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CARBOHYDRATESExamples of Furanose forms

Carbohydrates are the most abundant class of organic molecules found in nature. The name
carbohydrate arises from the basic molecular formula (CH2O)n, which can be re-written
(C.H2O). This shows that these substances are hydrates of carbon, where n =3 or more. Initially it
was believed that carbohydrates contain carbon, hydrogen and oxygen in which hydrogen and
oxygen exist in the same ratio as found in water (2:1). For example glucose “C6H12O6” contain
hydrogen and oxygen in 2:1 ratio (6(CH2O). But there are several compounds which contain
hydrogen and oxygen in the same ratio as found in water but still they are not carbohydrate, for
example acetic acid “C2H4O2” {2C(2H2O)}. Moreover, there are several compounds that do not
contain hydrogen and oxygen in the same ratio as found in water but still they are carbohydrate
in nature, e.g. rhaffinose.
In general carbohydrates are poly hydroxy aldehyde or poly hydroxy ketone and their
derivatives. This means that carbohydrates are organic compounds having several
hydroxyl groups along with aldehyde or ketonic functional group in their structures. Some
of the carbohydrates also contain nitrogen along with carbon, hydrogen and oxygen.
Carbohydrates constitute a versatile (multi purpose) class of molecules. Energy from the sun
captured by green plants, algae, and some bacteria during photosynthesis is stored in the form of
carbohydrates. In turn, carbohydrates are the metabolic precursors of virtually all other
biomolecules. Breakdown of carbohydrates provides the energy that sustains animal life. In
addition, carbohydrates are covalently linked with a variety of other molecules. Carbohydrates
linked to lipid molecules, (glycolipids) are common components of biological membranes.
Proteins that have covalently linked carbohydrates are called glycoproteins. These two classes of
bio-molecules, together called glycoconjugates, are important components of cell walls and extra
cellular structures in plants, animals, and bacteria. In addition to the structural roles such
molecules play, they also serve in a variety of processes involving recognition between cell types
or recognition of cellular structures by other molecules. Recognition events are important in
normal cell growth, fertilization, transformation of cells, and other processes. All of these
functions are made possible by the characteristic chemical features of carbohydrates: like,
(1) The existence of at least one and often two or more asymmetric centers,
(2) The ability to exist either in linear or ring structures,
(3) The capacity to form polymeric structures via glycosidic bonds, and
(4) The potential to form multiple hydrogen bonds with water or other molecules in their
●Carbohydrate classification:
Carbohydrates are generally classified into three groups: monosaccharides (and their
derivatives), oligosaccharides, and polysaccharides. The monosaccharides also called simple
sugars and have the formula (CH2O)n, where n represents the number of carbon atoms.
Monosaccharides cannot be broken down into smaller sugars under mild conditions. Oligosaccharides derive their name from the Greek word oligo, meaning “few,” and consist of
from two to ten simple sugar molecules. The common oligosaccharide found in nature is
disaccharides. Four to six-sugar-unit Oligo-saccharides are usually bound covalently to other
molecules, including glycoproteins. As their name suggests, polysaccharides are polymers of
the simple sugars and their derivatives. They may be either linear or branched polymers and
may contain hundreds or even thousands of monosaccharide units. Their molecular weights
range up to 1 million or more.
Monosaccharides are the carbohydrates which cannot be hydrolyzed into simpler sugar:
These are also called simple sugars and have the formula (CH2O)n, where n is the number of
carbon atoms. Monosaccharides are colorless, crystalline solids that are freely soluble in
water and have a sweet taste. They are further classified in to two classes based on their
functional group and number of carbon atoms. They may be classified as trioses, tetroses,
pentoses, hexoses, or heptoses, depending upon the number of carbon atoms; and as aldoses
or ketoses depending upon whether they have an aldehyde or ketone group. The simple of
monosaccharides are three carbon compounds, glycereldehyde and Dihydroxyacetone.
# Carbons Category Name
Relevant examples
Glyceraldehyde, Dihydroxyacetone
Ribose, Ribulose, Xylulose
Glucose, Galactose, Mannose, Fructose
Neuraminic acid, also called sialic acid
Open chain structures of monosaccharides.
Monosaccharides and other sugars will often be represented in this book by Fischer
projections. In a Fischer projection of a molecule, atoms joined to an asymmetric carbon
atom by horizontal bonds are in front of the plane of the page, and those joined by vertical
bonds are behind. Fischer projections are useful for depicting carbohydrate structures
because they provide clear and simple views of the stereochemistry at each carbon center.
Those molecules, which have multiple asymmetric carbons, they exist as diastereoisomers,
isomers that are not mirror images of each other, as well as enantiomers.
Pentoses and Hexoses Cyclize to Form Furanose and Pyranose Rings
The predominant forms of ribose, glucose, fructose, and many other sugars in solution are
not open chains. Rather, the open chain forms of these sugars cyclize into rings. In general,
an aldehyde can react with an alcohol to form a hemiacetal.
Hemiacetal is the linkage formed by the condensation of an aldehyde or ketonic group with
the hydroxyl group of its own molecule or with another molecule.
For an aldohexose such as glucose, the C-1 aldehyde group in the open-chain form of glucose
reacts with the C-5 hydroxyl group to form an intramolecular hemiacetal. The resulting
cyclic hemiacetal, a six-membered ring, is called pyranose because of its similarity to pyran.
Similarly, a ketone can react with an alcohol to form a hemiketal. The C-2 keto group in the
open-chain form of fructose can form an intramolecular hemiketal by reacting with either the
C-6 hydroxyl group to form a six-membered cyclic hemiketal or the C-5 hydroxyl group to
form a five-membered cyclic hemiketal. The five-membered ring is called a furanose because
of its similarity to furan. The reaction is catalyzed by acid (H) or base (OH) and is readily
The cyclic pyranose and furanose forms are the preferred structures for monosaccharides in
aqueous solution. At equilibrium, the linear aldehyde or ketone structure is only a minor
component of the mixture (generally much less than 1%). When hemiacetals and hemiketals
are formed, the carbon atom that carried the carbonyl group becomes an asymmetric carbon
atom and therefore an additional asymmetric carbon is formed.
