Download Chapter 20, Carbohydrates

Chemistry 110
Bettelheim, Brown, Campbell & Farrell
Ninth Edition
Introduction to General,
Organic and Biochemistry
Chapter 20
Carbohydrates
Polyhydroxy Aldehydes & Ketones
Carbohydrates
¾A Carbohydrate is a polyhydroxyaldehyde, a
polyhydroxyketone, or a substance that gives these compounds
on hydrolysis.
¾A Monosaccharide is a carbohydrate that cannot be
hydrolyzed to a simpler carbohydrate. Monosaccharides have
the general formula CnH2nOn, where n varies from 3 to 8.
¾Aldose is a monosaccharide
containing an aldehyde group.
¾Ketose is a monosaccharide
containing a ketone group.
¾Monosaccharides are
classified by their number of
carbon atoms.
Name
Formula
Triose
Tetrose
C3 H6 O3
C4 H8 O4
Pentose
C5 H1 0 O 5
Hexose
C6 H1 2 O 6
Heptose
Octose
C7 H1 4 O 7
C8 H1 6 O 8
Monosaccharides
¾There are only two Trioses: One aldotriose; One ketotriose
H C O
CH2OH
CH2OH
C O
CH2OH
D-Glyceraldehyde
Dihydroxyacetone
H C* OH
¾Glyceraldehyde has a tetrahedral stereocenter and exists as
two enantiomers. Only D-glyceraldehyde occurs naturally.
¾The D- and L- nomenclature proposed by Emil Fischer in
1891 is well entrenched in carbohydrate chemistry. The
enantiomer shown, with the *-OH on the right with +13.5E
rotation is assigned the D- (dextro, on the right) prefix.
¾All natural sugars have the penultimate –OH on the right.
1
Rules for Fischer Projections
¾ Visualize the molecule with its main carbon chain vertical
and with the bonds that hold the chain together projecting to
the rear at each stereocenter. Carbon 1 is at the top, at or
nearest the carbonyl group. You will find that in 3D space the
molecule will roll up into a ring.
¾Mentally flatten the structure, stereocenter by stereocenter,
onto a plane surface. Represent each stereocenter either as the
intersection of two lines or conventionally as a carbon atom.
¾The horizontal lines at a stereocenter actually represent
bonds that project forward, out of the plane of the paper, or
outward from ring.
¾The vertical lines at a stereocenter actually represent bonds
that project rearward, behind the plane. In 3D space the
carbons make a ring and the H & OH groups project outward.
D-Aldose Family of
Monosaccharides
H C O
H C OH
triose
CH2OH
D-Glyceraldehyde
H C O
H C O
tetroses
H C OH
H C OH
CH2OH
D-Erythrose
CH2OH
D-Threose
H C O
pentoses
HO C H
H C OH
H C O
H C OH
H C O
HO C H
H C OH
H C OH
H C OH
H C OH
H C OH
H C OH
CH2OH
CH2OH
CH2OH
D-Arabinose
D-Xylose
D-Ribose
HO C H
H C O
HO C H
HO C H
H C OH
CH2OH
D-Lyxose
All D-aldose sugars are built
D-Aldose Family of
up from glyceraldehyde.
Monosaccharides
pentoses
H C O
HO C H
H C OH
H C O
H C OH
H C OH
H C OH
CH2OH
D-Ribose
H
H
H
H
H
C
C
C
C
C
O
H C O
OH HO C H
OH
H C OH
OH
H C OH
OH
H C OH
CH2OH
D-Allose
CH2OH
D-Altrose
H
H
HO
H
H
C
C
C
C
C
H C OH
CH2OH
D-Arabinose
D-Xylose
O
OH
H
OH
OH
CH2OH
D-Glucose
H
HO
HO
H
H
H C O
HO C H
HO C H
H C O
H C OH
HO C H
H C OH
CH2OH
C
C
C
C
C
O
H
H
OH
OH
CH2OH
D-Mannose
H
H
H
HO
H
C
C
C
C
C
O
OH
OH
H
OH
CH2OH
D-Gulose
H
HO
H
HO
H
H C OH
CH2OH
D-Lyxose
C
C
C
C
C
O
H
OH
H
OH
CH2OH
D-Idose
H
H
HO
HO
H
C
C
C
C
C
O
OH
H
H
OH
CH2OH
D-Galactose
H
HO
HO
HO
H
C O
C H
C H
C H
C OH
CH2OH
D-Talose
hexoses
2
D-Ketose Family of
Monosaccharides
CH2OH
C O
CH2OH
triose
Dihydroxyacetone
CH2OH
C O
All ketose sugars are
built up from
dihydroxyacetone.
