Carbohydrates I & II Notes
(Move to outline here)
Saccharide is another name for a carbohydrate. Simple saccharides are the monosaccharides,
commonly called sugars. Glucose is an example of a monosaccharide. Others are shown in Figure
11.2 and Figure 11.3. We use the terms monosaccharide, oligosaccharide, or polysaccharide to
refer to compounds composed of a single sugar, several sugars linked together, or many sugars
linked together, respectively.
The term carbohydrate derives from the fact that many of them have a formula that can be
simplified to (CH2O)n. Some of these compounds are chemically modified, however, and do not fit
the formula due to the modification.
Saccharides play a variety of roles in living organisms, including energy storage
(monosaccharides and oligosaccharides), structural roles (polysaccharides), and cell identity
Monosaccharides are the simplest sugars, having the formula (CH2O)n. The smallest molecules
usually considered to be monosaccharides are those with n = 3.
Monosaccharides can be categorized according to their value of 'n,' as shown below:
Monosaccharides can exist as aldehydes or ketones and are called aldoses or ketoses,
respectively. For example, THIS shows the structures of glyceraldehyde, an aldo-triose, and
dihydroxyacetone, a keto-triose. Glyceraldehyde and dihydroxyacetone have the same atomic
composition, but differ only in the position of the hydrogens and double bonds.
Carbons in a monosaccharide are numbered such that the aldehyde group is carbon number one or
the ketone group is carbon number two.
The three dimensional arrangement of atoms around a carbon atom are such that if four different
groups are attached to it, they can be arranged in two different ways. Such a carbon is described as
chiral or asymmetric. The two molecules with different three-dimensional arrangement are mirror
images of each other, and the two different forms are called stereoisomers. For example, Dglyceraldehyde and L-glyceraldehyde (HERE) are mirror images of each other (stereoisomers) and
cannot be superimposed on each other. Such molecules with these properties are called
enantiomers. The designation 'D-' or 'L-' is an older nomenclature still used widely in
biochemistry. It originally described whether the compound rotated a plane of polarized light to the
right (D for dextro) or left (L for left). This is not absolute, however, because it depends on the
reference compound chosen. The R-S nomenclature, which is an absolute naming scheme for
organic chemistry will not be used here. The predominant monosaccharides found in nature have
the 'D' configuration.
Sugars with more than one asymmetric carbon have many possible three dimensional
configurations. In general a molecule with m chiral centers will have 2m stereoisomers. The
multiple stereoisomeric forms means that not all stereoisomers will be mirror images of each other.
Stereoisomers that are not mirror images of each other are called diastereomers.
Ketose-aldose pairs of sugars frequently are named by inserting the letters 'ul' in the name of the
corresponding aldose to derive the name of the ketose. An example is erythrose - erythrulose.
Sugar Ring Structures
When sugars cyclize, they typically form furanose or pyranose structures. These are molecules
with five-membered or six-membered rings, respectively. Cyclization creates a carbon with two
possible orientations of the hydroxyl around it. Cyclization of an aldose occurs by intramolecular
reaction with the aldehyde and alcohol groups to form a hemiacetal. Cyclization of a ketose occurs
by intramolecular reaction with the ketone and alcohol groups to form a hemiketal. In either case,
a new asymmetric carbon is created by the reaction and we refer to the carbon as the anomeric
carbon and the two possible configurations as anomers. The two possible configurations of the
hydroxyl group are called alpha and beta, which correspond to the hydroxyl being in the "down"
and "up" positions, respectively, in standard projections. Anomers are capable of interconverting
between alpha and beta positions in a process is called mutarotation IF the hydroxyl group of the
hemiacetal or hemiketal is unaltered.
Figure 11.7 shows that a pyranose, such as glucose, has two common conformational isomers,
referred to as the "boat" and "chair" form. For glucose (and most sugars), the chair form is more
stable because the hydroxyls of carbons 1 and 2 are further removed and thus have less steric
interference with carbons 3, 4, and 5. Figure 11.8 illustrates conformational isomers for the
furanose ribose. These structures are different forms of the so-called Envelope conformation,
designated as C3-endo and C2-endo.
Chiral carbons (carbons covalently linked to 4 different entities) give rise to stereoisomers.
Molecules that are stereoisomers have the same formula and the same structure, but have their
atoms arranged in different ways in 3D space. For example, compare the structures of Dglyceraldehyde and L-glyceraldehyde HERE. Notice that they are nonsuperimposable.
Common sugars typically have not one, but multiple chiral carbons. Glucose, for example,
contains four chiral carbons. For a carbon with 'm' chiral carbons, the number of possible
stereoisomers is 2m. Thus, for glucose, there are 16 possible stereoisomers. The form most
commonly found in living organisms, D-glucose, has only one mirror image. In fact, any
stereoisomer has only one mirror image. The other 14 stereoisomers of glucose that are not mirror
images of it are called diastereomers. That is, diastereomers are stereoisomers that are not mirror
images of each other.
Sugar ring structures can be written in a variety of ways. Figures 11.4 and 11.5 show both
glucose and fructose in linear and circular projections. Linear projections are called Fischer
projections and circular projections are called Haworth projections. Note that neither is exactly
"anatomically correct", but give an approximation of the structure of each form.
Derivatives of Monosaccharides
Monosaccharides can be chemically altered in several ways to provide new classes of compounds.
Acids and Lactones - made by mild oxidation of an aldose, for example, to form an aldonic acid
(see HERE). In metabolic pathways, oxidation at carbon 6 of glucose yields glucuronic acid.
