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.
C H O C H O H 2 C H O H C = O C H O H C H O H 2 2 Glyceraldehyde D ihydroxyacetone (an aldotriose) (a ketotriose) 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.
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.
C H O C H O C H O C H O O H O H H O O H O H O H O H O H O H C H O H C H O H C H O H C H O H 2 2 2 2 2-Deoxy-D- ribose D,L Monosaccharides • Here are the two most abundant D-aldotetroses and the two most abundant D-aldopentoses in the biological world. H H H H H H H H H H H D-Erythrose D-Threose D-Ribose
C H O C H O C H O H 2 O H O H H O H O H O O H H O O H O H O H O H C H O H C H O H C H O H 2 2 2 D - G l u c o s e D - F r u c t o s e D - G a l a c t o s e D,L Monosaccharides • And the three most abundant hexoses. H H C O H H H H H H H H H
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
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.
C H O H C H O H 2 2 O H O H H O O H L-Ascorbic acid (Vitamin C) 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. H H O O oxidation O O reduction H H O O 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.
C H O H C H O C H O H 2 C - O H C H O H C = O ( C H O H ) ( C H O H ) ( C H O H ) n n n C H O H C H O H C H O H 2 2 2 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). A 2-ketose An enediol An aldose
C H O C H O e n z y m e - c a t a l y z e d O H O H C O O H o x i d a t i o n H O H O H O O H O H O H H O O H O H O H C H O H C O O H 2 D - G l u c o s e D - G l u c u r o n i c a c i d ( a u r o n i c a c i d ) D. Oxidation to Uronic Acids • Enzyme-catalyzed oxidation of the terminal -OH group gives a -COOH group and is labelled as a uronic acid. H H O H H H H H H
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.
O H H O O H - H O 2 O H H O O H O H P eriodic acid O H H I O 3 4 O H O H I odic acid A cyclic periodic ester E. Oxidation by HIO4 • Periodic acid cleaves the C-C bond of a glycol. C O + I C A 1,2-diol C O C O O + I C O C O
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.
Galactose-glucose with a b linkage Disaccharides: B. Lactose • The principle sugar present in milk. • about 5 - 8% in human milk, 4 - 5% in cow’s milk.
Glucose-glucose with an linkage Disaccharides: C. Maltose • Disaccharide from starch or glycogen. From malt, the juice of sprouted barley and other cereal grains.
C H O H - 1 , 4 - Glucose-glucose with a b linkage g l y c o s i d i c b o n d 2 b H O H O C H O H O H 2 O H ( b ) H O O H Disaccharides: D. Cellibiose • Disaccharide from cellulose. Cellulose is the structural carbohydrate in plants. 4 O O O 1 (b )
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