Chapter 18: Carbohydrates

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Chapter 18Carbohydrates

• 18.3 - Types of Carbohydrates
• 18.4 and 18.6 - D and L Notations from Fischer
  Projections
• 18.8 - Classification of Monosaccharides
• 18.9 - Structures of Some Important
  Monosaccharides
• 18.10 and 18.11 - Cyclic Structures of
  Monosaccharides
• 18.12 - Chemical Properties of Monosaccharides
• 18.13 - Disaccharides
• 18.14 and 18.16 - Polysaccharides

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Carbohydrates

• Most abundant organic compounds in nature
• Produced by photosynthesis in plants
• Composed of the elements C, H and O
• Also called saccharides, which means sugars
• A major source of energy from our diet
• A source of C for synthesis of other biomolecules
• Can be linked to cell membranes or proteins

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Types of Carbohydrates

• Monosaccharides are the simplest carbohydrates
• Empirical formula CH2O
• Oligosaccharides contain a few monosaccharides
  (2-10)
• Disaccharides consist of two monosaccharides
• Polysaccharides contain many monosaccharides

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Chiral Molecules

• Chiral molecules
• Have handedness
• Nonsuperimposable on mirror images
• Ex hand, shoe
• Achiral molecules
• Do not have handedness
• Ex glass, spoon
• Chiral molecules contain carbons with 4 different
  groups
• Called chiral carbons or chiral center

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Fischer Projections

• Used to represent carbohydrates
• Places the most oxidized group at the top
• All chiral carbons are represented as
  intersection of lines
• C is not shown
• Implied 3D arrangement of atoms
• Horizontal lines for bonds that come forward
• Vertical lines for bonds that go back

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D and L Notations

• By convention, the letter L is assigned to the
  structure with the OH on the left
• The letter D is assigned to the structure with
• OH on the right

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D and L Monosaccharides

• The OH on the chiral atom farthest from the
  carbonyl group is used to assign the D or L
  configuration

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Learning Check

• Indicate whether each is the D or L
  isomer
• Ribose Threose
  Fructose

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Classification of Monosaccharides

• Monosaccharide unbranched chain of 3-8 C atoms
  one is carbonyl, others attached to -OH
• Aldose
• monosaccharide with an aldehyde group (1st
  carbon)
• Ketose
• monosaccharide with a ketone group (2nd
  carbon)
• Aldose Aldose
  Ketose

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Monosaccharides

• Monosaccharides are also classified according to
  the number of carbon atoms
• A triose has three carbons a tetrose has four
  carbons a pentose has five carbons and a hexose
  has six carbons.
• triose tetrose
  hexose
• aldotriose aldotetrose
  ketohexose

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Learning Check

• Identify each as tetrose, pentose or hexose, and
  as aldose or ketose

A B
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D-Glucose

• Most common hexose
• Found in fruits, corn syrup, and honey
• An aldohexose with the formula C6H12O6
• Known as blood sugar in the body
• Building block for many disaccharides and
  polysaccharides

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D-Galactose

• Aldohexose
• Differ from D-glucose at C4
• C1 is at the top
• Not found in the free form in nature
• Obtained from lactose, a disaccharide (milk
  products)
• Important in cellular membranes in CNS (brain
  sugar)

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D-Fructose

• Ketohexose C6H12O6
• Differ from glucose at C1 and C2 (location of
  carbonyl)
• The sweetest carbohydrate (2x sucrose)
• Found in fruit juices and honey
• Formed from hydrolysis of sucrose
• Converts to glucose in the body

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Learning Check

• Draw the structure of D-fructose

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Hemiacetal Review

• What is a hemiacetal?
• How is a hemiacetal formed?
• What if the alcohol and carbonyl are attached?

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Hexose hemiacetals

• Favor formation of 5- or 6-membered rings
• Hydroxyl group on C5 reacts with the aldehyde or
  ketone
• The Haworth structure can be written from the
  Fischer projection
• The cyclic structure of a D-isomer has the last
  CH2OH group located above the ring
• The OH group on the left (C3) is drawn up
• The OH groups on the right (C2, C4) are drawn
  down

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? and ? Anomers for D-Glucose

• Anomers are isomers which differ in placement of
  hydroxyl on C1
• The OH is drawn down for the ?-anomer, and up
  for the ?-anomer
• ?-D-Glucose ?-D-Glucose
• Mashed potatoes or mashed paper?