Examples of Furanose forms of Monosaccharides
Fructose (furanose)
Conformation of Pyranose and Furanose Rings
The six-membered pyranose ring is not planar, because of the tetrahedral geometry of its
saturated carbon atoms. Instead, pyranose rings adopt two classes of conformations, termed
chair and boat because of the resemblance to these objects. In the chair form, the substituents
on the ring carbon atoms have two orientations: axial and equatorial. Axial bonds are nearly
perpendicular to the average plane of the ring, whereas equatorial bonds are nearly parallel to
this plane. Axial substituents sterically hinder each other if they emerge on the same side of
the ring (e.g., 1,3-diaxial groups). In contrast, equatorial substituents are less crowded. The
chair form of D-glucopyranose predominates because all axial positions are occupied by
hydrogen atoms. The bulkier -OH and -CH2OH groups emerge at the less-hindered
periphery. The boat form of glucose is disfavored because it is quite sterically hindered. The
chair form is more stable because of less steric hindrance as hydrogen atoms occupy the axial
- D-Glucose (chair form)
-D-Glucose (boat form)
Isomerism in Monosaccharides
Compounds having the molecular formula but different structural formula are called isomers and
the phenomenon is known as isomerism. There are several kind of isomerism found in
monosaccharides, as listed bellow.
1). D and L isomerism
The orientation of the -H and -OH groups around the carbon atom adjacent to the terminal
primary alcohol carbon (carbon 5 in glucose) determines whether the sugar belongs to the D or L
series. When the –OH group on this carbon is on the right, the sugar is the D-isomer; when it is
on the left, it is the L-isomer. Most of the monosaccharides occurring in mammals are D sugars,
and the enzymes responsible for their metabolism are specific for this configuration. In solution,
glucose is dextrorotatory, hence the alternative name dextrose, often used in clinical practice.
The presence of asymmetric carbon atoms also confers optical activity on the compound. When a
beam of plane-polarized light is passed through a solution of an optical isomer, it will be rotated
either to the right, dextrorotatory (+); or to the left, laevorotatory (−). The direction of rotation is
independent of the stereochemistry of the sugar, so it may be designated D(−), D(+),L(−), or
L(+). For example, the naturally occurring form of fructose is the D(−) isomer while glucose is
D(+) isomer.
L- Glucose
D- Fructose
L- Fructose
2). Pyranose and Furanose Ring structures
When glucose and other monosaccharides are dissolved in water, they cyclize into 5 or 6
membered ring structures. This is due to the formation of hemiacetal and hemiketal linkage
between the carbonyl group and the hydroxyl group of its own molecule. The C-1 aldehyde
group in the open-chain form of glucose reacts with the C-5 hydroxyl group to form an
intramolecular hemiacetal. The resulting cyclic hemiacetal, a six-membered ring, is called
pyranose because of its similarity to pyran. Similarly, a ketone can react with an alcohol to form
a hemiketal. The C-2 keto group in the open-chain form of fructose can form an intramolecular
hemiketal by reacting with either the C-6 hydroxyl group to form a six-membered cyclic
hemiketal or the C-5 hydroxyl group to form a five-membered cyclic hemiketal. The fivemembered ring is called a furanose because of its similarity to furan.
Haworth Projection of α-D-Glucose
(Hemiacetal formation between C1 & C5
forming six membered ring.)
3). Alpha and beta anomers.
The carbon which carries the functional group is known is anomeric carbon. The C-1 in glucose
carries the aldehyde functional group and C-2 in fructose carries the ketonic group are anomeric
in nature. The isomers, which are formed as a result H and OH of configuration around the
anomeric carbon, are called anomers. In solution, the H and the OH groups mutually
interchanged around the anomeric carbon and thus form two kinds of anomers called alpha and
beta anomers. In alpha anomer, the OH group is bellow the plane of the ring and in beta anomer,
the –OH group is above the plane of the ring. An equilibrium mixture of glucose contains
approximately one-third alpha anomer, two-thirds beta anomer, and <1% of the open chain form.
The beta form is more stable than the alpha form due to less steric hindrance. In alpha anomer,
the two adjacent OH group on C1 & C2 are cis and close to each other, thus causing mutual
repulsion due to steric hindrance. While in beta anomer, the two OH groups on C1 & C2 are
transe to each other. Secondly, the OH group on anomeric carbon (C1) makes a hydrogen bond
with the ring’s oxygen and thus stabilizes the beta form.
In the Fischer projection, if OH group is on the right side of the anomeric carbon it will be called
alpha anomer and will be called beta anomer if OH is on the left side of the anomeric carbon.
The same nomenclature applies to the furanose ring form of fructose, except that alpha and beta
refer to the hydroxyl groups attached to C-2, the anomeric carbon atom. Fructose forms both
pyranose and furanose rings. The pyranose form predominates in fructose free in solution, and
the furanose form predominates in many fructose derivatives.
Fischer projections of glucose
4) Epimers: The carbons other than the anomeric carbon are known as epimeric carbons, usually
carbon 2, 3 and 4 in glucose. Isomers formed as a result of variations in configuration of the -OH
and -H around a single epimeric carbon atoms (2, 3, and 4) of glucose are known as epimers.
Biologically, the most important epimers of glucose are mannose and galactose, formed by
epimerization at carbons 2 and 4, respectively.
They differ from glucose in the configuration of –H and –OH around C-2 and C-4 respectively.
But they are not epimers of each other as they differ in configuration at two asymmetric carbon
atoms around C-2 and C-4. Similarly xylose is an epimer of ribose, which differ at C-3.
D- Mannose
D- Glucose
D- Galactose
Monosaccharides Are Reducing Agents
Those compounds, which reduce others but it self get oxidized are called reducing agents.