Sugars that react with cupric ion to become oxidized are called reducing sugars. Unmodified
aldoses will be reducing sugars because the aldehyde is readily oxidized to a carboxyl group.
Alditols - made by reducing the carbonyl group of a sugar. The resulting polyhydroxy compounds
are called alditols. Important ones include erythritol, D-mannitol, and D-glucitol (also called
Amino Sugars - made by replacing a hydroxyl of a sugar with an amine group. Two common
examples are beta-D-glucosamine and beta-D-galactosamine (see HERE). Common molecules
derived from these include beta-D-N-acetylglucosamine, muramic acid, N-acetylmuramic acid, D-N-acetylgalactosamine, and N-acetyl-neuraminic acid (also called sialic acid). Amino sugars
are often found in oligosaccharides and polysaccharides.
Glycosides - formed by elimination of water between the anomeric hydroxyl of a cyclic
monosaccharide and the hydroxyl group of another compound. Glycosides do NOT undergo
mutarotation in the absence of an acid catalyst, so they remain locked in the alpha or beta
configuration. (Remember that the FREE hydroxyl group on the anomeric carbon can undergo a
change in orientation from the alpha to beta position, or vice versa. This change is called
mutarotation). Glycosidic bonds are very common in plant and animal tissues. Many glycosides
are known. Some, such as ouabain or amygdalin are very poisonous. Others, such as the common
oligosaccarides and polysaccharides found in our cells, are not.
Glycosidic bonds between monosaccharides give rise to oligosaccharides and polysaccharides.
The simplest oligosaccharides, the disaccharides, include compounds such as sucrose and
lactose, which are referred to as sugars (like the monosaccharides). Other common disaccharides
include trehalose, maltose, gentiobiose, and cellobiose.
Four features distinguish disaccharides from each other:
The two specific sugar monomers and their stereoconfigurations
The carbons involved in the linkage
The order of the monomeric units, if they are different kinds
The anomeric configuration of the hydroxyl group on carbon 1 of each residue
Oligosaccharides are also found as part of glycoproteins and play a role in cell
recognition/identity. Oligosaccharides form the blood group antigens. In some cells, these
antigens are attached as O-linked glycans to membrane proteins. Alternatively, the oligosaccharide
may be linked to a lipid molecule to form a glycolipid. These oligosaccharides determine the
blood group types in humans.
Polysaccharides are polymers of monosaccharide units. The monomeric units of a
polysaccharide are usually all the same (called homopolysaccharides), though there are exceptions
(called heteropolysaccharides). In some cases, the monomeric units are modified monosaccharides.
Polysaccharides differ in the composition of the monomeric unit, the linkages between them, and
the ways in which branches from the chains occur. Common polymers, their monomeric units, and
linkages/branches are shown below:
alpha 1->4 links with
alpha 1->4 links with
some alpha 1->6
Linkages between the individual units require special enzymes to break them down. For example,
the alpha 1-> 4 linkages between glucose units in glycogen, amylose, and amylopectin, are readily
broken down by all animals, but only ruminants (cows) and related animals contain symbiotic
bacteria with an enzyme (cellulase) that can break down the beta 1-> 4 linkages between individual
glucose units in cellulose. As a result, the huge amount of cellulose in the biosphere is unavailable
as an energy source to most animals.
The secondary structure of the polysaccharides range from the bent structure of starch and
glycogen (HERE) to the planar structure of cellulose (see HERE).
Polysaccharides are used to some extent for energy storage in almost all higher organisms.
Animals use glycogen. Plants use starch, which is composed of amylose and amylopectin. In both
plants and animals, the polysaccharides used for energy storage are readily broken down into
monomeric units that can be rapily metabolized to produce ATP. In addition to polysaccharides
used for energy storage, plants use different polysaccharides, such as cellulose, for structural
purposes in their cell walls. The exoskeleton of many arthropods and mollusks is composed of
chitin, a polysaccharide of N-acetyl-D-glucosamine.
Polysaccharides containing a single sugar, such as glucose, are referred to as glucans. Others,
which contain only mannose, are called mannans. Still others, containing only xylose, are called
xylans. Glucans with structural roles include some in fungi, which have glucoses joined by beta 1>3 or beta 1->6 bonds.
Other plant polysaccharides include the xylans and the glucomannans. The xylans are polymers of
D-xylopyranose, often with substituent groups attached. The glucomannans, on the other hand, are
heteropolymers of glucopyranose and mannopyranose joined by beta 1->4 linkages with beta 1>6 branches to other substituents. The glucomannans and xylans are often grouped together and
Chitin is a homopolymer of N-acetyl-D-glucosamine, with units joined by beta 1-> 4 bonds.
Chitin is found in organisms as diverse as algae, fungi, insects, arthropods, mollusks, and insects.
Another group of polysaccharides of importance is the glycosaminoglycans. These are
heteropolysaccharides containing either N-acetylgalactosamine or N-acetylglucosamine as one of
their monomeric units. Examples include chondroitin sulfates and keratan sulfates of connective
tissue, dermatan sulfates of skin, and hyaluronic acid. All of these are acidic, through the
presence of either sulfate or carboxylate groups. Examples are shown in Figure 11.15.
A major function of glycosaminoglycans is formation of a matrix to hold together the protein
components of skin and connective tissue in animals. An example is the proteoglycan complex
(protein-carbohydrate complex) in cartilage
Hyaluronic Acid also acts in the body as a viscosity-increasing agent or lubricating agent in the
vitreous humor of the eye and synovial fluid of joints.
