CARBOHYDRATES
Carbohydrates are the most abundant organic constituents of
plants.
They serve as the major source of chemical energy for living
organisms (e.g. sugars and starch), as well as important constituents of
supporting tissues (e.g. cellulose in wood, cotton and flax).
The general name "Carbohydrates" is derived from that most of
these compounds have the formula Cx (H2O)y and thus, appear to be
"hydrates of carbon".
They usually contain hydrogen and oxygen in the same
proportion as in water (2:1).
Metabolism
Stored carbohydrates such as the starch of plant and
glycogen of animals are made available for energy
production through oxidation to carbon dioxide and
water.
C x (H2O) y + x O2
x CO2 + + y H2O + Energy
The energy released is converted to heat, and much of it is
conserved in a new chemical form .
Monosaccharides
These consist of only one saccharide or sugar unit and they are
non-hydrolysable.
They are subclassified according to:
1-The number of carbon atoms present in their molecule and,
2-The type of carbonyl group they contain.
• Thus, a monosaccharide containing three carbon atoms is called
a triose and that containing five is called a pentose and so on.
• A monosaccharide containing an aldehyde group is called an
aldose and one containing a keto group is called a ketose.
• These two classifications are frequently combined: e.g. a fivecarbon aldose, for example, is called an aldopentose, a sixcarbon ketose is called a ketohexose
Oligosaccharides
• These consist of 2 and up to 10 molecules of simple
sugars and are hydrolysable.
• They are sometimes called compound or complex sugars.
• They are subclassified into di-, tri- and tetrasaccharides
etc…, according to the number of molecules of simple
sugars they yield on hydrolysis.
Polysaccharides
• Polysaccharides are high molecular weight polymers of
monosaccharides of very complex nature.
• They are hydrolysable and yield a large number of
monosaccharides.
Configuration of Monosaccharides
D and L designations of monosaccharides
The simplest monosaccharide, which contains only
one stereocenter, is glyceraldehyde.
CHO
H
CHO
OH
CH2OH
D-Glyceraldehyde
HO
H
CH2OH
L-Glyceraldehyde
These two forms of glyceraldehyde have been chosen as
arbitrary standards for the D and L-series of
monosaccharides in the sugar chemistry
A monosaccharide in which the OH group attached to the
carbon atom next to the CH2OH (farthest asymmetric
carbon atom from the carbonyl group) is always to the
right is designated as a “D-sugar” and that with the same
OH to the left as “L -sugar”.
D and L designations are like (R) and (S) designations in
that they are not necessarily related to the optical
rotations of the sugars to which they are applied. Thus,
one may encounter sugars that are D (+) or D (-) and
others that are L (-) or L (+).
Reactions of monosaccharides
1-Reactions similar to alcohols
Ether Formation:
As alcohols, simple sugars form ethers. The methyl
derivatives (ethers) formed by reaction with
methylating agents are useful in determination of both
the ring size of monosaccharides and site of linkage in
oligo- and polysaccharides.
2-Reactions similar to carbonyl compounds
•
Glycoside formation ( acetal formation )
Each carbonyl group (aldehydic or ketonic) reacts with two molecules of alcohol to give an
acetal.
O
ROH
OH
OR
Carbonyl compound
Hemiacetal
ROH
OR
+ H2O
OR
Acetal
Oxidation
These reactions produce different products according to the reagent used.
i-Bromine water; is a mild oxidizing reagent. It selectively oxidizes the -CHO group into COOH, and converts aldoses to the corresponding aldonic acids, e.g. glucose is
transformed to gluconic acid.
ii-Nitric acid; is a strong oxidizing agent. It oxidizes both the -CHO and terminal -CH2OH of an
aldose to -COOH groups, and these dicarboxylic acids are known as aldaric acids.
Example, oxidation of glucose into glucaric acid (or saccharic acid) and galactose into
mucic acid.
2-Reactions similar to carbonyl compounds
•
Glycoside formation ( acetal formation )
Each carbonyl group (aldehydic or ketonic) reacts with two molecules of alcohol to give an
acetal.
