Chapter 18: Carbohydrates
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Transcript of Chapter 18: Carbohydrates
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Chapter 18:Carbohydrates
18.3 - Types of Carbohydrates18.4 and 18.6 - D and L Notations from Fischer Projections
18.8 - Classification of Monosaccharides18.9 - Structures of Some Important Monosaccharides
18.10 and 18.11 - Cyclic Structures of Monosaccharides18.12 - Chemical Properties of Monosaccharides
18.13 - Disaccharides18.14 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
CHO
C OH
CH3
Br
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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
CHOH OH
OHHOHH
CH2OH
D-Ribose
D
C
H OH
HHO
OHH
OHH
CH2OH
O
D-Glucose
H
D
L-Galactose
C
HO H
OHH
OHH
HHO
CH2OH
O
OH
L
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Learning Check Indicate whether each is the D or L isomer:
Ribose Threose Fructose
CHO
HO H
HHO
HHO
CH2OH
CHO
H OH
HHO
CH2OH
CH2OH
O
HHO
OHH
OH H
CH2OH
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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
CHO
HO H
CH2OH
CHO
HO H
OHH
CH2OH
CH2OH
O
HHO
OHH
OH H
OH
CH2OH
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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
CHO
HO H
CH2OH
CHO
HO H
OHH
CH2OH
CH2OH
O
HHO
OHH
OH H
OH
CH2OH
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Identify each as tetrose, pentose or hexose, and as aldose or ketose:
CHO
HO H
HHO
HHO
CH2OH
Learning Check
CH2OH
O
HHO
OHH
OHH
CH2OH
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)
CHO
C
C
OH
C
H
HHO
HO
C
CH2OH
OHH
H
D-Galactose
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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
D-Fructose
CH2OH
C
C
O
C
H
OHH
HO
C
CH2OH
OHH
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Learning CheckDraw 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?
C
O
HCH3 + CH3OH C
OH
HCH3
OCH3
H+
HOC
H
O
= C
OO
H
H
H+
C
OO
H
H
HOC
H
O
= C
OO
H
H
H+
C
OO
H
HCH2OH
HOCH2 HOCH2
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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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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?
OCH2OH
OHOH
OH
OH
OCH2OH
OHOH
OH
OH
and Anomers for D-Glucose
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Mutarotation In solution, -D-glucose is in equilibrium with β-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
aldehyde
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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
CH2OH
C
C
C
C
CH2OH
O
HO H
OHH
OHH
D-Fructose
CH2OH
OH
OH
OH
CH2OHO
CH2OH
OH
OH
CH2OH
OHO
-D-Fructose -D-Fructose
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Write the cyclic form of -D-galactose:Learning Check
C
C
C
C
C
CH2OH
OH
OHH
HO H
HHO
OHH
o
OH
OH
CH2OH
OH
OH
-D-galactose
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Review:Reactions of aldehydes1. Oxidation to form carboxylic acids2. Reduction to form alcohols3. Formation of hemiacetal4. Hemiacetal + alcohol → acetal
Now we’ll 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 Benedict’s text, D-glucose is oxidized to D-
gluconic acid. Glucose is a reducing sugar.
C
C
C
C
C
CH2OH
HO
OHH
HO H
OHH
OHH
C
C
C
C
C
CH2OH
OHO
OHH
HO H
OHH
OHH
+ Cu2+
D-Glucose D-Gluconic acid
+ Cu2O(s)
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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
glycosidic bond
O
OH
OH
CH2OH
O
OH
CH3
HOCH3+
O
OH
OH
CH2OH
OH
OH+ H2O
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Learning CheckWrite the products of the oxidation and reduction of D-mannose.
C
HO H
HHO
OHH
OHH
CH2OH
O
H
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 CheckIdentify 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 CheckIdentify the types of glycosidic bonds in:1) Amylose
2) Glycogen
3) Cellulose
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End of Chapter 18!