Heparin is yet another highly sulfated glycosaminoglycan. Part of the repeating unit of its complex
chain is shown here. Heparin is used medicinally to inhibit clotting in blood vessels.
Oligosaccharides and Polysaccharides as Cell Markers
Oligosaccharides play a role in cell recognition/identity. Oligosaccharides form the blood group
antigens (HERE) by linkage to proteins in blood cell membranes forming glycoproteins or, in
some cases, to lipids, forming glycolipids. Three different oligosaccharide structures give rise to
the blood groups - A, B, and O. The base structure of each contains the structure of the O antigen.
Specific glycosyltransferases add the extra monosaccharide to the O antigen to give rise to either
the A or B antigen.
Molecules of the blood group antigens represent only a special case of a much more general
phenomenon - cell marking by oligosaccharides. In multicellular organisms, different kinds of
cells must be marked on their surfaces so that they can interact properly with other cells and
molecules. The surface of many cells are nearly covered with polysaccharides, which are attached
to either proteins or lipids in the cell membrane. Some animal cells have an extremely thick coating
of polysaccharides called a glycocalyx.
The cell surfaces of many cancer cells are abnormal, which may account for the loss in tissue
specificity that such cells commonly exhibit.
Properties of oligosaccharides that aid in their role as cellular markers:
They can present a wide variety of structures in relatively short chains. The multiple possible
monomers, linkages, and branching patterns allow a vast, but specific vocabulary.
They are very potent antigens (antibodies can be elicited swiftly against them)
More than half of all eukaryotic proteins carry covalently attached oligosaccharide or
polysaccharide chains. In glycoproteins, sugars are attached either through the amide nitrogen
atom in the side chain of asparagine (termed N-linkage - see HERE) or to the oxygen atom in the
side chain of serine or threonine (called O-linkage - see HERE). In the case of N-linkage,
asparagine can accept an oligosaccharide only if the residue is part of an ASN-X-SER or ASN-XTHR sequence (X can be any residue). Thus, one can the predict possible N-glycoysylation sites in
a protein sequence. The common carbohydrate core of all N-linked oligosaccharides is shown in
A very important further use of N-linked oligosaccharides is in intracellular targeting in eukaryotic
organisms. Proteins destined for certain organelles or for excretion from the cell are marked
specifically by oligosaccharides during posttranslational processing to ensure they arrive at their
proper destinations. Some O-linked glycans appear to function in intracellular targeting and
molecular and cellular identification. An example is found in the blood group antigens.
Roles of Endoplasmic Reticulum and Golgi Complex
Protein glycosylation occurs inside the lumen of the endoplasmic reticulum and the Golgi complex
(Figure 11.21). An example is the enzyme called elastase (see HERE), which is synthesized by
ribosomes attached to the endoplasmic reticulum and secreted inwards, as shown in Figure 11.22.
Glycosylation occurs after the protein has entered the endoplasmic reticulum. N-linked
glycosylation begins in the endoplasmic reticulum and then continues in the Golgi apparatus. Olinked glycosylation occurs exclusively in the Golgi apparatus.
Dolichol phosphate (structure HERE) provides a lipid structure on which oligosaccharides
destined for attachment as N-linked glycosyl groups are synthesized. Figure 11.23 depicts the
assembly process. "Flipping" of the dolichol-phosphate-glycosyl complex facilititates its
movement into the lumen of the endoplasmic reticulum. Recycling of dolichol pyrophosphate
(released after the oligosaccharide complex is transferred to a protein) is targeted by the antibiotic
bacitracin (inhibits phosphatase action on dolichol phosphate). Another antibiotic, tunicamycin
inhibits the first step in synthesis of the process by inhibiting the addition of the first N-acetylglucosamine.
Transport vesicles carry proteins from the endoplasmic reticulum to the Golgi complex during the
glycosylation process, as shown in Figure 11.24.
Mannose-6-phosphate (M6P) targets lysosomal enzymes to their destinations. Lysosomes are
cellular organelles that degrade and recycle materials in cells. Synthesis of M6P is shown in Figure
11.25. People deficient in the phosphotransferase enzyme develop a disease called I-cell disease,
which is characterized by the absence of eight acid hydroloases normally present in the lysozomes.
Instead, these proteins lack M6P (containing mannose instead) and are found in abundance in the
blood and urine.
Figure 11.26 depicts the important role of oligosaccharides in the folding of some glycoproteins.
Properly folded proteins have glucose residues removed from the for passage to the Golgi complex.
Unfolded or improperly folded proteins bind to chaperone proteins known as calnexin or
calreticulin for proper folding.
For oligosaccharides or polysaccharides to serve as recognition signals, there must be proteins
that bind to them specifically. One such class is the immunoglobulins. Another very diverse group
of saccharide-binding proteins is the lectins. In plants, lectins appear to play defensive roles and aid
in adhering nitrogen-fixing bacteria to roots. In animals, lectins seem to be involved in interactions
between cells and proteins of the intercellular matrix, such as collagen, and help to maintain tissue
and organ structure. The molecular structures bound by various lectins is shown in Figure 11.28.
Viruses bind to specific structures on the surfaces of cells. In the case of the influenza virus, the
target residues are sialic acid (figure HERE) on cell surface glycoproteins. A viral protein called
hemagglutinin binds to these sugar residues. Release of the virus from the sialic acid (to allow it to
infect the cell) requires action of an enzyme called neuraminidase and this enzyme is the target of
White sugar is made by refining raw sugar obtained from sugar cane or sugar beet, removing all
Caster sugar is white, granulated sugar with
very fine sugar crystals. It is also called
superfine sugar, ultra fine sugar or bar
sugar. It is best used in baking and desserts, in
making of cakes, mousses and drinks, as well
as in foods and pastries that are sprinkled,
rolled or coated with sugar.
