Chapter 20: Carbohydrates Carbohydrate: 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

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

Chapter 20: Fischer projections Fischer projection: 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.

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

Chapter 20: Monosaccharides The most common monosaccharides: D-RiboseD- GlucoseD- GalactoseD- Fructose

Chapter 20: Cyclic Structure hemiacetals 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.

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

Chapter 20: Anomers The anomeric carbon of an aldose is C-1; The anomeric carbon of most ketoses is C-2. β β means that the -OH on the anomeric carbon is on the same side of the ring as the terminal -CH 2 OH. α α means that the -OH on the anomeric carbon is on the side of the ring opposite from the terminal -CH 2 OH. pyranose furanose 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.

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

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

Chapter 20: Monosaccharides Mutarotation: 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.

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

Chapter 20: Disaccharides Disaccharide: 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.

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.

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.

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

Chapter 20: Polysaccharides Polysaccharide: Polysaccharide: A carbohydrate consisting of large numbers of monosaccharide units joined by glycosidic bonds. Starch: 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.

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

Chapter 20: Polysaccharides Glycogen Glycogen is the energy-reserve carbohydrate for animals. Glycogen is a branched polysaccharide of approximately 10 6 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.

Chapter 20: Polysaccharides Cellulose 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.

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.