Carbohydrates containing free aldehyde or ketonic group (anomeric carbon) are reducing, as they
reduce mild oxidizing agents such as ferric (Fe3) or cupric (Cu2) ion .The carbonyl carbon is
oxidized to a carboxyl group. Glucose and other sugars capable of reducing ferric or cupric ion
are called reducing sugars. This property is the basis of Fehling’s reaction, a qualitative test for
the presence of reducing sugar.
Nomenclature of reducing disaccharides.
To name reducing disaccharides such as maltose and especially to name more complex
oligosaccharides, several rules are followed.
(1) Give the configuration (or) at the anomeric carbon joining the first monosaccharide unit (on
the left) to the second.
(2) (2) Name the nonreducing residue; to distinguish five- and six-membered ring structures,
insert “furano” or “pyrano” into the name.
(3) Indicate in parentheses the two carbon atoms joined by the glycosidic bond, with an arrow
connecting the two numbers; for example, (14) shows that C-1 of the first sugar is joined
to C-4 of the second.
(4) Name the second residue.
The simplest oligosaccharides are the disaccharides, which consist of two monosaccharide units
linked by a glycosidic bond. Each individual unit in an oligosaccharide is termed a residue. The
most common disaccharides found in nature are sucrose, maltose, and lactose. Each is a mixed
acetal, with one hydroxyl group provided intramolecularly and one hydroxyl from the other
monosaccharide. Except for sucrose, each of these structures possesses one free anomeric carbon
atom, and thus each of these disaccharides is a reducing sugar. The end of the molecule
containing the free anomeric car-bon is called the reducing end, and the other end is called the
non-reducing end. In the case of sucrose, both of the anomeric carbon atoms are substituted, that
is, neither has a free -OH group. The substituted anomeric carbons cannot be converted to the
aldehyde configuration and thus cannot participate in the oxidation–reduction reactions
characteristic of reducing sugars. Thus, sucrose is not a reducing sugar.
1). Maltose, Isomaltose, and cellobiose are all homodisaccharides because they each contain
only one kind of monosaccharide, namely, glucose. The disaccharide maltose contains two Dglucose residues joined by a glycosidic linkage between C-1 (the anomeric carbon) of one
glucose residue and C-4 of the other. Because the disaccharide retains a free anomeric carbon (C1 of the 2nd glucose residue), maltose is a reducing sugar. The configuration of the anomeric
carbon atom in the glycosidic linkage is  . The glucose residue with the free anomeric carbon is
capable of existing in - and -pyranose forms. Maltose is produced from starch (a polymer of D-glucose produced by plants) by the action of amylase enzymes and is a component of malt, a
substance obtained by allowing grain (particularly barley) to soften in water and germinate. The
enzyme diastase, produced during the germination process, catalyzes the hydrolysis of starch to
maltose. Maltose is used in beverages (malted milk, for example), and because it is fermented
readily by yeast, it is important in the brewing of beer. In both maltose and cellobiose, the
glucose units are 1-4 linked, meaning that the C-1 of one glucose is linked by a glycosidic bond
to the C-4 oxygen of the other glucose. The only difference between them is in the configuration
at the glycosidic bond. Maltose exists in the alpha configuration, whereas cellobiose is beta.
Isomaltose is obtained in the hydrolysis of some polysaccharides (such as dextran), and
cellobiose is obtained from the acid hydrolysis of cellulose. Isomaltose also consists of two
glucose units in a glycosidic bond, but in this case, C-1 of one glucose is linked to C-6 of the
other, and the configuration is alpha .The complete structures of these disaccharides can be
specified in short-hand notation by using abbreviations for each monosaccharide, or , to1nand
describe these simply as Glc4Glc and Glc6Glc, respectively. More complete names can also be
used, however, so that maltose would be O--D-glucopyranosyl-(1 4)-D-glucopyranose.
Cellobiose, because of its –glycosidic linkage, is formally O--D-glucopyranose-(14)-Dglucopyranose
2). D-lactose (O--D-Galactopyranosyl-(1 4)-D-glucopyranose) is the principal carbohydrate
in milk and is of critical nutritional importance to mammals in the early stages of their lives. It is
formed from D-galactose and D-glucose via  (14) linkage, and yields D-galactose and Dglucose on hydrolysis, Its abbreviated name is Gal(14)Glc. because it has a free anomeric
carbon, it is capable of mutarotation and is a reducing sugar. It is an interesting quirk of nature
that lactose cannot be absorbed directly into the bloodstream. It must first be broken down into
galactose and glucose by lactase, an intestinal enzyme that exists in young, nursing mammals but
is not produced in significant quantities in the mature mammal. Most humans, with the exception
of certain groups in Africa and northern Europe, produce only low levels of lactase. For most
individuals, this is not a problem, but some cannot tolerate lactose and experience intestinal pain
and diarrhea upon consumption of milk. Do you remember Mom who said "you mustn't
drink milk when you have a tummy ache (indigestion/ heart burn)"? She "knew" that
lactase production was reduced when the intestinal mucosa was enflamed. The result of drinking
milk and other fresh dairy products in the absence of lactase is the transport of the sugar to the
large intestine. Bacterial digestion of this leads to gas production. In addition, bacteria produce
two and three carbon compounds that increase the osmotic pressure of the intestinal contents,
thus retaining water. The result; "explosive diarrhea".
3). Sucrose (table sugar)
Sucrose is a white crystalline solid, soluble in water and with a melting point 180°C. When
heated above its melting point, it forms a brown substance known as caramel. It is dextrorotatory
and has a specific rotation of + 66.7°. It is by far the sweetest of the 3 common disaccharides
(sucrose, lactose, and maltose). It is also sweeter than glucose.