Regular granulated sugar has coarser crystals
than caster or superfine sugar. It may be used
in making preserves, jams, marmalades and
In dishes where sugar is to be whipped with
eggs, cream etc, it is best to use superfine
sugar (see left).
In making jams, marmalades, preserves etc,
superfine sugar can be replaced with coarser
granulated sugar (see right).
Icing sugar, also known as confectioners'
sugar, is made of white sugar ground into a
smooth, white powder and used in icings,
confections, drinks etc.
This white, large crystal sugar is unevenly
shaped and used to sprinkle on top of sweet
buns and other baked goods for garnish. It may
also be called pearl, sanding, coarse or
There is usually an amount of starch mixed in
icing sugar to prevent clumping. Also
differently coloured or flavoured icing sugars
can be found on sale.
There are also coloured decorating sugars on
A rather good substitute for real vanilla, vanilla
sugar is powdered or granulated white sugar
flavoured with real vanilla bean. Usually there
are little black dots of powdered vanilla bean
or seeds visible in the sugar.
Vanillin sugar consists of finely granulated
sugar flavoured with synthetic vanillin.
Vanilla sugar is used instead of vanilla bean to
give vanilla flavour to various sweet baked
goods, desserts, whipped cream and beverages.
It is added to foods only in small amount
(usually 1 - 2 teaspoons per a batch of batter,
dough etc). Vanilla sugar can be replaced with
the same or slightly smaller amount of goodquality vanillin sugar, see right.
Best varieties of vanilla
sugar should have a
pleasant, not bitter, taste of
In picture on right: a brand
of vanilla sugar sold in
Vanillin is a compound responsible for the
characteristic flavour of vanilla. Besides
vanilla beans, natural vanillin occurs widely in
the nature in other plants and essential oils, and
it is also produced synthetically. One source of
synthetic vanillin is lignin, a substance found
in trees, separated from the wood tissue by
chemical pulping process.
Vanillin sugar is slightly bitter in taste. It is
added to foods only in small amount (usually 1
- 2 teaspoons per a batch of batter, dough etc).
It should be added to custards and sauces that
are cooked on stovetop no sooner than just
after cooking, otherwise it will turn bitterer.
Most Finnish brands of vanillin sugar are not
too bad in substituting real vanilla. Vanillin
sugar can be replaced with the same or slightly
larger amount of good-quality vanilla sugar,
See also information about vanilla.
White sugar cubes
Brown sugar cubes
Also called lump sugar, sugar cubes are made by moulding and drying moistened, hot granulated
sugar. Coming in various forms and colours, lump sugar is mainly used to sweeten various hot drinks.
In cooking, lump sugar and sugar cubes may be used instead of granulated sugar in recipes where sugar
is melted, like syrups and caramel. Sugar cubes are also used in desserts like crêpes Suzette, where they
are rubbed against the zest of citrus fruit to absorb their essential oils, in order to flavour the dish.
Lump sugar can be ground into granules or powdered using a mortar, a blender or a food processor.
Jam sugar is a special gelling sugar used in
making jams, marmalades, jellies and other
preserves, instead of regular white sugar. It
consists of white, granulated sugar (about 98
%) added with natural fruit pectin (E440,
gelling agent), citric acid (E330, antioxidant)
and potassium sorbate (E202, preservative).
Jelly sugar is used to decorate desserts and
pastries and to make set, clear dessert jellies.
Jelly made with jelly sugar is spooned or
brushed over berry and fruit garnishes to give
them a thin and shiny, protective jelly coating.
Jelly sugar is not suitable to be used in milkbased jellies and puddings or in canning and
When using jam sugar, the cooking time of
various preserves is often reduced, thus better
maintaining the flavours, colours and vitamins
of the fruits and berries used. To determine
how much jam sugar to use in proportion to
fruits or berries, follow the instructions on the
sugar package. Jam sugar cannot be used
instead of regular sugar in baking or cooking,
but only in making of jams, marmalades and
fruit compotes or soups.
Jelly sugar consists of white, granulated sugar,
glucose syrup, natural fruit pectin (E440,
gelling agent) and citric acid (E330,
antioxidant). The sugar is mixed in boiling
water and left to cool for a couple of minutes
before spreading or pouring on the food. To
determine how much jelly sugar to use in
proportion to water or in making of jellied
berry or fruit desserts, follow the instructions
on the sugar package.
Traditional, natural brown sugars are made of partially refined raw cane sugar, containing certain
impurities, which give them their brown colour. Depending on the sugar type, the colour ranges from
very dark to pale brown, and the taste from a strong, almost liquorice-like to a lighter molasses flavour.
These types of brown sugars include the demerara and muscovado sugars.
Today, commercially produced brown sugars are mostly made by coating granulated, refined white
sugar with a thin layer of dark molasses, giving them a brown colour with molasses flavour.
Granulated brown sugar
Soft brown sugar
Regular granulated brown sugar is made by
coating white sugar with a layer of dark
molasses. It has loose, non-sticky sugar
crystals with the colour ranging from light to
dark brown. This type of brown sugar has a
light, clean molasses flavour and a coarser
texture than white, superfine sugar.
Soft brown sugar is made by coating white
sugar with a layer of dark molasses. It is firmly
packed, moist and slightly sticky, and has a
stronger molasses flavour than brown, loose
sugar (see left).
