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Chapter 20: Carbohydrates

Chapter 20: Carbohydrates. Carbohydrate: A polyhydroxyaldehyde or polyhydroxyketone , or a substance that gives these compounds on hydrolysis. Chapter Overview: Monosaccharides Fischer projections Haworth projections Hemiacetals and Acetals Oxidation and reduction Disaccharides

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Chapter 20: Carbohydrates

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  1. Chapter 20: Carbohydrates Carbohydrate: A polyhydroxyaldehyde or polyhydroxyketone, or a substance that gives these compounds on hydrolysis. Chapter Overview: • Monosaccharides • Fischer projections • Haworth projections • Hemiacetals and Acetals • Oxidation and reduction • Disaccharides • Polysaccharides

  2. Chapter 20: Monosaccharides Monosaccharide:A carbohydrate that cannot be hydrolyzed to a simpler carbohydrate. • Monosaccharides have the general formula CnH2nOn, where n varies from 3 to 8. • Aldose: A monosaccharide containing an aldehyde group. • Ketose: A monosaccharide containing a ketone group. • The prefixes tri-, tetra, penta, and so forth indicate the number of carbon atoms in the chain. Fructose ( ____________ )

  3. Chapter 20: Fischer projections Fischer projection:A two-dimensional representation for showing the configuration of tetrahedral stereocenters. • Horizontal lines represent bonds projecting forward from the stereocenter. • Vertical lines represent bonds projecting to the rear.

  4. Chapter 20: Monosaccharides In 1891, Emil Fischer made the arbitrary assignments of D- and L- to the enantiomers of glyceraldehyde. • D-monosaccharide: the -OH on its penultimate carbon is on the right in a Fischer projection. • L-monosaccharide: the -OH on its penultimate carbon is on the left in a Fischer projection.

  5. Chapter 20: Monosaccharides The most common monosaccharides: D-Ribose D- Glucose D- Galactose D- Fructose

  6. Chapter 20: Cyclic Structure • Aldehydes and ketones react with alcohols to form hemiacetals (Chapter 17). • Cyclic hemiacetals form readily when the hydroxyl and carbonyl groups are part of the same molecule and their interaction can form a five- or six-membered ring.

  7. Chapter 20: Haworth Projections • D-Glucose forms these two cyclic hemiacetals.

  8. Chapter 20: Anomers • The anomeric carbon of an aldose is C-1; • The anomeric carbon of most ketoses is C-2. • β means that the -OH on the anomeric carbon is on the same side of the ring as the terminal -CH2OH. • α means that the -OH on the anomeric carbon is on the side of the ring opposite from the terminal -CH2OH. • A six-membered hemiacetal ring is called a pyranose, and a five-membered hemiacetal ring is called a furanose because these ring sizes correspond to the heterocyclic compounds furan and pyran.

  9. Chapter 20: Monosaccharides The prefix “deoxy” means “without oxygen.”

  10. Chapter 20: Monosaccharides Fructose is a ketose and it also forms cyclic hemiacetals

  11. Chapter 20: Monosaccharides • Mutarotation: The change in specific rotation that accompanies the equilibration of a- and b-anomers in aqueous solution. • Example: When either a-D-glucose or b-D-glucose is dissolved in water, the specific rotation of the solution gradually changes to an equilibrium value of +52.7°, which corresponds to 64% beta and 36% alpha forms.

  12. Chapter 20: Monosaccharides Hemiacetal + Alcohol = Acetal • The acetal obtained from a monosaccharide is called glycoside • Mutarotation is not possible in glycosides

  13. Chapter 20: Disaccharides Disaccharide: a carbohydrate containing two monosaccharide units joined by a glycosidic bond. Sucrose (table sugar) = Glucose + Fructose • Sucrose is the most abundant disaccharide in the biological world; it is obtained principally from the juice of sugar cane and sugar beets. • Sucrose is a nonreducing sugar.

  14. Chapter 20: Monosaccharides Lactose = Galactose + Glucose • Lactose is the principal sugar present in milk; it makes up about 5 to 8 percent of human milk and 4 to 6 percent of cow's milk. • It consists of D-galactopyranose bonded by a β-1,4-glycosidic bond to carbon 4 of D-glucopyranose. • Lactose is a reducing sugar.

  15. Chapter 20: Monosaccharides Maltose = Glucose + Glucose • Present in malt, the juice from sprouted barley and other cereal grains. • Maltose consists of two units of D-glucopyranose joined by an a-1,4-glycosidic bond. • Maltose is a reducing sugar.

  16. Chapter 20: Physical Properties Monosaccharides are colorless crystalline solids, very soluble in water, but only slightly soluble in ethanol. Sweetness relative to sucrose:

  17. Chapter 20: Polysaccharides Polysaccharide: A carbohydrate consisting of large numbers of monosaccharide units joined by glycosidic bonds. Starch: A polymer of D-glucose. • Starch can be separated into amylose and amylopectin. • Amylose is composed of unbranched chains of up to 4000 D-glucose units joined by α-1,4-glycosidic bonds. • Amylopectin contains chains up to 10,000 D-glucose units also joined by α-1,4-glycosidic bonds; at branch points, new chains of 24 to 30 units are started by α-1,6-glycosidic bonds.

  18. Chapter 20: Polysaccharides Amylopectin, a branched polymer of approximately 10,000 units of D-glucose joined by -1,4-glycosidic bonds.

  19. Chapter 20: Polysaccharides Glycogen is the energy-reserve carbohydrate for animals. • Glycogen is a branched polysaccharide of approximately 106 glucose units joined by α-1,4- and α-1,6-glycosidic bonds. • The total amount of glycogen in the body of a well-nourished adult human is about 350 g, divided almost equally between liver and muscle.

  20. Chapter 20: Polysaccharides Cellulose is a linear polysaccharide of D-glucose units joined by β-1,4-glycosidic bonds. • It has an average molecular weight of 400,000 g/mol, corresponding to approximately 2200 glucose units per molecule. • Cellulose molecules act like stiff rods and align themselves side by side into well-organized water-insoluble fibers in which the OH groups form numerous intermolecular hydrogen bonds. • This arrangement of parallel chains in bundles gives cellulose fibers their high mechanical strength. • It is also the reason why cellulose is insoluble in water.

  21. Chapter 20: Polysaccharides Cellulose is a linear polymer containing as many as 3000 units of D-glucose joined by β-1,4-glycosidic bonds. • Humans and other animals can not digest cellulose because their digestive systems do not contain β-glycosidases, enzymes that catalyze the hydrolysis of β-glycosidic bonds. • Termites have such bacteria in their intestines and can use wood as their principal food. • Ruminants (cud-chewing animals) and horses can also digest grasses and hay. • Instead, we have only α-glucosidases; hence, the polysaccharides we use as sources of glucose are starch and glycogen. • Many bacteria and microorganisms have β-glucosidases.

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