It is formed from starch by diastase.
Lactose (milk sugar): β-D-galactose C1 linked to β-D-glucose C4; reducing.
Polysaccharides.
Starch, the main storage polysaccharide of plants, is a polymer of α-D-glucose with two components. Amylose (15-20%) is water-soluble, an unbranched chain of 200 to 1000 units linked C1-C4; it gives a blue colour with iodine. Amylopectin (80-85%) is water-insoluble and branched: C1-C4 chains with C1-C6 branch points.
Glycogen (animal starch) is stored in the liver, muscles and brain, and resembles amylopectin but is more highly branched. It is broken down to glucose when the body needs energy.
Cellulose is the main structural material of plant cell walls: an unbranched chain of β-D-glucose units linked C1-C4. Humans lack the enzyme to hydrolyse β-linkages, so cellulose is not digested by us, though ruminants digest it with the help of gut microbes.
Key Takeaways:
- Fructose: ketohexose, laevorotatory, furanose ring (C2-C5), yet a reducing sugar.
- Sucrose: α-Glc C1 - β-Fru C2, non-reducing; hydrolysis gives laevorotatory invert sugar.
- Maltose (α 1-4) and lactose (β 1-4) are reducing.
- Starch and glycogen are α-linked; cellulose is β-linked.
- Pitfall: "reducing" requires a free anomeric (hemiacetal or hemiketal) carbon, not merely an aldehyde in the formula.
Brain-Teaser 1: Using values glucose and fructose , find the specific rotation of invert sugar (equal masses).
Solution: For an equimolar (and equal-mass, since is the same) mixture, the specific rotation is the average: . The negative value is why hydrolysis of sucrose is called inversion.
Brain-Teaser 2: of sucrose is completely hydrolysed. What masses of glucose and fructose are obtained?
Solution: , so the sample is . gives of each, that is, of glucose and of fructose (total , the extra is the water added).
Brain-Teaser 3: Why do starch and cellulose, both polymers of D-glucose, have such different properties?
Solution: Starch has α(1-4) links (plus α(1-6) branches in amylopectin), which make the chain coil into helices that are easily hydrolysed by amylase. Cellulose has β(1-4) links, which give straight chains that stack side by side with many hydrogen bonds into strong, insoluble fibres. Human enzymes cannot hydrolyse the β link.
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