Granulated brown sugar can be replaced for
example with demerara sugar (see below).
Soft brown sugar should be stored wrapped
airtight to prevent it from drying and hardening
into a clump. Hardened sugar will soften in a
couple of days, when you add a few drops of
water in the sugar package and close it tightly.
Soft brown sugar can be replaced for example
with light muscovado sugar (see below).
Named after the Demerara area of Guyana, the
coarse-grained demerara sugar is brown,
partially refined raw sugar containing some
residual impurities. The colour of demerara
sugar varies from golden brown (eg turbinado
sugar) to dark brown, with a strong dark
Demerara sugar can be used to sweeten and
flavour various hot beverages, and it is used in
fruit and berry desserts or in making candies
and toffees. Depending on its colour, texture
and depth of flavour, it can be used to replace
granulated or soft brown sugar in many sweet
and savoury dishes.
Turbinado sugar is a further refined type of
demerara sugar with a pale colour and a mild
Muscovado sugar is the darkest of the partially
refined brown raw sugars. It has slightly sticky
crystals, with the colour varying from light to
Muscovado sugar can be used to flavour tea,
coffee and other beverages. It brings deep and
dusky flavour of molasses into various dishes
and desserts. Light muscovado sugar can be
used to replace soft brown sugar in cooking
Barbados sugar is a type of muscovado sugar
with a finer texture.
Carbohydrates and Nucleic Acids
of Glucose and Fructose
Figure 23-1 Glucose and fructose are monosaccharides. Glucose is an
Caption aldose (a sugar with an andehyde group), and fructose is a ketonse (a sugar
with a ketone group).
Notes Carbohydrate structures are commonly drawn usinf the Fischer projections.
Keywords glucose, fructose, carbohydrates, Fischer projection
Sugars with aldehyde groups are called aldoses, and those with ketone
groups are called ketoses. The number of carbon atoms in the sugar
Caption generally ranges from three to seven, designated by the term triose (three
carbons), tetrose (four carbons), pentose (five carbons), hexose (six
carbons), and heptose (seven carbons).
Notes Most ketones have the ketone on C2, the second carbon of the chain.
aldoses, ketoses, aldohexose, ketohexose, aldotetrose, ketotetrose, triose,
tetrose, pentose, hexose, heptose
and L Series of Sugars
The (+) enantiomer of glyceraldehyde has its OH group on the right of the
Fischer projection. Therefore sugars of the D series have the OH groups on
the bottom asymmetric carbon on the right of the Fischer projection; sugars
of the L series have the OH of the bottom asymmetric carbon on the left.
Notes D and L are enantiomers (mirror images).
Keywords asymmetric carbon
of an Aldose
Figure 23-2 Degradation of an aldose removes the aldehyde carbon atom to
give a smaller sugar.
Notes Sugars of the D series give (+)-glyceraldehyde in degradation to the triose.
Keywords degradation, aldose, glyceraldehyde, triose
D Family of Aldoses
Figure 23-3 The D family of aldoses. All these sugars occur naturally except
for threose, lyxose, allose, and gulose.
The family tree of D aldoses can be created by starting with D-(+)glyceraldehyde and adding carbons one by one.
Keywords threose, lyxose, allose, gulose, aldoses
and Threo Diastereomers
A diastereomer is called erythro if its Fischer projection shows similar groups
Caption on the same side of the molecule. It is called threo if similar groups are on
opposite sides of the Fischer projection.
Hydroxylation of trans-crotonic acid produces two threo enantiomers, while
Notes hydroxylation of cis-crotonic acid produces two erythro enantiomers. Th
erythro and threo forms are diastereomers.
Keywords erythro, threo, crotonic acid, hydroxylation
Figure 23-4 The term erythro and threo are used with disymmetric molecules
whose ends are different.
The terms meso and (+), (-), or (d),(l) are preferred with symmetric
Keywords erythro, threo, meso, disymmetric
Sugars that differ only by the stereochemistry at a single carbon are called
epimers, and the carbon atom they generally differ if generally stated.
Notes If the number of a carbon is not specified, it is assumed to be C2.
of Cyclic Hemiacetal Formation
If the aldehyde group and the hydroxyl group are part of the same molecule,
a cyclic hemiacetal results.
Cyclic hemiacetals are particularly stable if they result in five- or sixNotes
23-5 Glucose exists almost entirely as its cyclic hemiacetal form.
The Haworth projection is widely used to draw the hemiacetals, although it
Notes may give the impression of the ring being flat. The chair conformation is more
Keywords glucose, Haworth projection, chair conformation
Figure 23-6 Fructose form a five-membered cyclic hemiacetal. Fivemembered rings are usually represented as flat Haworth structures.
Notes Since five-membered rings are not puckered as much as six-membered
rings, they are usually depicted as flat Haworth projections.
Keywords fructose, Haworth projection
Figure 23-7 The anomers of glucose. the hydroxyl group on the anomeric
(hemiacetal) carbon is down (axial) in the a anomer and up (equatorial) in the
b anomer. The b anomer of glucose has all its subtituents in equatorial
The hemiacetal carbon is called the anomeric carbon, easily identified as the
only carbon atom bonded to two oxygens.
Keywords anomer, hemiacetal, anomeric carbon, equatorial position, axial position
Figure 23-8 The a anomer of fructose has the anomeric -OH group down,
to the terminal -CH2OH group. The b anomer has the anomeric -OH
group up, cis to the terminal -CH2OH.
anomer, anomeric carbon
Figure 23-9 An aqueous solution of D-glucose contains an equilibrium
mixture of a-D-glucopyranose, b-D-glycopyranose, and the intermediate
open-chain form, Cystallization below 98oC gives the a anomer, and
cystallization above 98oC gives the b anomer.
