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Chapter 20 Chapter 20 CarbohydratesCarbohydrates
CarbohydratesCarbohydrates
Carbohydrate:Carbohydrate: A polyhydroxyaldehyde or polyhydroxyketone, or a substance that gives these compounds on hydrolysis.
Monosaccharide:Monosaccharide: A carbohydrate that cannot be hydrolyzed to a simpler carbohydrate.• Monosaccharides have the general formula
CCnnHH2n2nOOnn, where nn varies from 3 to 8.
• AldoseAldose:: A monosaccharide containing an aldehyde group.
• KetoseKetose:: A monosaccharide containing a ketone group.
MonosaccharidesMonosaccharidesThe suffix -ose-ose indicates that a molecule is a
carbohydrate.The prefixes tri-tri-, tetratetra, pentapenta, and so forth
indicate the number of carbon atoms in the chain.
Those containing an aldehyde group are classified as aldosesaldoses. .
Those containing a ketone group are classified as ketosesketoses..
There are only two trioses:
MonosaccharidesMonosaccharides There are only two trioses:
◦ Often aldo- and keto- are omitted and these compounds are referred to simply as trioses.
◦ Although “triose” does not tell the nature of the carbonyl group, it at least tells the number of carbons.
HC
HC
H2C OH
O
Glyceraldehydean aldotrioses
OH C
H2C
H2C OH
OH
O
Dihydroxyacetonea ketotrioses
MonosaccharideMonosaccharide Monosaccharides with
◦ three carbons: trioses◦ Five carbons: pentose◦ Six carbons: hexose ◦ And so on …
M M M M M M
Polysaccharide
hydrolysisn M
monosaccharide
MonosacharidesMonosacharides
Figure 12.1 Glyceraldehyde, the simplest aldose, contains one stereocenter and exists as a pair of enantiomers.
EnantiomersEnantiomers Enantiomers: a molecule has a nonsuperimposable
mirror image◦ Chiral molecule – has four different groups
MonosaccharidesMonosaccharidesFischer projection:Fischer projection: A two-dimensional representation for showing the configuration of tetrahedral stereocenters.• Horizontal lines represent bonds projecting forward
from the stereocenter. • Vertical lines represent bonds projecting to the rear.• Only the stereocenter is in the plane.
MonosacharidesMonosacharidesIn 1891, Emil Fischer made the arbitrary assignments of D- and L- to the enantiomers of glyceraldehyde.
• D-monosaccharide:D-monosaccharide: the –OH is attached to the bottom-most assymetric center (the carbon that is second from the bottom) is on the right in a Fischer projection.
HC
HC
H2C OH
O
OH
achiral carbon
D-Glyceraldehyde
MonosacharidesMonosacharides
• L-monosaccharide:L-monosaccharide: the -OH is on the left in a Fischer projection.
CH
HC
H2C OH
O
HO
achiral carbon
L-Glyceraldehyde
D,L-MonosaccharidesD,L-Monosaccharides• The most common D-tetroses and D-pentoses are:
D,L-MonosaccharidesD,L-Monosaccharides The three most common D-hexoses are:
Amino SugarsAmino SugarsAmino sugars contain an -NH2 group in place of an -OH group. • Only three amino sugars are common in nature: D-
glucosamine, D-mannosamine, and D-galactosamine. N-acetyl-D-glucosamine is an acetylated derivative of D-glucosamine.
Cyclic StructureCyclic Structure• Aldehydes and ketones react with alcohols to form
hemiacetalshemiacetals • Cyclic hemiacetals form readily when the hydroxyl and
carbonyl groups are part of the same molecule and their interaction can form a five- or six-membered ring.
EpimersEpimers Diastereomers that differ in configuration at only on
asymmetric center
HC O
OHH
OHH
OH
CH2OH
H
HC O
HHO
OHH
OH
CH2OH
H
D-ribose D-arabinose
1
2
3
4
5
1
2
3
4
5
C2-epimers*dif ferent configuration at C2
HC O
HHO
OHH
HHO
HC O
HHO
HHO
OHHO
CH2OH CH2OH
H OH OHH
C3-epimers*different conf iguration at C3
D-iodose D-talose
Table 20-1 p532
Table 20-2 p532
ExamplesExamples Draw Fisher projections for all 2-ketopentoses.