O
ROH
OH
OR
Carbonyl compound
Hemiacetal
ROH
OR
+ H2O
OR
Acetal
Oxidation
These reactions produce different products according to the reagent used.
i-Bromine water; is a mild oxidizing reagent. It selectively oxidizes the -CHO group into COOH, and converts aldoses to the corresponding aldonic acids, e.g. glucose is
transformed to gluconic acid.
ii-Nitric acid; is a strong oxidizing agent. It oxidizes both the -CHO and terminal -CH2OH of an
aldose to -COOH groups, and these dicarboxylic acids are known as aldaric acids.
Example, oxidation of glucose into glucaric acid (or saccharic acid) and galactose into
mucic acid.
Reaction with oxidising cations
•
All monosaccharides and reducing disaccharides (i.e. all
sugars containing free hemiacetal or hemiketal groups) are
readily oxidized by metal ions such as Cu+2 (Fehling’s and
Benedict’s reagents), Bi+3 and Hg+2 in alkaline medium.
• These reactions are used for identification and quantification
of reducing sugars.
CHO
H
HO
OH
CH 2OH
O
OH
OH
H
H
OH
H
OH
O
HO
HO
-D-glucose
CH 2OH
O
CH 2OH
HO
H
H
H
OH
H
OH
CH 2OH
O
H
H
OH
H
OH
HO
HO
H
CH 2OH
OH
OH
-D-galactose
OH
O
HO
HO
O
CH 2OH
CHO
OH OH
OH
-D-mannose
HO
OH
CH 2OH
H
OH
OH
H
HO
OH
CH 2OH
O
CHO
HO
H
O
-D-glucose
H
OH
HO
OH
HO
HO
CH 2OH
O
CHO
OH
O
HO
HO
OH
OH
-L-galactose
OH
Ketohexoses
β-D-Fructose (levulose or fruit sugar)
Source
It is found free in honey and in fruits juices, or as constituent of polysaccharide e.g.
inulin.
Preparation
• The best large-scale source of fructose is the “inversion” (hydrolysis) of sucrose by
acid or invertase enzyme.
•
•
•
•
•
Properties
Fructose occurs as colourless crystals with intense sweet taste.
It is freely soluble in water.
It reduces Fehling's, and Barfoed's solutions
It forms an osazone similar to that of glucose
It gives a positive Seliwanoff’s test for ketoses (rapid furfural).
Uses
• Fructose is used as food for diabetics (in emergencies of diabetic acidosis) and in
infant feeding formulae (more easily digested than glucose).
The Mucopolysaccharides
Mucopolysaccharides are polysaccharides which on hydrolysis yields
amino sugar units.
• Amino sugars are derived from monosaccharides by replacement of
a hydroxyl group by an amino group.
ex. Heparin
Source
• Heparin is the powerful blood anticoagulant; it can be isolated from
the mast cells of liver, heart and lung.
Biologically Active Carbohydrates
• According to recent researches carbohydrates promise to be a
major source of drug discovery. The diversity and complexity of
carbohydrates explain their wide range of biological functions.
• There are several established carbohydrates-based products
with “biopharmaceutical” application, as well as, other new
products with potential application in medicine, e.g.,
development of specific cancer vaccines, new non-steroidal
anti-inflammatory drugs, and many other examples.
Sweeteners
• Carbohydrates play an important factor in increasing the
incidence of diabetes, obesity and dental caries.
• There is increasing need for other alternatives to sucrose as
sweetening agents for medical purposes, especially in case of
diabetes, and for diet improvement.
• These agents should have high solubility in water, good stability,
and a relative sweetness close or equivalent to that of sucrose.
• They should be safe, low caloric, non-cariogenic and noncarcinogenic.
Bulk sweeteners
• These are the traditional sweeteners such as sucrose, glucose,
fructose and the polyols or sugar alcohols e.g. sorbitol, mannitol,
xylitol and lactitol.
Intense sweeteners
• These are either synthetic e.g. saccharin, aspartame, cyclamate
and or natural e.g. steviol glycosides.
.
Artificial sweeteners
• These are synthetic sweeteners, non-caloric,
non-nutritive and must be used at very low
concentrations.
• Examples: Cyclamate: 300 times sweeter
than sucrose.
Aspartame: 1500 times sweeter
than sucrose.
Saccharin: 3500 times sweeter than
sucrose