It contain glucose and fructose which are linked together by 12 glycosidic linkage. As the
anomeric carbon of both glucose and fructose are utilized in glycosidic linkage, therefore it has
no free anomeric carbon and is non-reducing in nature. Nonreducing disaccharides are named as
glycosides; in this case, the positions joined are the anomeric carbons. In the abbreviated
nomenclature, a double-headed arrow connects the symbols specifying the anomeric carbons and
their configurations. For example, the abbreviated name of sucrose is either Glc(12)Fru or
Sucrose is dextrorotatory in nature because both the glucose and fructose in sugar are
dextrorotatory. In sugar, fructose exists in furanose ring form, which is dextrorotatory. On
hydrolysis, sugar is changed in to laevorotatory. The reason is that on hydrolysis fructose is
changed in to pyranose ring form. The pyranose form of fructose is laevorotatory and its
laevorotatory activity is greater than glucose. The mixture of fructose and glucose obtained as
a result of sucrose hydrolysis is known as invert sugar and the process in which the sign of
rotation is reversed is known as inversion. This reaction occurs in the presence of enzyme
invertase or acid hydrolysis. The honey is rich in invert sugar that’s why honey is much sweeter
than any other carbohydrate. Honey is similar to invert sugar, consisting roughly of 38%
fructose, 31% glucose, 9% disaccharides such as maltose and 17%water.
Derivatives of Monosaccharides
A variety of chemical and enzymatic reactions produce derivatives of the simple sugars. These
modifications produce a diverse array of saccharide derivatives. Some of the most common
derivations are discussed here.
Sugar Acids: Sugars with free anomeric carbon atoms are reasonably good reducing agents and
will reduce hydrogen peroxide, ferricyanide, certain metals (Cu+2 and Ag), and other oxidizing
agents. Such reactions convert the sugar to a sugar acid. For example, addition of alkaline
CuSO4 (called Fehling’s solution) to an aldose sugar produces a red cuprous oxide (Cu2O)
precipitate: and converts the aldose to an aldonic acid, such as gluconic acid.
Formation of a precipitate of red Cu2O constitutes a positive test for an aldehyde. Carbohydrates
that can reduce oxidizing agents in this way are referred to as reducing sugars. Diabetes mellitus
is a condition that causes high levels of glucose in urine and blood, and frequent analysis of
reducing sugars in diabetic patients is an important part of the diagnosis and treatment of this
disease. . Monosaccharides can be oxidized enzymatic ally at C-6, yielding uronic acids, such as
D-glucuronic. Oxidation at both C-1 and C-6 produces aldaric acids, such as D-glucaric acid.
Sugar Alcohols: Sugar alcohols, another class of sugar derivative, can be prepared by the mild
reduction (with NaBH4or similar agents) of the carbonyl groups of aldoses and ketoses. Sugar
alcohols, or alditols, are designated by the addition of –itol to the name of the parent sugar. The
alditols are linear molecules that cannot cyclize in the manner of aldoses. However, alditols are
characteristically sweet tasting, and sorbitol, mannitol, and xylitol are widely used to sweeten
sugarless gum and mints. Sorbitol buildup in the eyes of diabetics is concerned in cataract
Sweet Sorbitol
Sorbitol is a sugar alcohol produced in our bodies by reduction of glucose and which is further
metabolized to fructose. It is readily oxidized and should be a good source of energy. BUT, it is
not taken up in the small intestine since no sorbitol carrier is found here. Therefore, ingested
sorbitol can give food a sweet taste without contribution to the energy content of food. Sorbitol
is used in chewing gum and some "drops" as a "non-fattening" sweetener. As long as one takes
less than those 4-5 grams all is well. More can give "stomach pain" and diarrhea.
Deoxy Sugars
The deoxy sugars are monosaccharides with one or more hydroxyl groups replaced by hydrogen
atoms. In other words they are deficient by one oxygen than the parent sugar. 2-Deoxy-D-ribose
whose systematic name is 2-deoxy-D-erythropentose, is a constituent of DNA in all living
things. Deoxy sugars also occur frequently in glycoproteins and polysaccharides. L-Fucose and
L-rhamnose, both 6-deoxy sugars, are components of some cell walls, and rhamnose is a
component of Ouabain, a highly toxic cardiac glycoside found in the bark and root of the
ouabaio tree. Ouabain is used by the East African Somalis as an arrow poison. The sugar moiety
is not the toxic part of the molecule.
Sugar Esters
Phosphate esters of glucose, fructose, and other monosaccharides are important metabolic
intermediates, and the ribose moiety of nucleotides such as ATP and GTP is phosphorylated at
the 5-position. Several sugar esters are important in metabolism.
Amino Sugars
Amino sugars, including D-glucosamine and D-galactosamine, contain an amino group (instead
of a hydroxyl group) at the C-2 position. They are found in many oligo and polysaccharides,
including chitin, a polysaccharide in the exoskeletons of crustaceans and insects. Muramic acid
and neuraminic acid, which are components of the polysaccharides of cell membranes of higher
organisms and also bacterial cell walls, are glucosamine linked to three-carbon acids at the C-1
or C-3 positions. In Muramic acid (an amine isolated from bacterial cell wall polysaccharides;
murus is Latin for “wall”), the hydroxyl group of a lactic acid moiety makes an ether linkage to
the C-3 of glucosamine. Neuraminic acid (an amine isolated from neural tissue) forms a C-C
bond between the C-1 of N-acetylmannosamine and the C-3 of pyruvic acid. The N-acetyl and
N-glycolyl derivatives of neuraminic acid are collectively known as sialic acid sand are
distributed widely in bacteria and animal systems.
Acetals, Ketals, and Glycosides
Hemiacetals and hemiketals can react with alcohols in the presence of acid to form acetals and
Ketals. Formation of an acetal occurs when the hydroxyl group of a hemiacetal becomes
protonated and is lost as water. The carbocation ion that is produced is then rapidly attacked by a
molecule of alcohol. Loss of the proton from the attached alcohol gives the acetal. Acetals are
stable compared to hemiacetals but their formation is reversible as with esters. As a reaction to
create an acetal proceeds, water must be removed from the reaction mixture or it will hydrolyse
the product. This reaction is another example of a dehydration synthesis and is similar in this
respect to the reactions under-gone by amino acids to form peptides and nucleotides to form
nucleic acids. The pyranose and furanose forms of monosaccharides react with alcohols in this
way to form glycosides with retention of the - or - configuration at the C-1 carbon. The new
bond between the anomeric carbon atom and the oxygen atom of the alcohol is called a
glycosidic bond.
