Carbohydrates Chapter 25 Chapter 25

Carbohydrates Carbohydrate: a polyhydroxyaldehyde, a polyhydroxyketone, or a compound that gives either of these compounds after hydrolysis. Monosaccharide: a carbohydrate that cannot be hydrolyzed to a simpler carbohydrate. they 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.

25.1 A. Monosaccharides Monosaccharides are classified by their number of carbon atoms.

Monosaccharides There are only two trioses. Often the designations aldo- and keto- are omitted and these compounds are referred to simply as trioses, tetroses, and so forth. although these designations do not tell the nature of the carbonyl group, they at least tell the number of carbons. C H O C H 2 O C H O C = O C H 2 O C H 2 O Glyceraldehyde (an aldotriose) D ihydroxyacetone (a ketotriose)

Monosaccharides Glyceraldehyde contains a stereocenter and exists as a pair of enantiomers.

B. Fischer Projections Fischer projection: a two dimensional representation for showing the configuration of carbohydrates. horizontal lines represent bonds projecting forward. vertical lines represent bonds projecting to the rear. the only atom in the plane of the paper is the stereocenter.

C. D,L Monosaccharides In 1891, Emil Fischer made the arbitrary assignments of D- and L- to the enantiomers of glyceraldehyde.

D,L Monosaccharides According to the conventions proposed by Fischer. D-monosaccharide: a monosaccharide that has the same configuration at its penultimate carbon as D-glyceraldehyde; that is, its -OH is on the right when written as a Fischer projection. L-monosaccharide: a monosaccharide that has the same configuration at its penultimate carbon as L-glyceraldehyde; that is, its -OH is on the left when written as a Fischer projection.

D,L Monosaccharides Here are the two most abundant D-aldotetroses and the two most abundant D-aldopentoses in the biological world. C H O C H O C H O C H O H O H H H H O H H O H H O H H O H H O H H O H H O H H O H C H 2 O C H 2 O C H 2 O C H 2 O D-Erythrose D-Threose D-Ribose 2-Deoxy-D- ribose

D,L Monosaccharides And the three most abundant hexoses. C H O C H O C 2 O H O H H O H C O H O H H O H H O H H O H H O H H O H H O H H O H H O H C H 2 O C H 2 O C H 2 O D - G l u c o s e D - G a l c t o s e D - F r u c t o s e

D. Amino Sugars Amino sugar: a sugar that contains an -NH2 group in place of an -OH group. only three amino sugars are common in nature. N-acetyl-D-glucosamine is a derivative of D-glucosamine.

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

25.2 Cyclic Structure Monosaccharides have hydroxyl and carbonyl groups in the same molecule and exist almost entirely as five- and six-membered cyclic hemiacetals. anomeric carbon: the new stereocenter created as a result of cyclic hemiacetal formation. anomers: carbohydrates that differ in configuration at their anomeric carbons.

A. Haworth Projections Haworth projections: five- and six-membered hemiacetals are represented as planar pentagons or hexagons, as the case may be, viewed through the edge. they are most commonly written with the anomeric carbon on the right and the hemiacetal oxygen to the back right. the designation - means that the -OH on the anomeric carbon is cis to the terminal -CH2OH; - means that it is trans to the terminal -CH2OH.

Haworth Projections six-membered hemiacetal rings are shown by the infix -pyran- so become pyranose. five-membered hemiacetal rings are shown by the infix -furan- so become furanose. O O Furan Pyran

Haworth Projections

B. Conformational Formulas five-membered rings are so close to being planar that Haworth projections are adequate to represent furanoses.

Conformational Formulas other monosaccharides also form five-membered cyclic hemiacetals. here are the five-membered cyclic hemiacetals of D-fructose.

Ascorbic Acid (Vitamin C) L-Ascorbic acid (vitamin C) is synthesized both biochemically and industrially from D-glucose.

Ascorbic Acid (Vitamin C) L-Ascorbic acid is very easily oxidized to L-dehydroascorbic acid. Both are physiologically active and are found in most body fluids. C H 2 O C H 2 O H O H O H O H O oxidation O O reduction H H H O O H O O L-Ascorbic acid (Vitamin C) L-Dehydroascorbic acid

Conformational Formulas for pyranoses, the six-membered ring is more accurately represented as a chair conformation.

Conformational Formulas if you compare the orientations of groups on carbons 1-5 in the Haworth and chair projections of -D-glucopyranose, you will see that in each case they are up-down-up-down-up respectively.

C. Mutarotation Mutarotation: the change in specific rotation that occurs when an  or  form of a carbohydrate is converted to an equilibrium mixture of the two.

25.3 A. Glycosides Glycoside: a carbohydrate in which the -OH of the anomeric carbon is replaced by –OR. methyl -D-glucopyranoside (methyl -D-glucoside).

