Download Chapter 12 Lecture Notes: Carbohydrates

Chemistry 108
Chapter 12 Lecture Notes
Carbohydrates
Chapter 12 Lecture Notes: Carbohydrates
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 D-sugar 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.
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.
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Chemistry 108
Chapter 12 Lecture Notes
Carbohydrates
Introduction to Carbohydrates
Carbohydrates are also known as_______________________.
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__________________, a simple sugar
6 CO2 + 6 H2O + Energy à C6H12O6 + 6 O2
Carbohydrates are a large class of naturally occurring polyhydroxy ___________________and
________________________.
•
________________________(also known as simple sugars) are the simplest carbohydrates containing
3-7 carbon atoms. A sugar containing:
–
an aldehyde is known as an ______________________
–
a ketone is known as a _________________________
Classification of Carbohydrates
Carbohydrates are grouped into 3 classes:
•
___________________________ are the simplest sugars and serve as the building blocks of larger
molecules
– Example: Glucose
•
__________________________ contain 2-10 monosaccharides bonded together
block = residue)
– Example: Sucrose
•
_____________________________contain more than 10 residues
– Example: Complex Carbohydrates
(building
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Chemistry 108
Chapter 12 Lecture Notes
Carbohydrates
Monosaccharides
Monosaccharides are _____________________________ 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:
______________________
or _______________________
• ______________________ of
carbons: triose, tetrose, etc.
• Both: aldotriose, ketotriose, and so
on…
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Chemistry 108
Chapter 12 Lecture Notes
Carbohydrates
Examples:
You try it:
Name each of the following monosaccharides as an aldose or ketose & according to its number of C atoms.
_________________________
______________________________
_____________________
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Chemistry 108
Chapter 12 Lecture Notes
Carbohydrates
Stereoisomers in Carbohydrates
Carbohydrates are ________________________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 have the same _______________ physical properties except they behave differently in
the way they rotate polarized light and the way they are affected by catalysts.
Remember: Compounds with n chiral carbon atoms has a maximum of 2n possible stereoisomers and half that
many pairs of enantiomers (mirror images).
•
The aldotetrose (below), has 2 chiral carbon atoms and a total of 22 = 4 possible stereoisomers (2 pairs
of enantiomers).
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Chemistry 108
Chapter 12 Lecture Notes
Carbohydrates
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
___________________of two crossed lines.
•
Bonds that point __________________ the page are shown as horizontal lines.
•
Bonds that curve _________________ _______ ______________the page are shown as vertical
lines.
In a Fischer projection, the aldehyde or ketone carbonyl group of a monosaccharide is always placed
toward the top of the page.
•
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Chemistry 108
Chapter 12 Lecture Notes
Carbohydrates
Example:
Glucose
Monosaccharides are divided into _________________ 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_______________________.
•
L: the –OH group on the chiral C furthest from the C=O comes out of the plane of paper
and points to the ______________________.
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Chemistry 108
Chapter 12 Lecture Notes
Carbohydrates
Monosaccharides
We will briefly survey some important pentoses and hexoses, and their
derivatives.
D-glucose, also called ___________________or_________________
___________________________, is the most important monosaccharide
in human metabolism.
D-fructose, or fruit sugar, is most common natural ketose
•
Honey is 40% fructose
Monosaccharide Derivatives
In ________________ __________ 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.
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Chemistry 108
Chapter 12 Lecture Notes
Carbohydrates
Monosaccharide Derivatives (continued)
In __________________ _______________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
In __________ ________________the carbonyl group of a monosaccharide has been reduced to an
alcohol group.
o Sorbitol is an example.
Sorbitol and Xylitol are used as sweeteners
Ribitol is found in the coenzyme FAD
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Chemistry 108
Chapter 12 Lecture Notes
Carbohydrates
Monosaccharide Derivatives
(continued)
•
In ______________ ______ sugars, an
aldehyde group of a monosaccharide has
been oxidized to form a carboxyl group
D-Gluconic Acid is an example.
Reactions of Monosaccharides
Reactions of monosaccharides are reactions of carbonyl and hydroxyl groups (chapter 11).
• Aldehyde and ketone groups can be reduced
• Aldehyde and alcohol groups can be oxidized
Reduction of Monosaccharides
• The reduction of the C=O group in
an aldehyde or ketone produces
_____________
_______________.
Oxidation of Monosaccharides
The oxidation of the aldehyde C=O group
produces __________________________
_______________ _________________ .
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Chemistry 108
Chapter 12 Lecture Notes
Carbohydrates
Oxidation of Monosaccharides (continued)
Benedict’s reagent is a copper compound that will oxidize only _________________ ________________
(aldoses) and not alcohols.
A sugar that reacts with
Benedict’s solution is called a
reducing sugar since it reduces
the ion Cu2+ à Cu+
NOTE: Some ketoses give positive results for Benedict’s test because they ______________________ to
aldehydes in the strongly basic Benedict solution.
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Chemistry 108
Chapter 12 Lecture Notes
Carbohydrates
Monosaccharides: Their Cyclic Form
Review:
A hydroxyl group in a monosaccharide can react with the carbonyl to form a _________ _____________
•
Hemiacetals are made by the reaction of an _________________with an ___________________.
A hemiacetal contains a C atom bonded to an ______________ and an ________________group.
A monosaccharide contains both an alcohol and an aldehyde group.
•
It can react with _____________ to form a cyclic hemiacetal.
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Chemistry 108
Chapter 12 Lecture Notes
Carbohydrates
Open Chain to Cyclic Form Mechanism (MECHANISM NOT ON EXAM)
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”
The pair of cyclic hemiacetals with the OH on the hemiacetal carbon in different positions are called
__________________.
• For D-sugars, the α-anomer has the OH pointing ________________
• For D-sugars, the β-anomer has the OH pointing ________________
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Chemistry 108
•
Chapter 12 Lecture Notes
Carbohydrates
Example: The open-chain form of D-galactose with its cyclic anomers.
In solution, the open chain and cyclic forms of a monosaccharide are in______________________:
•
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
________________________.
The cyclic forms of monosaccharides
named as derivatives of the
heterocyclic ethers furan (5 members)
pyran (6 members).
can be
and
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Chemistry 108
Chapter 12 Lecture Notes
Carbohydrates
Example: The aldopentose D-ribose forms a cyclic _____________ (the deoxy form is also shown below)
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Chemistry 108
Chapter 12 Lecture Notes
Carbohydrates
Oligosaccharides
Oligosaccharides are short polymers containing 2-10 monosaccharide _________________________.
The residues are bonded to each other by
___________________ _____________.
A glycosidic bond is the ether linkage formed when an acetal is made by reacting a
__________________ of a monosaccharide with a __________________on another sugar.
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Chemistry 108
Chapter 12 Lecture Notes
Carbohydrates
The glycosidic bond in maltose is referred to as an __________________ 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 ________ or_________.
–
Example: Cellobiose is a disaccharide formed when the polysaccharide cellulose is broken down.
– A disaccharide is composed of two monosaccharide residues.
– 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.
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Chemistry 108
Chapter 12 Lecture Notes
Carbohydrates
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.
Sucrose, or table sugar, is a disaccharide with two twists: ___________________ _______________
and___________ ____________________________.
• 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
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Chemistry 108
Chapter 12 Lecture Notes
Carbohydrates
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
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Chemistry 108
Chapter 12 Lecture Notes
Carbohydrates
Most common oligosaccharides are disaccharides
The following are found in peas and beans
Raffinose
(a trisaccharide composed of three monosaccharide residues)
Stachyose
(a tetrasaccharide composed of four monosaccharide residues)
Verbascose (a pentasaccharide composed of four monosaccharide residues)
These oligosaccharides are indigestible since they contain galactopyranose residues involved in α-(1à6)
glycosidic bonds that humans lack the enzyme to hydrolyze.
_____________________ 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 alcohol group of a lipid by a ________________ bond.
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Chemistry 108
Chapter 12 Lecture Notes
Carbohydrates
“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!
Artificial Sweeteners:
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Chemistry 108
Chapter 12 Lecture Notes
Carbohydrates
Polysaccharides
___________________________ contain 10 or more residues
o In a_________________________, all the residues are the same monosaccharide
o In a_________________________, the residues are built from more than one type of
monosaccharide
o The primary functions of polysaccharides are to:
•
•
___________________________ (e.g. cellulose)
•
__________________ ________________ (e.g. starch and glycogen)
Cellulose is a homopolysaccharide consisting of long, linear chains of glucose residues joined by β-(1à4)
bonds
•
Cellulose is so strong because the linear chains can form _________________ 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
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Chemistry 108
Chapter 12 Lecture Notes
Carbohydrates
Starch is a homopolysaccharide used by some plants to store energy; there are 2 components of starch:
•
1) _____________________________
•
2) _____________________________
Starch: Amylose
Amylose contains chains of glucose residue connected by α-(1à4) glycosidic bonds.
•
Unlike cellulose, amylose chains are not linear but coil
into a_________________.
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 ____________________ through additional α-(1à6) glycosidic bonds
to residues in other chains.
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Chemistry 108
Chapter 12 Lecture Notes
Carbohydrates
Starch: Amylopectin (continued)
•
This branched polysaccharide can be “pruned” simultaneously at
numerous points allowing the glucose residues (and their energy)
to be released more quickly!
________________________, or animal starch, is very similar to amylopectin, except that the chains in
glycogen branch more frequently.
•
•
In amylopectin (right), branches occur every 25 to 30 residues
In glycogen (left), branches occur every 8 to 12 residues
________________________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.
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Chemistry 108
Chapter 12 Lecture Notes
Carbohydrates
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.
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Chemistry 108
Chapter 12 Lecture Notes
Carbohydrates
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