chapter 17: carbohydrates 17.1 ─ classes of carbohydrates 17.2 and 17.3 ─ stereochemistry and...
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Chapter 17:Carbohydrates
17.1 ─ Classes of Carbohydrates
17.2 and 17.3 ─ Stereochemistry and Fischer Projections
17.4 and 17.6 ─ Monosaccharides
17.5 ─ Cyclic Structures and Chemical Properties of Monosaccharides
17.7 ─ Disaccharides
17.8 ─ Polysaccharides
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17.1 Carbohydrates Most abundant organic compounds in nature
Composed of the elements C (“carbo”), H and O (“hydrate”)
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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Carbohydrate Structure Polyhydroxy aldehydes or ketones
Examples:
C
CH OH
C
C
H OH
CH2OH
H OH
O H
CH2OH
C O
C
C
H OH
CH2OH
H OH
ribose ribulose
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Types of Carbohydrates Monosaccharides are the simplest carbohydrates
Empirical formula = CH2O
Contain one aldehyde or ketone unit (saccharide)
Disaccharides consist of two monosaccharides Oligosaccharides contain a few monosaccharides (3-10) Polysaccharides contain many monosaccharides
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17.2 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
or stereocenter
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Chiral Molecules Compounds can have more
than one chiral carbon Example:
Glucose contains 6 carbons. How many of these carbons
are chiral?
C
CH OH
C
C
HO H
C
H OH
H O
CH2OH
OHH
1
2
3
4
5
6
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Chiral Molecules
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17.3 Fischer Projections Used to represent carbohydrates Places the most oxidized group (carbonyl)
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
C
CH OH
C
C
HO H
C
H OH
H O
CH2OH
OHH
1
2
3
4
5
6
CHO
H OH
HO H
H OH
CH2OH
OHH
1
2
3
4
5
6
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D and L Notations The —OH on the chiral atom farthest from the carbonyl
group is used to assign a D or L configuration 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
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D and L Monosaccharides
DD 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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17.4 Classification of Monosaccharides
Monosaccharide = unbranched chain of 3 or more carbon atoms one C is carbonyl, other C’s 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 triose, tetrose, pentose or hexose, and as aldose or ketose:
Learning Check
CH2OH
O
HHO
OHH
OHH
CH2OH
A B
CHO
HO H
HHO
HHO
CH2OH
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17.6 Important Monosaccharides Ribose and deoxyribose Glucose Galactose Fructose
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D-Ribose and D-Deoxyribose
Pentoses Used in the synthesis of DNA (deoxyribose) and
RNA (ribose)
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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 C6H12O6
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 (73% sweeter than 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: