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Carbohydrates
Chapter 12
Educational Goals
1.  Given a Fischer projection of a monosaccharide, classify it as either
aldoses or ketoses.
2.  Given a Fischer projection of a monosaccharide, classify it by the
number of carbons it contains.
3.  Given a Fischer projection of a monosaccharide, identify it as a Dsugar or L-sugar.
4.  Given a Fischer projection of a monosaccharide, identify chiral
carbons and determine the number of stereoisomers that are
possible.
5.  Identify four common types of monosaccharide derivatives.
6.  Predict the products when a monosaccharide reacts with a reducing
agent or with Benedict’s reagent.
7.  Define the term anomer and explain the difference between α and β
anomers.
8.  Understand and describe mutarotation.
Educational Goals
9.  Given its Haworth projection, identify a monosaccharide either a
pyranose or a furanose.
10. Identify the anomeric carbon in Haworth structures.
11. Compare and contrast monosaccharides, disaccharides,
oligosaccharides, and polysaccharides.
12. Given the structure of an oligosaccharide or polysaccharide,
identify the glycosidic bond(s) and characterize the glycosidic
linkage by the bonding pattern [for example: β(1⟶4)].
13. Given the Haworth structures of two monosaccharides, be able to
draw the disaccharide that is formed when they are connected by a
glycosidic bond.
14. Understand the difference between homopolysaccharides and
heteropolysaccharides.
15. Compare and contrast the two components of starch.
16. Compare and contrast amylopectin and glycogen.
17. Identify acetal and hemiacetal bonding patterns in carbohydrates.
Introduction to Carbohydrates
•  Carbohydrates are also known as sugars.
•  Carbohydrates are an abundant biomolecule.
– More than 50% of the carbon in organic
compounds is found in carbohydrates
– Plants use photosynthesis to store energy in
glucose, a simple sugar
6 CO2 + 6 H2O + Energy à C6H12O6 + 6 O2
Introduction to Carbohydrates
•  Carbohydrates are a large class of naturally occurring
polyhydroxy aldehydes and ketones.
•  Monosaccharides (also known as simple sugars) are the
simplest carbohydrates containing 3-7 carbon atoms. A
sugar containing:
–  an aldehyde is known as an aldose
–  a ketone is known as a ketose
Classification of Carbohydrates
•  Carbohydrates are grouped into 3 classes:
– Monosaccharides are the simplest sugars and serve
as the building blocks of larger molecules
Example: Glucose
– Oligosaccharides contain 2-10 monosaccharides
bonded together (building block = residue)
Example: Sucrose
– Polysaccharides contain more than 10 residues
Example: Complex Carbohydrates
Monosaccharides
•  Monosaccharides are polyhydroxy ketones
or aldehydes with 3 or more carbons.
Naming Monosaccharides
•  Carbohydrate nomenclature is unique to
“sugar chemistry” — we do not name
monosaccharides using the IUPAC rules.
•  Monosaccharide names end in “ose”
•  Monosaccharides can be classified by:
• Carbonyl group: aldose or ketose
• Number of carbons: triose, tetrose, etc.
• Both: aldotriose, ketotriose, and so on…
Naming Monosaccharides
Naming Monosaccharides
Examples:
You try it:
Name each of the following monosaccharides as an
aldose or ketose & according to its number of C atoms.
Stereoisomers in Carbohydrates
•  Carbohydrates are chiral molecules since they have
carbon atoms carrying four different groups.
•  The simplest three carbon sugar is glyceraldehyde.
This sugar exists as a pair of enantiomers.
Stereoisomers in Carbohydrates
•  Enantiomers have the same physical properties except they
behave differently in the way they rotate polarized light
and the way they are affected by catalysts.
Stereoisomers in Carbohydrates
•  Remember: Compounds with n chiral carbon atoms has a
maximum of 2n possible stereoisomers and half that many
pairs of enantiomers (mirror images).
• This aldotetrosose, has 2 chiral carbon atoms and a total of 22
= 4 possible stereoisomers (2 pairs of enantiomers).
The D and L Families of Sugars:
Drawing Sugar Molecules
•  Fischer Projections represent three-dimensional
structures of stereoisomers on a flat page.
•  A chiral carbon atom is represented in the Fisher
projection as the intersection of two crossed lines.
The D and L Families of Sugars:
Drawing Sugar Molecules
•  Bonds that point above the page are shown as
horizontal lines.
The D and L Families of Sugars:
Drawing Sugar Molecules
Bonds that curve behind and below the page are
shown as vertical lines.
Fischer Projections of Sugar Molecules
In a Fischer projection, the aldehyde or ketone
carbonyl group of a monosaccharide is always
placed toward the top of the page.
Fischer Projections of Sugar Molecules
Example:
Glyceraldehyde
Conventional
representation
Fischer Projections of Sugar Molecules
Example:
Glucose
Fischer Projections of Sugar Molecules
Monosaccharides are divided into two families:
D form and L form sugars.
•  D: the –OH group on the chiral C furthest from the C=O
comes out of the plane of paper and points to the right.