Notes Mutarotation occurs because the two anomers interconvert in solution.
Keywords mutarotation, interconversion, glucopyranose
Epimerization of Glucose
Figure 23-10 Under basic conditions, stereochemistry is lost at the carbon
atom next to the carbonyl group.
The enolate intermediate is not chiral so reprotonation can produce either
Notes stereomer. Because a mixture of epimers results, this stereochemical change
is called epimerization.
Keywords epimer, epimerization, stereochemistry
Figure 23-11 Under basic condictions, the carbonyl group can isomerize to
carbon atoms. Aldoses equilibrate with ketoses via enediol
Under strongly basic conditions, the combination of enediol rearragements
and epimerization leads to a complex mixture of sugars.
Keywords enediol rearragement, epimerization
of Aldoses Using Bromine Water
Bromine water oxidizes the aldehyde group of an aldose to a carboxylic acid.
Caption Bromine water is used for this oxidation because it does not oxidize the
alcohol groups of the sugar and it does not oxidize ketoses.
Notes The reaction can be used as a qualitative method to identify aldoses.
Keywords bromine, aldose, ketose
Nitric acid is a stronger oxidizing agent than bromine water, oxidizing both the
group and the terminal -CH2OH group of an aldose to a carboxylic
Treatment of aldose with nitric acid produces aldaric acid. Glucose is
oxidized to glucaric acid.
Keywords nitric acid, aldose, aldaric acid, glucaric acid
In its open form, an aldose has an aldehyde group, which reacts with Tollens
to give an aldonic acid and a silver mirror. Sugars that reduce
Tollens reagent to give a silver miror are called reducing sugars.
Notes Tollens test is used as a qualitative test for the identification of aldehydes.
Keywords Tollens test, silver mirror, reducing sugars
Figure 23-12 Sugars that are full acetals are stable to Tollens reagent and
are nonreducing. Such sugars are called glycosides.
Notes Nonreducing sugars (glycosides) are acetals, and they do not mutarotate.
Keywords nonreducing sugar, mutarotate
Figure 23-13 The group bonded the anomeric carbon o a glycoside is called
Some aglycones are bonded through an oxygen atom (a true acetal), and
others are bonded through other atoms such as nitrogen.
Figure 23-14 Treatment of an aldose or a ketose with methyl iodide and silver
oxide gives the totally methylated ether.
If the conditions are carefully controlled, the stereochemistry at the anomeric
carbon is usually preserved.
Keywords aldose, methyl iodide, silver oxide, anomeric carbon
Figure 23-15 Acetic anhydride and pyridine convert all the hydroxyl groups on
Caption a sugar to acetate esters. The stereochemistry at the anomeric carbon is
Notes Sugar esters are readily crystallized and purified, and they dissolve in
common organic solvents.
Keywords acetate ester
Two molecules of phenylhydrazine condense with each molecule of the sugar
give an osazone, in which both C1 and C2 have been converted to
Most osazones are easily crystallized, with sharp melting points. Melting
Notes points of osazone derivatives provide valuable clues for the identification and
comparison of sugars.
Keywords osazones, phenylhydrazine, phenylhydrazone
The Ruff degradation is a two-step process that begins with bromine water
oxidation of the aldose to its aldonic acid. Treatment of the aldonic acid with
hydrogen peroxide and ferric sulfate oxidizes the carboxyl group to CO2 and
gives an aldose with one less carbon atom.
The Ruff degradation is used mainly for structure determination and
synthesis of new sugars.
Keywords Ruff degradation, oxidation, hydrogen peroxide, ferric sulfate, aldose
The Kiliani-Fischer synthesis lengthens an aldose carbon chain by adding
one carbon atom to the aldehyde end of the aldose.
This synthesis is useful both for determining the structure of existing sugars
and for synthesizing new sugars.
Keywords Kiliani-Fischer synthesis
Acid Cleavage of Carbohydrates
Because ether and acetal groups are not affected, periodic acid cleavage of a
glycoside can help to determine the size of the ring.
Cleavage of methyl b-D-glucopyranoside produces 4 different products,
implying an original six-membered ring.
Keywords periodic acid, cleavage, diol
Figure 23-16 A sugar reacts with an alcohol to give an acetal called
Caption glycoside. When the alcohol is part of another sugar, the product is a
Notes A disaccharide is a sugar composed of two monosaccharide units.
Keywords glycoside, disaccharide
In cellobiose, the anomeric carbon of one glucose unit is linked through an
equatorial (b) carbon-oxygen bond to C4 of another glucose unit.
The monosaccharides are joined together by the equatorial position of C1
and the equatorial position of C4'.
Like cellobiose, maltose contains a 1,4' glucosidic linkage between the two
Caption glucose units. The difference in maltose is that the stereochemistry of the
glucosidic linkages is a rather than b.
The monosaccharides are joined together by the axial position of C1 and the
equatorial position of C4'.
Keywords glucosidic linkage, maltose
of Two Anomeric Carbons: Sucrose
Some sugars are joined by a direct glycosidic linkage between their anomeric
carbon atoms: a 1,1' linkage.
Sucrose is composed of one glucose unit and one fructose unit by an oxygen
atom linking their anomeric carbon atoms.
