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.