•  L: the –OH group on the chiral C furthest from the C=O
comes out of the plane of paper and points to the left.
Fischer Projections of Sugar Molecules
D: the –OH group on the chiral C furthest from the C=O comes out
of the plane of paper and points to the right.
L: the –OH group on the chiral C furthest from the C=O comes out
of the plane of paper and points to the left.
Monosaccharides
•  We will briefly survey some important pentoses and
hexoses, and their derivatives.
D-glucose, also called
dextrose or blood sugar, is
the most important
monosaccharide in human
metabolism.
Monosaccharides
D-fructose, or fruit sugar, is most common
natural ketose
• Honey is 40% fructose
Monosaccharide Derivatives
•  In deoxy sugars a hydrogen atom replaces one or more of
the -OH groups in a monosaccharide.
–  D-ribose and its derivative D-2-deoxyribose (deoxy
= minus one oxygen atom) are found in various
coenzymes and in DNA.
Monosaccharide Derivatives
•  In amino sugars an -OH group of a monosaccharide has
been replaced by an amino (-NH2) group. D-glucosamine is
an example.
• D-glucosamine is an amino derivative in which an amino group
replaces one hydroxyl group
Monosaccharide Derivatives
•  In alcohol sugars the carbonyl group of a monosaccharide
has been reduced to an alcohol group. Sorbitol is an
example.
Monosaccharide Derivatives
• Sorbitol and Xylitol are used as sweeteners
• Ribitol is found in the coenzyme FAD
Monosaccharide Derivatives
•  In carboxylic acid sugars, an aldehyde or alcohol group of
a monosaccharide has been oxidized to form a carboxyl
group.
[O]
Reactions of Monosaccharides
•  Reactions of monosaccharides are rxns of
carbonyl and hydroxyl groups (chapter 11).
–  Aldehyde and ketone groups can be reduced
–  Aldehyde and alcohol groups can be oxidized
Reduction Monosaccharides
The reduction of the C=O group in an aldehyde or
ketone produces alcohol sugars.
Example:
Oxidation of Monosaccharides
– The oxidation of the aldehyde C=O group
produces carboxylic acid sugars.
[O]
Oxidation of Monosaccharides
– Benedict’s reagent is a copper compound that
will oxidize only aldehyde groups (aldoses)
and not alcohols.
Benedict’s
Reagent
A sugar that reacts with Benedict’s solution is called a
reducing sugar since it reduces the ion Cu2+ à Cu+
Oxidation of Monosaccharides
NOTE: Some ketoses give positive results for
Benedict’s test because they rearrange to
aldehydes in the strongly basic Benedict solution.
Oxidizable Aldehydes
Monosaccharides: Their Cyclic Form
•  A hydroxyl group in a monosaccharide can react
with the carbonyl to form a cyclic hemiacetal.
–  Hemiacetals are made by the reaction of an aldehyde
with an alcohol.
A hemiacetal contains a C atom bonded to an -OH and
an -OR group.
Monosaccharides: Their Cyclic Form
–  A monosaccharide contains both an alcohol and an
aldehyde group.
–  It can react with itself to form a cyclic hemiacetal.
Open Chain to Cyclic Form Mechanism
Mechanism will not be
on the Exam
Monosaccharides: Their Cyclic Form
–  Cyclic forms of monosaccharides are usually drawn
with the Haworth Projection in which the ring is
viewed from the side at an angle.
The edge of the ring closest to the viewer is drawn with
a bold line for perspective.
Substituents on the ring in a Haworth projection are
either “up” or “down”
Monosaccharides: Their Cyclic Form
– The pair of cyclic hemiacetals with the OH on the
hemiacetal carbon in different positions are called
anomers.
– For D-sugars:
• The α-anomer has the OH pointing down.
• The β-anomer has the OH pointing up.
Monosaccharides: Their Cyclic Form
Example: The open-chain form of D-galactose
with its cyclic anomers.
Monosaccharides: Their Cyclic Form
– In solution, the open chain and cyclic forms of a
monosaccharide are in equilibrium:
• If we start with a pure open chain or cyclic form in
solution, the optical rotation of the solution will change
until equilibrium is achieved and the concentrations of the
different forms remain constant.
The change in optical rotation observed as the system
approaches equilibrium is called mutarotation.
Monosaccharides: Their Cyclic Form
– The cyclic forms of monosaccharides can be
named as derivatives of the heterocyclic ethers
furan (5 members) and pyran (6 members).
Monosaccharides: Their Cyclic Form
–  Example: The aldopentose D-ribose forms a cyclic
furanose (the deoxy form is also shown below)
Drawing Haworth Projection in Online
Homework
–  Note that Haworth Projection can be drawn with or
without some of the hydrogens bound to ring-carbons:
Oligosaccharides
•  Oligosaccharides are short polymers containing
2-10 monosaccharide residues.
The residues are bonded to each other by
glycosidic bonds.
–  A glycosidic bond is the ether linkage formed
when an acetal is made by reacting a hemiacetal of
a monosaccharide with a hydroxyl on another
sugar.
