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Chapter 25 Biomolecules: Carbohydrates

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Chapter 25 Biomolecules: Carbohydrates. 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).

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the importance of carbohydrates
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).
chemical formula
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.

D+ Glucose

(C6H12O6)

sources of carbohydrates
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.
classifications of carbohydrates
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
aldose and ketose
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.
fischer projections
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.
minimal fischer projections
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.
stereochemical references
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
d and l sugars
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.
configurations of aldoses
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.
aldotetrose
Aldotetrose
  • Aldotetroses have two chiral centers; therefore, there are two pairs of enantiomers.
  • There and four sterioisomeric aldotertroses.
aldopentose
Aldopentose
  • Aldopentoses have three chiral centers, four enantiomers and eight stereoisomer.
  • Only D enantiomers are shown.
aldohexose
Aldohexose
  • Aldohexose has eight pairs of enantiomers: allose, altrose, glucose, mannose, gulose, idose, galactose, talose.
hemiacetal formation
Hemiacetal Formation
  • Alcohols add reversibly to aldehydes and ketones to form hemiacetals.
hemiacetals in sugar
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
williamson ether synthesis
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.
glycosides
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.
glycoside formation
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.
reduction of monosaccharides
Reduction of Monosaccharides
  • Treatment of an aldose or ketose with NaBH4 reduces it to a polyalcohol (alditol).
oxidation of monosaccharides
Oxidation of Monosaccharides
  • Br2 in water is an effective oxidizing reagent for converting an aldose to an aldonic acid (carboxylic acid).
maltose and cellobiose
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
lactose
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.
sucrose
Sucrose
  • A disaccharide that hydrolyzes to glucose and fructose.
cellulose
Cellulose
  • Cellulose: thousands of D-glucopyranosyl 1-4’-β-glucopyranosides
  • Cellulose molecules form a large aggregate structure held together by hydrogen bonds.
starch
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
glycogen
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.
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