Which are D-2-ketopentoses, which are L-2-ketopentoses? Prefer to table 12.2 (your textbook) to write their names
Haworth ProjectionsHaworth Projections
• Figure 12.2 D-Glucose forms these two cyclic hemiacetals.
Haworth ProjectionsHaworth Projections
• A five- or six-membered cyclic hemiacetal is represented as a planar ring, lying roughly perpendicular to the plane of the paper.
• Groups bonded to the carbons of the ring then lie either above or below the plane of the ring.
• The new carbon stereocenter created in forming the cyclic structure is called the anomeric carbonanomeric carbon.
• Stereoisomers that differ in configuration only at the anomeric carbon are called anomersanomers.
• The anomeric carbon of an aldose is C-1; that of the most common ketose is C-2.
Haworth ProjectionsHaworth Projections In the terminology of carbohydrate chemistry,
◦ means that the -OH on the anomeric carbon is on the same side of the ring as the terminal -CH2OH.
◦ means that the -OH on the anomeric carbon is on the side of the ring opposite from the terminal -CH2OH.
◦ A six-membered hemiacetal ring is called a pyranosepyranose, and a five-membered hemiacetal ring is called a furanosefuranose because these ring sizes correspond to the heterocyclic compounds furan and pyran.
Haworth ProjectionsHaworth Projections◦ Aldopentoses also form cyclic hemiacetals.◦ The most prevalent forms of D-ribose and other pentoses
in the biological world are furanoses.
◦ The prefix “deoxydeoxy” means “without oxygen.” at C2
Haworth ProjectionsHaworth ProjectionsD-Fructose (a 2-ketohexose) also forms a five-membered cyclic hemiacetal.
ExamplesExamples Give structure of the cyclic hemiacetal formed by
◦ 4-hydroxybutanal
◦ 5-hydroxypentanal
Chair ConformationsChair Conformations• For pyranoses, the six-membered ring is more accurately
represented as a strain-free chair conformationstrain-free chair conformation.
Chair ConformationsChair Conformations• In both Haworth projections and chair conformations, the
orientations of groups on carbons 1- 5 of -D-glucopyranose are up, down, up, down, and up.
O
CH2OH
OH
OH
OH
1
23
4
5
6
OH
HOH2C
HOHO
OH123
45
6
-D-glucose
OH
opposite = trans
Chair ConformationsChair Conformations
O
OH
CH2OH
OH
OH
OH
HOH2C
1
23
4
5
6
HOHO
OHOH
123
45
6
D-glucosesame = cis
ExamplesExamples Which OH groups are in the axial position in β-D-mannopyranose
β-D-idopyranose
MutarotationMutarotation Mutarotation: Mutarotation: The change in specific rotation that
accompanies the equilibration of - and -anomers in aqueous solution.◦ Example: When either -D-glucose or -D-glucose is
dissolved in water, the specific rotation of the solution gradually changes to an equilibrium value of +52.7°, which corresponds to 64% beta and 36% alpha forms.
Formation of GlycosidesFormation of Glycosides• Treatment of a monosaccharide, all of which exist almost
exclusively in cyclic hemiacetal forms, with an alcohol gives an acetal.
Formation of GlycosidesFormation of Glycosides
• A cyclic acetal derived from a monosaccharide is called a glycosideglycoside.
• The bond from the anomeric carbon to the -OR group is called a glycosidic bondglycosidic bond.
• Mutarotation is not possible for a glycoside because an acetal, unlike a hemiacetal, is not in equilibrium with the open-chain carbonyl-containing compound.
Formation of GlycosidesFormation of Glycosides• Glycosides are stable in water and aqueous
base, but like other acetals, are hydrolyzed in aqueous acid to an alcohol and a monosaccharide.
• Glycosides are named by listing the alkyl or aryl group bonded to oxygen followed by the name of the carbohydrate in which the ending -ee is replaced by -ide-ide.
ExamplesExamples Draw a Haworth projection and a chair conformation
for methyl -D-mannopyranoside. Label the anomeric carbon and glycosidic bond