tetrose
H C OH
CH2OH
D-Erythrulose
CH2OH
C O
CH2OH
C O
H C OH
H C OH
HO C H
H C OH
CH2OH
D-Xylulose
CH2OH
D-Ribulose
CH2OH
C O
CH2OH
C O
H C OH
H C OH
HO C H
H C OH
CH2OH
C O
H C OH
HO C H
H C OH
CH2OH
H C OH
CH2OH
H C OH
CH2OH
D-Psicose
D-Fructose
D-Sorbose
pentose
CH2OH
C O
HO C H
HO C H
hexose
H C OH
CH2OH
D-Tagatose
Monosaccharides
¾The most common Tetroses
and Pentoses:
H C O
H C OH
H C O
H C O
H C O
H C OH
H C H
HO C H
H C OH
H C OH
H C OH
H C OH
H C OH
H C OH
CH2OH
CH2OH
CH2OH
D-Erythrose
D-Ribose
D-Threose
CH2OH
2-Deoxy-D-ribose
¾The most common Hexoses:
H C O
H
H
HO
HO
H
H C OH
HO C H
H C OH
H C OH
CH2 OH
D-Glucose
CH2OH
C O
C OH
C H
C H
C OH
CH2OH
C O
HO C H
H C OH
H C OH
CH2OH
D-Galactose
D-Fructose
Amino Sugars
¾Amino sugars contain an -NH2 group in place of an -OH group.
¾Only three amino sugars are common in nature:
D-glucosamine, D-mannosamine, and D-galactosamine
H C O
H C NH2
HO C H
H C O
H2N C H
HO C H
H C OH
H C OH
H C OH
H C OH
CH2 OH
D-Glucosamine
CH2 OH
H
H
HO
HO
H
C O
C NH2
C H
C H
C OH
CH2OH
D-Manosamine
D-Galactosamine
(C-2 stereoisomer
of D-glucosamine)
(C-4 stereoisomer
of D-glucosamine)
H C O O
H C NH CCH3
HO C H
H C OH
H C OH
CH2 OH
N-AcetylD-glucosamine
¾The last is an amido derivative of D-glucosamine.
3
Addition Reactions to the Carbonyl Group
¾When an alcohol and aldehyde (ketone) functionality are
present in the same molecule and a 5- or 6-membered ring can
form, the cyclic hemiacetal form is predominate.
O
C
*
HO
OH
*
O
cyclic hemiacetal
H
HO
O
HO
C
*
H
* O
H
cyclic hemiacetal
Cyclic Forms of Monosaccharides
Hemiacetals
¾These are called Haworth Projections.
HOCH2 O OH
CH2OH
*
HO
frontside
*C O
CH2OH
β-D-fructose OH
HO C H
backside
¾β
means
the OH is on the same side
H C OH
frontside
of the ring as the CH2OH at C-4.
H C OH
HOCH2 O CH2OH
backside
CH2OH
*
HO
D-fructose
OH
α-D-fructose
¾α-D-fructofuranose is
commonly abbreviated to
α-D-fructose.
OH
¾α means the OH is on the opposite
side of the ring as the CH2OH at C-4.
Cyclic Forms of D-Glucose - Hemiacetals
CH2OH
β-D-glucopyranose
backside
*
H C O
OH
frontside
OH
H C OH
HO C H
O
¾These are called
Haworth
OH Projections.
*
OH
CH2OH
¾β means the OH
is on the same side
of the ring as the
CH2OH at C-5.
¾α means the OH
is on the opposite
side of the ring as
CH2OH
the CH2OH at C-5.
*
OH
Left – Up : Right - Down OH
OH ¾Pyranose means
a 6-membered
OH
α-D-glucopyranose
ring.
¾α- D-glucopyranose is commonly abbreviated to α-D-glucose.
H C OH
H C OH
O
4
Understanding Haworth Projections
¾A five- or six-membered cyclic hemiacetal is represented as
a planar ring, lying nearly horizontally, i.e. roughly
perpendicular to the plane of the paper.