Hemiacetal + alcohol + acid (catalyst) ↔ acetal + water
Formation of hemiacetals and hemiketals.
An aldehyde or ketone can react with an alcohol in a 1:1 ratio to yield a hemiacetal or hemiketal,
respectively, creating a new chiral center at the carbonyl carbon. Substitution of a second alcohol
molecule produces an acetal or ketal. When the second alcohol is part of another sugar molecule,
the bond produced is a glycosidic bond.
Glycosides are formed by condensation between the hydroxyl group of the anomeric
carbon of a monosaccharide and a second compound that may or may not be another
monosaccharide. Glycoside is a carbohydrate for which the OH group attached to the anomeric
carbon has been replaced by an –OR group. The sugar component of glycosides is called glycon
and the non sugar component is called aglycon. Glycosides are named according to the parent
monosaccharide. For example, methyl--D-glucoside can be considered a derivative of -Dglucose.
Example and biomedical importance of glycosides
For example, a-D-glucose can react with methanol to produce a compound called methyl α-Dglucoside. This is a condensation reaction that takes place in the presence of an acid catalyst,
with water being given off as a by-product. The end result is that the OH group attached to the
anomeric carbon is replaced by OCH3.
Structure of methyl glycoside?
Salicin, one of the oldest herbal remedies known, was the model for the synthetic analgesic
aspirin. A large class of hydroxylated, aromatic oxonium cations called anthocyanins provides
the red, purple and blue colors of many flowers, fruits and some vegetables. Peonin is one
example of this class of natural pigments, which exhibit pronounced pH color dependence. The
oxonium moiety is only stable in acidic environments and the color changes or disappears when
base is added. The complex changes that occur when wine is fermented and stored are in part
associated with glycosides of anthocyanins.
Most of the carbohydrates found in nature occur in the form of high molecular weight polymers
called polysaccharides. The monomeric building blocks used to generate polysaccharides can be
varied; in all cases, however, the predominant monosaccharide found in polysaccharides is Dglucose. Polysaccharides, also called glycans, consist of monosaccharides and their derivatives.
If a polysaccharide contains only one kind of monosaccharide molecule, it is a
homopolysaccharide, or homoglycan, whereas those containing more than one kind of
monosaccharide are heteropolysaccharides. The most common constituent of polysaccharides is
D-glucose, but D-fructose, D-galactose, D-mannose, L-abrasions, and D-xylose are also found.
Common monosaccharide derivatives in polysaccharides include the amino sugars (Dglucosamine and D-galactosamine), their derivatives (N-acetylneuraminic acid and Nacetylmuramic acid), and simple sugar acids (glucuronic and iduronicacids).
Homopolysaccharides are often named for the sugar unit they contain, so that glucose
homopolysaccharides are called glucans, while mannose homopolysaccharides are mannans.
They can be classified into three separate groups, based on their different functions. Structural
polysaccharides provide mechanical stability to cells, organs, and organisms. Water binding
polysaccharides are strongly hydrated and prevent cells and tissues from drying out. Finally,
reserve polysaccharides serve as carbohydrate stores that release monosaccharides as required.
Due to their polymeric nature, reserve carbohydrates are osmotically less active, and they can
therefore be stored in large quantities within the cell.
Storage polysaccharides: These are an important form of carbohydrate in plants and animals.
The organisms store carbohydrates in the form of polysaccharides rather than as
monosaccharides so as to lower the osmotic pressure of the sugar reserves. Because osmotic
pressures depend only on numbers of molecules, the osmotic pressure is greatly reduced by
formation of a few polysaccharide molecules out of thousands (or even millions) of
monosaccharide units.
Starch is the major form of stored carbohydrate in plants. It is composed of a mixture of two
substances: amylose, an essentially linear polysaccharide, and amylopectin, a highly branched
polysaccharide. Both forms of starch are polymers of α-D-Glucose. Natural starches contain 1020% amylose and 80-90% amylopectin. Amylose forms a colloidal dispersion in hot water
(which helps to thicken gravies) whereas amylopectin is completely insoluble.
Amylose molecules consist typically of 200 to 20,000 glucose units which form a helix as a
result of the bond angles between the glucose units. Most forms of starch in nature are 10 to 30%
-amylose and 70to 90% amylopectin. Typical corn starch produced in the United States is about
25% -amylose and 75% amylopectin.  -Amylose is composed of linear chains of D-glucose in
(14) linkages. The chains are of varying length, having molecular weights from several
thousand to half a million. As can be seen from the structure in Figure, the chain has a reducing
end and a non reducing end. Although poorly soluble in water, -amylose forms micelles in
which the polysaccharide chain adopts a helical conformation. Iodine reacts with -amylose to
give a characteristic blue color, which arises from the insertion of iodine into the middle of the
hydrophobic amylose helix.
Amylopectin: amylopectin is a highly branched chain of glucose units. Branches occur in these
chains every 12 to 30 residues. The average branch length is between 24 and 30 residues, and
molecular weights of amylopectin molecules can range up to 100 million. The linear linkages in
amylopectin are (-14), whereas the branch linkages are (16). As is the case for amylose, amylopectin forms micellar suspensions in water; iodine reacts with such suspensions
to produce a red-violet color.
Starch is stored in plant cells in the form of granules in the stroma of plastids (plant cell
organelles) of two types: chloroplasts, in which photosynthesis takes place and
amyloplasts, plastids that are specialized starch accumulation bodies. When starch is to
be mobilized and used by the plant, it must be broken down into its component
monosaccharides. Starch is split into its monosaccharide elements by stepwise
phosphorolytic cleavage of glucose units, a reaction catalyzed by starch phosphorylase.
