chapter 25 biomolecules: carbohydrates. 2 the importance of carbohydrates carbohydrates are…...
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Chapter 25Biomolecules: Carbohydrates
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The Importance of Carbohydrates
• Carbohydrates are…– widely distributed in nature.– key intermediates in metabolism (sugar).– structural components of plants (cellulose).– key components of industrial products (wood,
fibers).– key components of food sources (sugar,
flour).
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Chemical Formula
• Carbohydrates are highly oxidized.– They have approximately as many oxygen atoms as
carbon atoms.
• Carbons of carbohydrates are usually bond to an alcohol and hydrogen atom; therefore, the empirical formula is roughly (C(H2O))n.
O
H
HO
H
HO
H
OHOHH H
OH
D+ Glucose(C6H12O6)
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Sources of Carbohydrates
• Glucose is produced in plants from CO2 and H2O via photosynthesis.
• Plants convert glucose into other small sugars and polymers (cellulose, starch).
• Dietary carbohydrates provide the major source of energy required by organisms.
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Classifications of Carbohydrates
• Monosaccharide: simple sugars that can not be converted into smaller sugars by hydrolysis
• Carbohydrate (Oligosaccharide, Polysaccharide): two or more simple sugars connected as acetals
• Sucrose: disaccharide of two monosaccharides (glucose linked to fructose)
• Cellulose: polysaccharide of several thousand glucose units connected by acetal linkages
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Aldose and Ketose
• The prefixes aldo- and keto- identify the nature of the carbonyl group.– Aldo: carbonyl is located at the end of the
chain– Keto: carbonyl is located within the chain
• The suffix -ose denotes a carbohydrate.
• The number of carbons is indicated by the root.
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Aldose and Ketose
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Fischer Projections
• Carbohydrates have multiple chiral centers.
• A chiral center carbon is projected into the plane of the paper and other groups are drawn as horizontal and vertical lines.
• The oxidized end of the molecule is always “up” on the paper.
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Fischer Projections
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Minimal Fischer Projections
• In order to work with the structure of an aldose more easily, only the essential components are shown.
• An alcohol is designated by a “-” and a carbonyl is designated by an “↑”.
• The terminal OH in the CH2OH is not shown.
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Stereochemical References
• The reference compounds for stereochemistry are the two enantiomers of glyceraldehyde (C3H6O3).
• The stereochemistry depends on the hydroxyl group attached to the chiral center farthest from the oxidized end of the sugar.– D: hydroxyl group is on the right– L: hydroxyl group is on the left
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Stereochemical References
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The “D” Sugar Family
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D and L Sugars
• The two enantiomers of glyceraldehyde were first identified by their opposite rotation of plane polarized light.
• Naturally occurring glyceraldehyde rotates light in a clockwise rotation and is denoted as “+”.
• The enantiomer rotates light counterclockwise and is denoted as “-”.
• The direction of the rotation of light does not correlate to structural features.
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Configurations of Aldoses
• Because R and S designations are difficult to work with when multiple chiral centers are present, the D,L designations are used with aldoses.
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Aldotetrose
• Aldotetroses have two chiral centers; therefore, there are two pairs of enantiomers.
• There and four sterioisomeric aldotertroses.
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Aldopentose
• Aldopentoses have three chiral centers, four enantiomers and eight stereoisomer.
• Only D enantiomers are shown.
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Aldohexose
• Aldohexose has eight pairs of enantiomers: allose, altrose, glucose, mannose, gulose, idose, galactose, talose.
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Hemiacetal Formation
• Alcohols add reversibly to aldehydes and ketones to form hemiacetals.
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Hemiacetals in Sugar
• Intramolecular nuclephillic addition creates a cyclic hemiacetal in sugars.
• Five- and six-membered rings are stable.• The formation of a cyclic hemiactal creates an additional
chiral center creating two diasteromeric forms called anomer, which are designated α and β.– α: the OH at the anomer center is on the same side as the
hydroxyl that determines D,L naming in the Fischer projection– β: the OH at the anomer center is on the opposite side of the
hydroxyl that determines D,L naming in the Fischer projection
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Fischer Projections of Anomers
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Williamson Ether Synthesis
• Treatment with a alkyl halide in the presence of a base
• Silver oxide is used as a catalyst for base-sensitive compounds.
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Glycosides
• Carbohydrate acetals are named by sighting the alkyl group and replacing the -ose ending of the sugar with -oside.
• Glycosides are stable in water; therefore, they require an acid catalyst for hydrolysis.
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Glycoside Formation
• Treatment of a monosaccharide hemiacetal with an alcohol and an acid catalyst yields an acetal in which the anomeric -OH has been replace with an -OR group.
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Reduction of Monosaccharides
• Treatment of an aldose or ketose with NaBH4 reduces it to a polyalcohol (alditol).
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Oxidation of Monosaccharides
• Br2 in water is an effective oxidizing reagent for converting an aldose to an aldonic acid (carboxylic acid).
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Maltose and Cellobiose
• Maltose: two D-glycopyranose units with a 1,4’-α-glycoside bond– Formed from the hydrolysis of starch
• Cellobiose: two D-glycopyranose units with a 1,4’-β-glycoside bond– Formed from the hydrolysis of cellulose
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Lactose
• Lactose: 1,4-D-galactopyranosyl-D-glucopyranoside
• Lactose is a disaccharide that occurs naturally in milk.
• Lactose is cleaved during digestion to form glucose and galactose.
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Sucrose
• A disaccharide that hydrolyzes to glucose and fructose.
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Cellulose
• Cellulose: thousands of D-glucopyranosyl 1-4’-β-glucopyranosides
• Cellulose molecules form a large aggregate structure held together by hydrogen bonds.
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Starch
• Starch: 1,4--glupyranosyl-glucopyranoside polymer
• Starch is digested into glucose
• Starch is made of two components– Amylose
• insoluble in water – 20% of starch
– Amylopectin• soluble in water – 80% of starch
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Glycogen
• Glycogen is a polysaccharide that serves the same energy storage function in animals that starch does in plants.
• Glycogen is highly branched and contain up to 100,000 glucose units.