¾Groups bonded to the carbons of the ring then lie either
above or below the plane of the ring.
¾The new carbon stereocenter created in forming the cyclic
hemiacetal structure is called an anomeric carbon.
¾Stereoisomers that differ in configuration only at the
anomeric carbon are called anomers.
¾The anomeric carbon of an aldose is C-1; that of the most
common ketoses is C-2.
¾aldopentoses also form cyclic hemiacetals.
¾the most prevalent forms of D-ribose and other pentoses in
the biological world are furanoses.
Cyclic Forms of Monosaccharides - Mutarotation
backside
*
H C O
frontside
H C OH
CH2OH
O
OH
OH
CH2OH
O
OH
HO C H
*
*
OH
OH
H C OH
OH
H C OH
OH
D-α-glucose
20
o
α D = 113
HOH2C
HO
HO
O
OH
CH2OH
OH
< .05%
D-β-glucose
20
o
α D = 19
36% α
α
20
D =
HOH2C
HO
HO
*
OH
64% β
o
52
HOH2C
HO
* H HO
OH O
OH
O
OH
OH *
Cyclic Forms of Ribose & Deoxyribose
HOCH2 O
OH
β-D-ribose
*
H
*C O
OH OH
H C OH
H C OH
H C OH
CH2OH
HOCH2 O
β-D-2-deoxyribose
OH
H
*
The ring
is locked!
OH H
5
Reduction of Monosaccharides - Alditols
¾The carbonyl group of a monosaccharide can be reduced to
an hydroxyl group by a variety of reducing agents, including
NaBH4 and H2 in the presence of a transition metal catalyst.
¾the reduction product is called an alditol.
alditol.
¾D-Sorbitol is
H
found in berries,
H C O
H C OH
plums, apples,
H C OH
H C OH
seaweed, & algae.
HO C H
HO C H
NaBH4
H C OH
H C OH
H C OH
H C OH
CH2 OH
¾It is used to make
candies and as a
sugar substitute for
diabetics.
CH2 OH
D-Glucose
D-Glucitol
(D-Sorbitol)
¾It is 60% as sweet
as sucrose.
¾Other common alditols are erythritol, mannitol and xylitol.
Oxidation of Monosaccharides – Aldonic Acids
¾The aldehyde group of an aldose is oxidized under basic
conditions to a carboxylate anion.
¾The oxidation product is called an aldonic acid.
¾Any carbohydrate that reacts with an oxidizing agent to form
an aldonic acid is classified as a reducing sugar (it reduces the
oxidizing agent).
O
O¾Any sugar which
C
H C O
mutarotates will be
H C OH
H C OH
a reducing sugar
HO C H
HO C H
[O]
because the cyclic
H C OH
H C OH
OHhemiacetal will
H C OH
H C OH
open to the chain
CH2 OH
CH2 OH
form during the
D-Glucose
D-Gluconate
interconversion.
D-Gluconic Acid anion
¾Even 2-ketoses can form an aldehyde by tautomerism! See text!
Oxidation of Monosaccharides – Uronic Acids
H
C
O
H
C
O
H
C
OH
H
C
OH
HO
C
H
HO
C
H
H
C
OH
H
C
OH
H
C
OH
H
C
OH
enzyme
oxidation
CH 2 OH
C
OH
O
D-Glucuronic Acid
D-Glucose
¾D-Glucuronic Acid is formed by enzyme oxidation of D-glucose.
It is widely distributed in the plant and animal world. In humans
it is found in connective tissues and is used by the liver to detoxify
foreign phenols so they can be excreted in the urine.
H2COH
COOH
HO
O
HO
O
enzyme
HO
OH
HO
β-D-Glucose
oxidation
HO
OH
HO
β-D-Glucuronic Acid
6
Monosaccharide Hemiacetals Form Acetals
O H
O
R C O R' + H O R"
H
hemiacetal
R"
R C O R' + H O H
H
acetal
alcohol
water
Acetals of Sugars are called Glycosides
CH2OH
CH2OH
O
O
H OH
+
H3C OH
*
*
OH
OH
OH
O CH3
α-D-glucose
methyl α-glucoside
OH
OH
The acetal bond at the anomeric carbon is called a glycosidic bond.