Glycosides Glycosidic bond: the bond from the anomeric carbon of the glycoside to an -OR group. Glycosides are named by listing the name of the alkyl or aryl group bonded to oxygen followed by the name of the carbohydrate with the ending -e replaced by –ide. methyl -D-glucopyranoside. methyl -D-ribofuranoside.

N-Glycosides The anomeric carbon of a cyclic hemiacetal also undergoes reaction with the N-H group of an amine to form an N-glycoside. N-glycosides of the following purine and pyrimidine bases are structural units of nucleic acids.

N-Glycosides the b-N-glycoside formed between D-ribofuranose and cytosine.

B. Reduction to Alditols The carbonyl group of a monosaccharide can be reduced to an hydroxyl group by a variety of reducing agents, including NaBH4 and H2/M.

Reduction to Alditols other alditols common in the biological world are:

C. Oxidation to Aldonic Acids The -CHO group can be oxidized to –COOH.

Oxidation to Aldonic Acids 2-Ketoses are also oxidized to aldonic acids. under the conditions of the oxidation, 2-ketoses equilibrate with isomeric aldoses. This equilibration occurs in base (NaOH). C H O C H O C H 2 O C - O H C H O C = O ( C H O ) n ( C H O ) n ( C H O ) n C H 2 O C H 2 O C H 2 O A 2-ketose An enediol An aldose

D. Oxidation to Uronic Acids Enzyme-catalyzed oxidation of the terminal -OH group gives a -COOH group and is labelled as a uronic acid. C H O C H O H O H e n z y m - c a t l d o x i H O H C O H H O H H O H O H O H O H H O H H O O H O H H O H H O H C H 2 O C O H D - G l u c o s e D - G l u c r o n i a d ( )

Oxidation to Uronic Acids in humans, D-glucuronic acid is an important component of the acidic polysaccharides of connective tissue. it is also used by the body to detoxify foreign hydroxyl-containing compounds, such as phenols and alcohols; one example is the intravenous anesthetic propofol.

E. Oxidation by HIO4 Periodic acid cleaves the C-C bond of a glycol. + 2 O + I O H C O H H O O H A 1,2-diol P eriodic acid O H C O C O O I + H 3 I O 4 O H C O C O O H I odic acid A cyclic periodic ester

Oxidation by HIO4 it also cleaves -hydroxyketones. and -hydroxyaldehydes.

Oxidation by HIO4 Oxidation of methyl -D-glucoside consumes 2 moles of HIO4 and produces 1 mole of formic acid, which indicates 3 adjacent C-OH groups.

25.4 Disaccharides: A. Sucrose Table sugar, obtained from the juice of sugar cane and sugar beet.

Disaccharides: B. Lactose The principle sugar present in milk. about 5 - 8% in human milk, 4 - 5% in cow’s milk. Galactose-glucose with a b linkage

Disaccharides: C. Maltose Disaccharide from starch or glycogen. From malt, the juice of sprouted barley and other cereal grains. Glucose-glucose with an  linkage

Disaccharides: D. Cellibiose Disaccharide from cellulose. Cellulose is the structural carbohydrate in plants. b - 1 , 4 g l y c o s i d n C H 2 O C H 2 O 4 O O H O O H O H O O H ( b ) O H 1 O H (b ) Glucose-glucose with a b linkage

25.5 Homopolysaccharides: A. Starch Starch is used for energy storage in plants. it can be separated into two fractions; amylose and amylopectin; each on complete hydrolysis gives only D-glucose. Amylose is composed of continuous, unbranched chains of up to 4000 D-glucose units joined by -1,4-glycosidic bonds. amylopectin is a highly branched polymer of D-glucose; chains consist of 24-30 units of D-glucose joined by -1,4-glycosidic bonds and branches created by -1,6-glycosidic bonds.

Homopolysaccharaides: B. Glycogen Glycogen is the reserve carbohydrate for animals. like amylopectin, glycogen is a nonlinear polymer of D-glucose units joined by -1,4- and -1,6-glycosidic bonds bonds. the total amount of glycogen in the body of a well-nourished adult is about 350 g (about 3/4 of a pound) divided almost equally between liver and muscle.

Homopolysaccharaides: C. Cellulose Cellulose is a linear polymer of D-glucose units joined by -1,4-glycosidic bonds. it has an average molecular weight of 400,000 g/mol, corresponding to approximately 2800 D-glucose units per molecule. both rayon and acetate rayon are made from chemically modified cellulose.

25.6 Heteropolysaccharides: A. Acidic Hyaluronic acid: an acidic polysaccharide present in connective tissue, such as synovial fluid and vitreous humor.

B. Acidic Heteropolysaccharides Heparin: its best understood function is as an anticoagulant. following is a pentasaccharide unit of heparin.

Carbohydrates End Chapter 25