Chapter 25 Biomolecules: Carbohydrates

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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.

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 a and ß.
• a the OH at the anomer center is on the same
  side as the hydroxyl that determines D,L naming
  in the Fischer projection
• ß 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-a-glycoside bond
• Formed from the hydrolysis of starch
• Cellobiose two D-glycopyranose units with a
  1,4-ß-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-ß-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.