Keywords sucrose, glucose, fructose, anomeric carbon
Figure 23-17 Cellulose is a b-1,4' polymer of D-glucose, systematically
Cellulose is the most abundant organic material. Cellulose is synthesized by
plants as a structural material to support the weight of the plant.
Figure 23-18 Amylose is an a-1,4' polymer of glucose, systematically named
Amylose differs from cellulose only in the stereochemistry of the glycosidic
Keywords amylose, cellulose
Figure 23-19 The amylose helix forms a blue charge-transfer complex with
Notes This is the basis of the starch-iodide test for oxidizers.
Keywords amylose, starch-iodide complex
Figure 23-20 Amylopectin is a branched a-1,6' polymer of glucose. At the
points, there is a single a-1,6' linkage that provides the attachment
point for another chain.
Notes Glycogen has a similar structure, except that its branching is more extensive.
Keywords amylopecting, a-1,6' linkage, glycogen
23-21 A short segment of the RNA polymer. Nucleic acids are
assembled on a backbone made up of ribofuranoside units linked by
DNA and RNA each contain four monomers, called nucleotides that differ in
the structure of the bases bonded to the ribose units.
Keywords RNA, nucleosides, ribofuranoside
Uridine, Adenosine, and Guanosine
Figure 23-22 The four common ribonucleosides are cytidine, uridine,
adenosine, and guanosine.
Ribonucleosides are components of RNA based on glycosides of the
furanose form of D-ribose.
Keywords cytidine, uridine, adenosine, guanosine, ribonucleosides
Figure 23-23 Four common ribonucleotides. These are ribonucleosides
Caption esterified by phosphoric acid at their 5'-position, the -CH2OH at the end of
the ribose chain.
Notes Ribonucleosides are joined together by phosphate esters linkages.
Keywords ribonucleotides, ribonucleosides, phosphorilation
Figure 23-24 Two nucleotides are joined by a phosphate linkage between the
5'-phosphate group of one and the 3'-hydroxyl group of the other.
A molecule of RNA always has two ends (unless it is in the form of a large
ring); one end has a free 3' group, and the other end has a free 5' group.
Keywords phosphate linkage
common deoxyribonucleosides that make up DNA.
The structure of the DNA polymer is similar to that of RNA, except there are
no hydroxyl groups on the 2' carbon atoms of the ribose rings.
Pairing in DNA and RNA
Figure 23-25 Base pairing in DNA and RNA. Each purine forms a stable
hydrogen-bonded pair with a specific pyrimidine base.
Guanine hydrogen bonds to cytosine in three place; adenine hydrogen bonds
to thymine in two places.
Keywords purine, pyrimidine, guanine, cytosine, adenine, thymine
Figure 23-26 DNA usually consists of two complimentary strands, with all the
base pairs hydrogen-bonded together.
The two strands are anti-parallel, running in opposite directions. One strand
Notes is arranged 3' to 5', while the other runs in the opposite direction, 5' to 3' from
left to right.
Figure 23-27 Double helix of DNA. Two complimentary strands are joined by
hydrogen bonds between the base pairs.
Notes This double strand coils into a helical arrangement.
Keywords double helix
Figure 23-28 Replication of the double strand of DNA. A new strand is
on each of the original strands, with the DNA polymerase enzyme
forming the phosphate ester bonds of the backbone.
A similar process transcribes DNA into a complimentary molecule of
messenger RNA for use by ribosomes as template for protein synthesis.
Keywords replication, DNA polymerase enzyme, ribosomes
Caractérisation des sucres réducteurs
par le réactif à la phénylhydrazine
Les photographies ci-dessous présentent les cristaux caractéristiques obtenus avec différents glucides.
Ces cristaux peuvent présenter des formes variables en fonction de la concentration en sucre et du
mode de refroidissement. Pour en savoir plus, cliquer sur le nom du glucide souhaité.
Pourquoi le glucose et le fructose donnent-ils des cristaux identiques ?
Caractérisation des sucres réducteurs
par le réactif à la phénylhydrazine
Les oses réducteurs possèdent une fonction aldéhyde (-C=O). Cette fonction réagit avec les molécules
de phénylhydrazine pour donner naissance à des cristaux appelés "osazones". Les atomes de carbone
des oses sont numérotés en fonction de leur place dans la molécule. Ce sont toujours les carbones 1 et
2 qui participent à la formation des cristaux.
exemple du glucose :
étapes de la réaction
formule du glucose
ajout d'une molécule de
(R correspond à la partie non active de
réaction entre la molécule de
phénylhydrazine et la molécule de
ajout d'une deuxième molécule de
phénylhydrazine et réaction
d'oxydation entre le carbone 2 du
glucose et la phénylhydrazine
ajout d'une troisième molécule de
phénylhydrazine et réaction de cette
molécule avec le carbone 2 du glucose
La molécule formée est un osazone qui
cristallise sous la forme d'un épi. Pour
le glucose, il porte le nom de
NH2-H + R-NH-H
Le glucose et le fructose étant des isomères qui ont la même configuration au niveau des atomes de
carbone porteurs des fonctions alcool (-COH) possèdent le même osazone qui est le glucosazone
(cristaux en épi).
Pourquoi le saccharose, sucre non réducteur, donne-t'il naissance à des
Le saccharose est constitué par l'association d'un glucose et d'un fructose liés par les carbones C 1 et C2
portant les fonctions réductrices. Ce sucre n'est donc pas réducteur et ne devrait donc pas former
d'osazone. Au cours de la réaction, la présence d'acide acétique entraîne une hydrolyse du saccharose
en molécules de glucose et de fructose qui réagissent avec la phénylhydrazine.
