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Chapter 8 (part 1)
Carbohydrates
Carbohydrates
• Most abundant class of biological
molecules on Earth
• Originally produced through CO2
fixation during photosynthesis
Roles of Carbohydrates
• Energy storage (glycogen,starch)
• Structural components
(cellulose,chitin)
• Cellular recognition
• Carbohydrate derivatives include
DNA, RNA, co-factors,
glycoproteins, glycolipids
Carbohydrates
• Monosaccharides (simple sugars)
cannot be broken down into simpler
sugars under mild conditions
• Oligosaccharides = "a few" - usually
2 to 10
• Polysaccharides are polymers of the
simple sugars
Monosaccharides
• Polyhydroxy ketones (ketoses)
and aldehydes (aldoses)
• Aldoses and ketoses contain
aldehyde and ketone
functions, respectively
• Ketose named for “equivalent
aldose” + “ul” inserted
• Triose, tetrose, etc. denotes
number of carbons
• Empirical formula = (CH2O)n
O
H
C
CH2OH
H
C*
OH
HO
C*
H
H
C*
OH
CH2OH
D-ribose
C
O
HO
C*
H
H
C*
OH
CH2OH
D-ribulose
Monosaccharides are chiral
• Aldoses with 3C or more and
ketoses with 4C or more are
chiral
• The number of chiral carbons
present in a ketose is always
one less than the number
found in the same length
aldose
• Number of possible
steroisomers = 2n (n = the
number of chiral carbons)
O
H
C
CH2OH
H
C*
OH
HO
C*
H
H
C*
H
C*
C
O
HO
C*
H
OH
H
C*
OH
OH
H
C*
OH
CH2OH
CH2OH
D-glucose
D-fructose
Stereochemistry
Enantiomers
O
H
O
C
O
H
C
HO
C*
H
H
C*
OH
HO
C*
HO
C*
Epimers
Diastereomers
O
H
C
C
C*
OH
HO
C*
H
H
C*
OH
HO
C*
H
HO
C*
H
HO
C*
H
H
C*
OH
H
C*
OH
HO
H
H
C*
OH
H
C*
OH
H
CH2OH
L-glucose
D-glucose
O
H
C
H
CH2OH
O
H
H
C
H
C*
OH
HO
C*
H
H
HO
C*
H
HO
C*
H
C*
H
H
C*
OH
H
C*
OH
C*
OH
H
C*
OH
H
C*
OH
CH2OH
CH2OH
D-mannose
D-galactose
CH2OH
D-glucose
CH2OH
D-mannose
•Enantiomers = mirror images
•Pairs of isomers that have opposite configurations at
one or more chiral centers but are NOT mirror images
are diastereomers
•Epimers = Two sugars that differ in configuration at
only one chiral center
Cyclization of aldose and ketoses
introduces additional chiral center
• Aldose sugars (glucose) can cyclize to form a
H
cyclic hemiacetal H
NEW CHIRAL
ALDEHYDE
O
O
C
H
ALCOHOL
H
R1
C*
R1
O
R2
O
H
CARBON
R2
HEMIACETAL
• Ketose sugars (fructose) can cyclize to form a
H
H
cyclic hemiketal
NEW CHIRAL
KETONE
O
O
CARBON
C
R
ALCOHOL
R
R1
R1
O
R2
O
H
C*
R2
HEMIKETAL
Haworth Projections
O
H
-OH up = beta
-OH down = alpha
C1
H
C2
OH
HO
C3
H
H
C4
OH
H
C5
OH
CH2OH
6
5
4
1
3
2
Anomeric carbon
(most oxidized)
For all non-anomeric carbons, -OH groups
point down in Haworth projections if
pointing right in Fischer projections
Monosaccharides can cyclize to
form Pyranose / Furanose forms
a = 64%
b = 36%
a = 21.5%
b = 58.5%
a = 13.5%
b = 6.5%
Conformation of Monosaccharides
Pyranose sugars not planar molecules, prefer to be in
either of the two chair conformations.
Reducing Sugars
• When in the uncyclized
form, monosaccharides
act as reducing agents.
• Free carbonyl group from
aldoses or ketoses can
reduce Cu2+ and Ag+ ions
to insoluble products
Derivatives of
Monosaccharides
Sugar Phosphates
Deoxy Acids
Amino Sugars
Sugar alcohols
Monosaccharide structures
you need to know
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•
•
•
•
Glucose
Fructose
Ribulose
Glyceraldehyde
Dihydroxyacetone
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