– The glycosidic bond in maltose is referred to as
an α-(1à4) bond since the monosaccharide on
the left reacts it’s α-anomer hemiacetal at C-1
with a hydroxyl at C-4 on the second
monosaccharide
Formation of α and β anomers
•  The glycosidic bond can be either α or β.
A β(1→4) Glycosidic bond
An α(1→4)
Glycosidic bond
Oligosaccharides
–  Example: Cellobiose is a disaccharide formed when the
polysaccharide cellulose is broken down.
•  Cellobiose is made by connecting two glucose molecule by a
β(1à4) glycosidic bond.
•  Cellobiose cannot be used as a source of glucose by humans
since we lack the enzyme to hydrolyze the glycosidic bond.
Oligosaccharides
Example: Lactose is a disaccharide found in milk
• Lactose consists of a galactose connected to a glucose
residue by a β(1à4) glycosidic bond.
– 
Sounds like cellobiose! But the OH on C-4 is up in
galactose and down in glucose.
• Lactose intolerance is the inability to hydrolyze lactose due to an
enzyme deficiency.
Oligosaccharides
–  Sucrose, or table sugar, is a disaccharide with two twists:
different residues and no hemiacetal.
• The glycosidic bond in sucrose is formed between the hemiacetal
C of α-glucopyranose and the hemiacetal C of β-fructofuranose
– This is an α,β-(1↔2) glycosidic bond
Note that, as is the case with monosaccharides, the
oligosaccharides can be in equilibrium with their
anomers.
Only the end, hemiacetal residue can open and close
Oligosaccharides
–  Most common oligosaccharides are disaccharides
–  The following are found in peas and beans
Raffinose (trisaccharide)
Stachyose (tetrasaccharide)
Verbascose (pentasaccharide)
Raffinose
Stachyose
Oligosaccharides
– 
Raffinose (trisaccharide)
Stachyose
(tetrasaccharide)
Verbascose (pentasaccharide)
– These oligosaccharides are indigestible
since they contain galactopyranose
residues involved in α-(1à6) glycosidic
bonds that humans lack the enzyme to
hydrolyze.
Oligosaccharides
Glycolipids are sugar-containing lipids that:
-Are present in nerve cell membranes.
-Serve as identifying markers on cell surfaces.
The hemiacetal of a sugar residue is connected to an
alcohol group of a lipid by a glycosidic bond.
Oligosaccharides
•  “How Sweet It Is!”
– Sweetness is rated in comparison to sucrose,
which is assigned a sweetness = 100
– Sweeteners used in our foods can be divided
into two classes: natural and artificial
• Natural Sweeteners are sugars or derivatives
• Artificial Sweeteners may bear no similarity
to sugars!
Relative Sweetness
Artificial Sweeteners
Polysaccharides
•  Polysaccharides contain 10 or more residues
–  In a homopolysaccharide, all the residues are the
same monosaccharide
–  In a heteropolysaccharide, the residues are built
from more than one type of monosaccharide
–  The primary functions of polysaccharides are to:
•  Provide structure (e.g. cellulose)
•  Store energy (e.g. starch and glycogen)
Polysaccharides
–  Cellulose is a homopolysaccharide consisting of long,
linear chains of glucose residues joined by β-(1à4)
bonds
Polysaccharides
•  Cellulose is so strong because the linear chains can form many
hydrogen bonds with adjacent chains forming sheets of the
polymer
•  Wood is about 50% cellulose
•  Bacteria in horses, cows, and termites have enzyme cellulase to
hydrolyze β-(1à4) bonds
Polysaccharides
– Starch is a homopolysaccharide used by
some plants to store energy; there are 2
components of starch:
• 1) Amylose
• 2) Amylopectin
Starch: Amylose
– Amylose contains chains of glucose residue
connected by α-(1à4) glycosidic bonds.
Starch: Amylose
– Unlike cellulose, amylose chains are not
linear but coil into a helix.
Starch: Amylopectin
• Amylopectin is the other component of starch.
•  Amylopectin is similar to amylose in that it contains glucose
residues linked by α-(1à4) glycosidic bonds, BUT in amylopectin
this chain branches through additional α-(1à6) glycosidic bonds
to residues in other chains.
Starch: Amylopectin
•  This branched polysaccharide can be “pruned”
simultaneously at numerous points allowing the glucose
residues (and their energy) to be released more quickly!
Polysaccharides
• Glycogen, or animal starch, is very similar to
amylopectin, except that the chains in glycogen
branch more frequently.
• In amylopectin (left), branches occur every 25 to 30
residues
• In glycogen (right), branches occur every 8 to 12 residues
Polysaccharides
• Chitin is a homopolysaccharide of the glucose derivative
N-acetyl-D-glucosamine
.
-Chitin makes up the hard exoskeleton of crustaceans and
insects.
-The polymer chains hydrogen bond to each other leading
to chitin’s rigidity.
Polysaccharides
– An example of a heteropolysaccharide is
hyaluronic acid.
Hyaluronic acid is found in the lubricating fluid that
surrounds joints, and also in the vitreous humor
inside the eye.