OH
OH
Both alpha and beta Glycosides Form
CH2OH
O
CH2OH
O
OH
*
OH
OH
H3C OH
methyl β-glucoside
O CH3
*
OH
OH
+
H OH
β-D-glucose
OH
OH
Both reactions require an acid catalyst or an enzyme.
CH2OH
CH2OH
O
O
H OH
+
H3C OH
*
*
OH
OH
OH
O CH3
α-D-glucose
methyl α-glucoside
OH
OH
Mutarotation is not possible here; the α & β are not in equilbrium.
OH
OH
Disaccharides are Acetals - Maltose
CH2OH
CH2OH
O
O
OH
*
*
OH
OH OH
OH
OH
OH
OH
It is this orientation of
the OH which makes
this maltose "β"
H OH
+
β-maltose
α-D-glucose
β-D-glucose
CH2OH
CH2OH
This glycoside, maltose, or
O
O
“malt sugar” found in
OH
germinating seeds has an
*
*
OH
α-glycosidic linkage.
OH
O
OH
Because it has one
anomeric center free it is a
OH
OH
reducing sugar.
7
Disaccharides are Acetals - Lactose
CH2OH
O
CH2OH
OH
OH
*
OH
O
OH
OH
*
OH
It is this orientation of
the OH which makes
this lactose "β"
H OH
+
CH2OH
OH
β-D-glucose
OH
O
CH2OH
OH
O
OH
β-D-galactose
Note the glycosidic bond here is β
like in cellulose. This is a factor
in lactose intolerance. Many adult
people cannot digest this sugar.
O
*
OH
*
OH
OH
β-lactose
OH
Disaccharides are Acetals - Sucrose
CH2OH
O
Because there is no free
hemiacetal to open sucrose
is a non-reducing sugar.
HOCH2 O
*
*
OH HO
OH
OH
OH
α-D-glucose
HO
CH2OH
OH
β-D-fructose
H OH
+
sucrose
O
HOCH2 O
* *
HO
O
CH2OH
CH2OH
Note that in sucrose both
anomeric carbons are locked
in the glycosidic bond.
OH
OH
OH
OH
A Non-nutritive Sweetener Derived from Sucrose
¾Sucralose has an intensely sweet
taste, much greater than sucrose.
CH2OH
O
OH
OH
HOCH2 O
* *
HO
O
OH
¾Toxicity is quite low compared to
dosage.
CH2OH
Sucralose
OH
CH2OH
Sucrose
O
Cl
IUPAC: 1,6-Dichloro-1,6dideoxy-β-D-fructofuranosyl4-chloro-4-deoxy-α-Dgalactopyranoside
ClCH2 O
* *
O
OH
LD50 = 16 g/kg
"Splenda"
OH
HO
CH2Cl
OH
8
This Disaccharide is Not Digestible
CH2OH
O
CH2OH
OH
*
OH
O
It is this orientation of
the OH which makes
this cellobiose "β"
H OH
+
CH2OH
OH
OH
OH
*
OH
O
β-D-glucose
OH
CH2OH
O
OH
β-D-glucose
*
OH
O
Like Lactose the glycosidic
OH
bond is β. Only a few kinds of
bacteria can digest this sugar.
OH
*
OH
OH
β-cellobiose
OH
Polysaccharides - Starch
¾Polysaccharide: a carbohydrate consisting of large numbers of
monosaccharide units joined by glycosidic bonds.
¾Starch: a polymer of D-glucose is edible/digestible.
Starch consists of two components:
- amylose - unbranched chains having up to 4000 glucose units.
- amylopectin has branches of 24-30 glucose units at these points.
CH2OH
CH2OH
CH2OH
CH2OH
O
O
O
O
*
O
OH
OH
OH
α-D-glucose
*
O
OH
OH
α-D-glucose
*
OH
*
OH
OH
O
OH
OH
α-D-glucose
α-D-glucose
(repeat unit - number up to 4000)
Polysaccharides - Cellulose
CH2OH
Structural unit in plant fibers such as cotton
and cellulose.
CH2OH
O
O
CH2OH
β-D-glucose
O
O
CH2OH
O
O
OH
OH
OH
*
*
OH
β-D-glucose
OH
OH
O
*
OH
*
OH
OH
β-D-glucose
β-D-glucose
(repeat unit - up to 2200)
OH
The β 1-4 glycosidic bond is digestible by only a few organisms.
9