La photographie ci-dessous correspond aux cristaux obtenus avec le saccharose.
Pour visualiser ces cristaux aux différentes concentrations, cliquer sur "saccharose".
Matériel nécessaire :
- solution de sucres réducteurs à tester.
- tubes à essai (autant que de solution à tester).
- réactif à la phénylhydrazine :
- 90 g d'acétate de sodium
- 180 ml d'acide acétique
- 900 ml d'eau distillée
- 45 ml de phénylhydrazine
- 45 ml de bisulfite de sodium molaire
- bain thermostaté à 80°C.
- lames et lamelles (autant que de solution à tester).
- pipettes graduées et propipettes (autant que de solution à tester).
Protocole expérimental :
- Mettre, dans un tube à essai, 2 mL de réactif à la phénylhydrazine et 2 mL de la solution de sucres à
- Placer le tube au bain thermostaté à 80°C pendant 1 heure.
- Sortir le tube du bain et le laisser refroidir, en fonction du temps disponible (temps idéal : 30 min)
- Prélever au compte-goutte une petite quantité de la solution contenant les cristaux. Réaliser une
préparation microscopique de cette solution.
- Observer les cristaux au microscope et identifier le sucre en le comparant à la fiche résultat
Carbohydrates are the most abundant class of organic compounds found in living organisms. They
originate as products of photosynthesis, an endothermic reductive condensation of carbon dioxide
requiring light energy and the pigment chlorophyll.
n CO2 + n H2O + energy
CnH2nOn + n O2
As noted here, the formulas of many carbohydrates can be written as carbon hydrates, C n(H2O)n, hence
their name. The carbohydrates are a major source of metabolic energy, both for plants and for animals
that depend on plants for food. Aside from the sugars and starches that meet this vital nutritional role,
carbohydrates also serve as a structural material (cellulose), a component of the energy transport
compound ATP, recognition sites on cell surfaces, and one of three essential components of DNA and
Carbohydrates are called saccharides or, if they are relatively small, sugars. Several classifications of
carbohydrates have proven useful, and are outlined in the following table.
sugars having an aldehyde function or an acetal equivalent.
sugars having a ketone function or an acetal equivalent.
sugars oxidized by Tollens' reagent (or Benedict's or Fehling's reagents).
sugars not oxidized by Tollens' or other reagents.
Carbohydrates have been given non-systematic names, although the suffix ose is generally used. The
most common carbohydrate is glucose (C6H12O6). Applying the terms defined above, glucose is a
monosaccharide, an aldohexose (note that the function and size classifications are combined in one
word) and a reducing sugar. The general structure of glucose and many other aldohexoses was
established by simple chemical reactions. The following diagram illustrates the kind of evidence
considered, although some of the reagents shown here are different from those used by the original
Hot hydriodic acid (HI) was often used to reductively remove oxygen functional groups from a molecule,
and in the case of glucose this treatment gave hexane (in low yield). From this it was concluded that the
six carbons are in an unbranched chain. The presence of an aldehyde carbonyl group was deduced from
cyanohydrin formation, its reduction to the hexa-alcohol sorbitol, also called glucitol, and mild oxidation to
the mono-carboxylic acid, glucuronic acid. Somewhat stronger oxidation by dilute nitric acid gave the
diacid, glucaric acid, supporting the proposal of a six-carbon chain. The five oxygens remaining in
glucose after the aldehyde was accounted for were thought to be in hydroxyl groups, since a pentaacetate derivative could be made. These hydroxyl groups were assigned, one each, to the last five
carbon atoms, because geminal hydroxyl groups are normally unstable relative to the carbonyl
compound formed by loss of water. By clicking on the above diagram, it will change to display the
suggested products and the gross structure of glucose. The four middle carbon atoms in the glucose
chain are centers of chirality and are colored red.
Glucose and other saccharides are extensively cleaved by periodic acid, thanks to the abundance of
vicinal diol moieties in their structure. This oxidative cleavage, known as the Malaprade reaction is
particularly useful for the analysis of selective O-substituted derivatives of saccharides, since ether
functions do not react. The stoichiometry of aldohexose cleavage is shown in the following equation.
HOCH2(CHOH)4CHO + 5 HIO4
H2C=O + 5 HCO2H + 5 HIO3
The Configuration of Glucose
The four chiral centers in glucose indicate there may be as many as sixteen (2 4) stereoisomers having
this constitution. These would exist as eight diastereomeric pairs of enantiomers, and the initial challenge
was to determine which of the eight corresponded to glucose. This challenge was accepted and met in
1891 by the German chemist Emil Fischer. His successful negotiation of the stereochemical maze
presented by the aldohexoses was a logical tour de force, and it is fitting that he received the 1902 Nobel
Prize for chemistry for this accomplishment. One of the first tasks faced by Fischer was to devise a
method of representing the configuration of each chiral center in an unambiguous manner. To this end,
he invented a simple technique for drawing chains of chiral centers, that we now call the Fischer
projection formula. Click on this link for a review.
At the time Fischer undertook the glucose project it was not possible to establish the absolute
configuration of an enantiomer. Consequently, Fischer made an arbitrary choice for (+)-glucose and
established a network of related aldose configurations that he called the D-family. The mirror images of
these configurations were then designated the L-family of aldoses. To illustrate using present day
knowledge, Fischer projection formulas and names for the D-aldose family (three to six-carbon atoms)
are shown below, with the asymmetric carbon atoms (chiral centers) colored red. The last chiral center in