At each step, the products are one molecule of glucose-1-phosphate and a starch
molecule with one less glucose unit. In -amylose, this process continues all along the
chain until the end is reached. Neither -amylase nor -amylase, can cleave the 
(16) branch points of amylopectin, and once again, (16)-glucosidase is required to
cleave at the branch points and allow complete hydrolysis of starch amylopectin.
However, when suspensions of starch granules are heated, the granules swell, taking up
water and causing the polymers to become more accessible to enzymes. Thus, cooked
starch is more digestible.
The hydrolysis of starch is done by amylases. With the aid of an amylase (such as pancreatic
amylase), water molecules enter at the 1  4 linkages, breaking the chain and eventually
producing a mixture of glucose and maltose. Beta-amylase cuts starch into maltose units.
The resulting products are assigned a Dextrose Equivalent (DE) value which is related to the
degree of hydrolysis. A DE value of 100 corresponds to completely hydrolyzed starch, which
is pure glucose (dextrose). Maltodextrin is partially hydrolyzed starch that is not sweet
and has a DE value less than 20. Syrups, such as corn syrup made from corn starch, have
DE values from 20 to 91. Commercial dextrose has DE values from 92 to 99. Corn syrup
solids are mildly sweet semi-crystalline or powdery amorphous products with DEs from 20
to 36 made by drying corn syrup in a vacuum or in spray driers. High fructose corn syrup
(HFCS), commonly used to sweeten soft drinks, is made by treating corn syrup with
enzymes to convert a portion of the glucose into fructose. Commercial HFCS contains from
42% to 55% fructose, with the remaining percentage being mainly glucose. Modified
starch is starch that has been changed by mechanical processes or chemical treatments to
stabilize starch gels made with hot water. Without modification, gelled starch-water
mixtures lose viscosity or become rubbery after a few hours. Hydrogenated glucose
syrup (HGS) is produced by hydrolyzing starch, and then hydrogenating the resulting syrup
to produce sugar alcohols like maltitol and sorbitol, along with hydrogenated oligo- and
polysaccharides. Polydextrose (poly-D-glucose) is a synthetic, highly-branched polymer
with many types of glycosidic linkages created by heating dextrose with an acid catalyst and
purifying the resulting water-soluble polymer. Polydextrose is used as a bulking agent
because it is tasteless and is similar to fiber in terms of its resistance to digestion. The
name resistant starch is applied to dietary starch that is not degraded in the stomach and
small intestine, but is fermented by microflora in the large intestine.
The major form of storage polysaccharide in animals is glycogen. Glycogen is found mainly in
the liver (where it may amount to as much as 10% of liver mass) and skeletal muscle (where it
accounts for 1 to 2% of muscle mass). Liver glycogen consists of granules containing highly
branched molecules, with (16) branches occurring every 8 to 12 glucose units. Like
amylopectin, glycogen yields a red-violet color with iodine. Glycogen can be hydrolyzed by both
- and -amylases, yielding glucose and maltose, respectively,
Chemistry. Glycogen is a branched-chain polysaccharide and resembles amylopectin very
much in structure, but has somewhat more glucose residues per molecule and about one and-ahalf times as many branching points. Also the chains are shorter (10–20 glucose units), and
hence the molecule is even more highly branched and more compact. This crucial molecule is a
homopolymer of glucose in α–(14) linkage; it is also highly branched, with α(16) branch
linkages occurring every 8-10 residues. Glycogen is a very compact structure that results from
the coiling of the polymer chains. This compactness allows large amounts of carbon energy to be
stored in a small volume, with little effect on cellular osmolarity.
Section of Glycogen Showing α–14– and α–16–Glycosidic Linkages
Another important family of storage polysaccharides is the dextrans. These are bacterial and
yeast polysaccharides made up of (α 16)-linked poly-D-glucose; all have (α _13) branches,
and some also have (α 12) or (α 14) branches. The degree of branching and the average
chain length between branches depend on the species and strain of the organism. Bacteria
growing on the surfaces of teeth produce extracellular accumulations of dextrans, an important
component of dental plaque. Synthetic dextrans are used in several commercial products (for
example, Sephadex) that serve in the fractionation of proteins by size-exclusion chromatography.
Structural Polysaccharide
The structural polysaccharides have properties that are dramatically different from those of the
storage polysaccharides, even though the compositions of these two classes are similar.
Structural polysaccharides provide mechanical stability to cells, organs, and organisms. Water
binding polysaccharides are strongly hydrated and prevent cells and tissues from drying out.
Cellulose is a linear homopolymer of D-glucose units having 14 glycosidic linkage, just as in
α-amylose. The structural difference, which completely alters the properties of the polymer, is
that in cellulose the glucose units are linked by  (14)-glycosidic bonds, whereas in α amylose the linkage is α (14). In cellulose, the -CH2OH groups are alternating above and
below the plane of the cellulose molecule thus producing long, unbranched chains. The absence
of side chains allows cellulose molecules to lie close together and form rigid structures (sheet
like). The flattened sheets of the chains lie side by side and are joined by hydrogen bonds. These
sheets are laid on top of one another in a way that staggers the chains, just as bricks are staggered
to give strength and stability to a wall. Cellulose is the major structural material of plants. Wood
is largely cellulose, and cotton is almost pure cellulose. As a polymer of glucose, cellulose has
the formula (C6H10O5)n where n ranges from 500 to 5,000, depending on the source of the
polymer. The glucose units in cellulose are linked in a linear fashion. The beta-glycoside bonds
permit these chains to stretch out, and this conformation is stabilized by intramolecular hydrogen
bonds. Cellulose is extremely resistant to hydrolysis, whether by acid or by the digestive tract
amylases. As a result, most animals (including humans) cannot digest cellulose to any significant
degree. Ruminant animals, such as cattle, deer, giraffes, and camels, are an exception because
bacteria that live in the rumen secrete the enzyme cellulase, a -glucosidase effective in the
hydrolysis of cellulose. The resulting glucose is then metabolized in a fermentation process to
the benefit of the host animal. Termites and shipworms (Teredo navalis) similarly digest
cellulose because their digestive tracts also contain bacteria that secrete cellulase. Most animals
cannot digest cellulose as a food, and in the diets of humans this part of our vegetable functions
Cellulose may be modified in the laboratory by treating it with nitric acid (HNO3) to replace all
the hydroxyl groups with nitrate groups (-ONO2) to produce cellulose nitrate (nitrocellulose or
guncotton) which is an explosive component of smokeless powder. Partially nitrated cellulose,
known as pyroxylin, is used in the manufacture of colluding, plastics, lacquers, and nail polish.