?
?
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Mutarotation

• In solution, ?-D-glucose is in equilibrium with
  ß-D-glucose
• Mutarotation involves the conversion of the
  cyclic anomers into the open chain
• At any time, there is only a small amount of
  open chain

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Cyclic Structure of Fructose

• As a ketohexose, fructose forms a cyclic
  structure when the OH on C5 reacts with the
  ketone on C2
• Result is 5-atom ring
• Anomeric carbon is C2

?-D-Fructose
?-D-Fructose
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Learning Check

• Write the cyclic form of ?-D-galactose

?-D-galactose
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ReviewReactions of aldehydes

• Oxidation to form carboxylic acids
• Reduction to form alcohols
• Formation of hemiacetal
• Hemiacetal alcohol ? acetal
• Now well see all of these with monosaccharides

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1. Oxidation of Monosaccharides

• Monosaccharides are reducing sugars if their
  carbonyl groups oxidize to give carboxylic acids.
  
• In the Benedicts text, D-glucose is oxidized
  to D-gluconic acid. Glucose is a reducing sugar.

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2. Reduction of Monosaccharides

• The reduction of the carbonyl group produces
  sugar alcohols, or alditols
• D-Glucose is reduced to D-glucitol (also called
  sorbitol)

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3. Monosaccharides alcohol

• Formation of hemiacetal (cyclic structures)

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4. Monosaccharide hemiacetals alcohol

• When a cyclic monosaccharide reacts with an
  alcohol
• A glycoside is produced (acetal)
• The bond is a glycosidic bond or glycosidic
  linkage
• ?-D-Glucose Methanol
  Methyl-?-D-glucoside

H2O
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Learning Check

• Write the products of the oxidation and
  reduction of D-mannose.

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Disaccharides

• A disaccharide consists of two monosaccharides
• Disaccharide Monosaccharides
• Maltose H2O Glucose Glucose
• Lactose H2O Glucose Galactose
• Sucrose H2O Glucose Fructose

H
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Maltose

• Malt sugar
• A disaccharide in which two D-glucose molecules
  are joined by an ?-1,4-glycosidic bond
• Obtained from starch
• Used in cereals, candies, and brewing
• A reducing sugar (has a hemiacetal)

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Lactose

• Milk sugar
• Composed of galactose and glucose linked by a
  ?-1,4-glycosidic bond
• Lactose intolerance
• A reducing sugar

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Sucrose

• Table sugar
• Is composed of glucose and fructose molecules
  joined by ?,?-1,2-glycosidic bond
• Has no isomers because mutarotation is blocked
• Not a reducing sugar (no hemiacetal)

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Sweetness of Sweeteners

• Sugars and artificial sweeteners differ in
  sweetness
• Each sweetener is compared to sucrose (table
  sugar), which is assigned a value of 100

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Learning Check

• Identify the monosaccharides in lactose, maltose,
  and sucrose as glucose, fructose, and/or
  galactose
• A. Lactose
• B. Maltose
• C. Sucrose

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Polysaccharides

• Polysaccharides are polymers of monosaccharides
• Complex carbohydrates
• Important polysaccharides of D-glucose are
• Starch (Amylose and Amylopectin)
• Glycogen
• Cellulose

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Starch and Glycogen

• Storage polysaccharides
• Form monosaccharides used for energy
• Starch
• Plants
• Amylose is a continuous chain of glucose
  molecules linked by ?-1,4 glycosidic bonds.
• Amylopectin is a branched chain of glucose
  molecules linked by ?-1,4- and ?-1,6-glycosidic
  bonds.
• Glycogen
• Humans, animals
• Similar to amylopectin, but more highly branched.

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Structures of Amylose and Amylopectin

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Cellulose

• Structural polysaccharide
• Plant cell walls (cellulose) and animal
  exoskeletons (chitin)
• Cellulose is a polymer of glucose molecules
  linked by ?-1,4-glycosidic bonds
• Enzymes in saliva can hydrolyze ?-1,4-glycosidic
  bonds in starch, but not ?-1,4-glycosidic bonds
  in cellulose

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Learning Check

• Identify the types of glycosidic bonds in
• 1) Amylose
• 2) Glycogen
• 3) Cellulose

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End of Chapter 18!