Cellulose is commonly accompanied by a lower molecular weight, branched, amorphous
polymer called hemicellulose. In contrast to cellulose, hemicellulose is structurally weak
and is easily hydrolyzed by dilute acid or base. The term "hemicellulose" is applied to
the polysaccharide components of plant cell walls other than cellulose, or to
polysaccharides in plant cell walls which are extractable by dilute alkaline
solutions.Also, many enzymes catalyze its hydrolysis. Hemicelluloses are composed of
many D-pentose sugars, with xylose being the major component. Mannose and mannuronic
acid are often present, as well as galactose and galacturonic acid. Hemicelluloses may be
found in fruit, plant stems, and grain hulls. Although hemicelluloses are not
digestible, they can be fermented by yeasts and bacteria. The polysaccharides
yielding pentoses on hydrolysis are called pentosans. Xylan is an example of a
pentosan consisting of D-xylose units with 1β→4 linkages. Hemicelluloses comprise
almost one-third of the carbohydrates in woody plant tissue.
Some plants store carbohydrates in the form of inulin as an alternative, or in addition, to
starch. Inulins are present in many vegetables and fruits, including onions, leeks, garlic,
bananas, asparagus, chicory, and Jerusalem artichokes. Inulins are polymers consisting of
fructose units that typically have a terminal glucose. Oligofructose has the same structure
as inulin, but the chains consist of 10 or fewer fructose units. Oligofructose has
approximately 30 to 50 percent of the sweetness of table sugar. Inulin is less soluble than
oligofructose and has a smooth creamy texture that provides a fat-like mouthfeel. Inulin
and oligofructose are nondigestible by human intestinal enzymes, but they are totally
fermented by colonic microflora. The short-chain fatty acids and lactate produced by
fermentation contribute 1.5 kcal per gram of inulin or oligofructose. Inulin and oligofructose
are used to replace fat or sugar and reduce the calories of foods like ice cream, dairy
products, confections and baked goods.
Inulin n= approx 35
Pectin is a polysaccharide that acts as a cementing material in the cell walls of all plant
tissues. The white portion of the rind of lemons and oranges contains approximately 30%
pectin. Pectin is the methylated ester of polygalacturonic acid, which consists of chains of
300 to 1000 galacturonic acid units joined with 1α→4 linkages. The Degree of Esterification
(DE) affects the gelling properties of pectin. The structure shown here has three methyl
ester forms (-COOCH3) for every two carboxyl groups (-COOH), hence it is has a 60%
degree of esterification, normally called a DE-60 pectin. Pectin is an important ingredient of
fruit preserves, jellies, and jams. When heated together with sugar, it causes a thickening that
is characteristic of jams and jellies. Pectin is a long chain of pectic acid and pectinic
acid molecules. Because these acids are sugars, pectin is a polysaccharide.
Pectin is a polymer of α-galacturonic acid
with a variable number of methyl ester groups
A homopolysaccharide that is similar to cellulose, both in its biological function and its primary,
secondary, and tertiary structure, is chitin. Chitin is present in the cell walls of fungi and is the
fundamental material in the exoskeletons of crustaceans, insects, and spiders. The structure of
chitin is identical to cellulose, except that the -OH group on each C-2 is replaced by NHCOCH3, so that the repeating units are N-acetyl-D-glucosamines in  (14) linkage. It may
be regarded as a derivative of cellulose, in which the hydroxyl groups of the second carbon
of each glucose unit have been replaced with acetamido group ( NHCOCH3) Like cellulose,
the chains of chitin form extended ribbons and pack side by side in a crystalline, strongly
hydrogen-bonded form. One significant difference between cellulose and chitin is whether the
chains are arranged in parallel(all the reducing ends together at one end of a packed bundle and
all the non reducing ends together at the other end) or anti parallel(each sheet of chains having
the chains arranged oppositely from the sheets above and below). Natural cellulose seems to
occur only in parallel arrangements. Chitin, however, can occur in three forms, some-times all in
the same organism. -Chitin is an all-parallel arrangement of the chains, whereas -chitin is an
anti parallel arrangement. In -chitin, the structure is thought to involve pairs of parallel sheets
separated by single anti parallel sheets.
Chitin is the earth’s second most abundant carbohydrate polymer (after cellulose), and its ready
availability and abundance offer opportunities for industrial and commercial applications. Chitinbased coatings can extend the shelf life of fruits, and a chitin derivative that binds to iron atoms
in meat has been found to slow the reactions that cause rancidity and flavor loss. Without such a
coating, the iron in meats activates oxygen from the air, forming reactive free radicals that attack
and oxidize polyunsaturated lipids, causing most of the flavor loss associated with rancidity.
Chitin-based coatings coordinate the iron atoms, preventing their interaction with oxygen.
FIGURE 7–18 Chitin. (a) A short segment of chitin,
a homopolymer of N-acetyl-D-glucosamine units in
(_14) linkage.
Some hetero polysaccharides
Glycosaminoglycans Are Heteropolysaccharides
of the Extracellular Matrix
A class of polysaccharides known as glycosaminoglycans is involved in a variety of
extracellular (and sometimes intracellular) functions. Glycosaminoglycans are found in the
lubricating fluid of the joints and as components of cartilage, synovial fluid, vitreous humor,
bone, and heart valves. Glycosaminoglycans are long unbranched polysaccharides
containing repeating disaccharide units that contain either of two amino sugar compounds -N-acetylgalactosamine or N-acetylglucosamine, and an uronic acid such as glucuronate
(glucose where carbon six forms a carboxyl group). Glycosaminoglycans are negatively
charged, highly viscous molecules sometimes called mucopolysaccharides. The
physiologically most important glycosaminoglycans are hyaluronic acid, dermatan sulfate,
condroitin sulfate, heparin, heparin sulfate, and keratan sulfate.
The repeating disaccharide
structures found commonly in glycosaminoglycans are given bellow,
1) Hyaluronic acid (Greek hyalos means “glass”; hyaluronates can have a glassy appearance)
Hyaluronic acid (hyaluronate at physiological pH) contains alternating residues of D-glucuronic
acid and N-acetylglucosamine. With up to 50,000 repeats of the basic disaccharide unit,
hyaluronates have molecular weights greater than 1 million; they form clear, highly viscous
solutions that serve as lubricants in the synovial fluid of joints and give the vitreous humor of the
vertebrate eye its jellylike consistency (the. Hyaluronate is also an essential component of the
extracellular matrix of cartilage and tendons, to which it contributes tensile strength and
elasticity as a result of its strong interactions with other components of the matrix. It also makes
a resistive coating around the cell which prevents the entry of pathogenic microbes in to the cell.
Hyaluronidase, an enzyme secreted by some pathogenic bacteria, can hydrolyze the glycosidic
linkages of hyaluronate, rendering tissues more susceptible to bacterial invasion. In many
organisms, a similar enzyme in sperm hydrolyzes an outer glycosaminoglycan coat around the
ovum, allowing sperm penetration.
2) Chondroitin sulfate (Greek chondros, cartilage”)
It contributes to the tensile strength of cartilage, tendons, ligaments, and the walls of the aorta.
Dermatan sulfate (Greek derma, “skin”) contributes to the pliability of skin and is also present in
blood vessels and heart valves. In this polymer, many of the glucuronate (GlcA) residues present
in chondroitin sulfate are replaced by their epimer, iduronate (IdoA). Chondroitin sulfate is
composed of D-glucuronic acid and N-acetyl galactosamine sulfate joined together by beta 13
3)Keratan sulfates (Greek keras, “horn”) have no uronic acid and their sulfate content is
variable. They are present in cornea, cartilage, bone, and a variety of horny structures formed of
dead cells: horn, hair, hoofs, nails, and claws. Keratan sulfate is composed of repeating units of
D-galactose and N-acetyl glucose amin-6-sulfate residues which are held together by beta 14
4)Heparin (Greek he–par, “liver”) is a natural anticoagulant made in mast cells (a type of leukocyte)
and released into the blood, where it inhibits blood coagulation by binding to the protein
antithrombin. Heparin binding causes antithrombin to bind to and inhibit thrombin, a protease
essential to blood clotting. The interaction is strongly electrostatic; heparin has the highest
negative charge density of any known biological macromolecule. Purified heparin is routinely
added to blood samples obtained for clinical analysis, and to blood donated for transfusion, to
prevent clotting. Heparin is composed of D-glucuronic- 2- sulfate and N- sulfo-D-glucoseamin
6-sulfate which are joined together by alpha 14 linkage.
Chemical properties of monosaccharides.
1) Oxidation: The aldehyde group of an aldose can be oxidized readily to a carboxyl group to
form an aldonic acid. For example, glucose form gluconic acid. Oxidation at the terminal end
produces uronic acid. For example when the last –OH of the glucose is oxidized, it form
glucuronic acid. Sugar chains with –COOH at both ends are called aldaric acids, e.g., glucose
form glucaric acid.
1) Gluconic acid. Its formation needs mild oxidation with bromine water which brings
oxidation at C1.
2) Glucuronic acid. Its formation needs oxidation at C6 while preventing oxidation at C1.
3) Glucaric acid: its formation need oxidation at both C1 and C6 using strong oxidizing
agent like HNO3.
2. Reduction. The sugars may be reduced in various ways depending upon the type of reducing
agent used.
(a) With sodium amalgam: The monosaccharides are reduced to their corresponding alcohols
by treating them with reducing agents like Na-amalgam. Thus, glucose yields sorbitol (glucitol),
mannose yields mannitol, galactose yields dulcitol, fructose yields a mixture of sorbitol and
mannitol, and glycereldehyde yields glycerol.
Sorbitol and mannitol both are important and are used in the manufacture of surface-active
agents and explosives, respectively.
3. Reaction with phenyl hydrazine. ( formation of osazone)
Reaction with phenyl hydrazine involves only 2 carbon atoms viz., the carbonyl (i.e., the
aldehyde or ketone group) and the adjacent one. One mole of phenyl hydrazine reacts with one
mole of aldose (or ketose) to form a mole of hydrazone. With a second mole of phenyl
hydrazine, the hydrazone is oxidized to aldohydrazone (new aldehyde group is produced) and
the phenyl hydrazine itself is reduced to aniline and ammonia. Finally, a third mole of phenyl
hydrazine reacts with the aldohydrazone to produce osazone. The hydrazone may, in fact, be
regarded as a special type of Schiff's base and the osazone as a double Schiff’s base
The reaction with ketose will also take place in a likewise manner. The osazones of reducing
(mutarotating) sugars are crystalline, yellow and usually insoluble compounds and hence
may also be recovered. These have characteristic crystal structure and melting points and are,
therefore, frequently used in the identification of sugars. It may, however, be noted that
glucose, fructose, and mannose would yield the same osazone owing to their similar structure
in the unaffected part of the molecule (from C3 to C6). Moreover, the asymmetry at C2 is
during the reaction. Obviously, galactose would, then, form a different osazone as it has different
configuration below C2.
4) Reaction
with hydroxylamine. Simple sugars react with hydroxylamine to yield oximes.
Fermentation. Monosaccharides such as glucose, fructose and mannose are readily fermented
by yeast. The process of yeast fermentation is very complex. During this process, sugar
phosphate and sugar acid phosphate are formed. Ordinarily, this process results in the formation
of alcohol and carbon dioxide.