Download Carbohydrate chemistry

Carbohydrate chemistry
 Definition
 Nomenclature
 Cyclic form of sugar
 Optical isomerism
 Glycosidic link
 Polysaccharides
 Carbohydrate derivatives
Definition
 Carbohydrate are polyhydroxy
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aldehydes,polyhydroxyketones or compounds that
can be hydrolysed to them
Monosaccharides
Disaccharides
Oligosaccharides
polysaccharides
Nomenclature
 Based on the no. of carbons with the suffix –ose
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added.
CnH2On,n= no. of carbons
Triose= three carbons
Tetrose= four carbon
Pentose = five carbon
Hexose= six carbon
cont.
 Nomenclature can also indicate the reactive groups.
 A) Aldoses are monosaccharides with aldehyde.
 B) ketoses are monosaccharides with a group
containing a ketone.
 Sugar with aldehyde=aldohexoses.
 System of numbering the carbon . The carbon are
numbered sequentially with aldehyde or ketone
group being on the carbon with the lowest possible
number.
Cyclic forms of sugar
 Chemical and physical properties of many sugars
have shown that cyclic forms predominate over
open-chain structure, both in liquid and solid state.
 Cyclic form of sugar = pyranoses; they resemble
pyran.
 Frutose= Furanose.
 The intermolecular hemiacetals.When an aldehyde
reacts with alcohol. They are unstable compounds.
Open chain and cyclic form
Optical isomerism
 Asymmetric carbons: carbohydrates contain
asymmetric carbons,those bounded to four different
atoms or groups of atoms. Because of this
asymmetry,carbohydrates are optically active and
rotate the plane of polarized light.
Optically active substances
An optically active substance is one which can rotate the plane of polarisation of plane polarised light. if you shine a beam of
polarised monochromatic light (light of only a single frequency - in other words a single colour) through a solution of an optically
active substance, when the light emerges, its plane of polarisation is found to have rotated.
The rotation may be either clockwise or anti-clockwise. Assuming the original plane of polarisation was vertical, you might get
either of these results.
Cont.
 If plane polarized light is rotated to the right
(clockwise) ,the compound is dextrorotatory.(+).
 If plane polarized light is rotated to the left
(counterclockwise) , the compound is called
levorotatory. (-).
configuration
 The simplest carbohydrate are the monosaccharides
trioses for e.g glyceraldehydes,which has two
optically active forms designated L and D form
L- Glecarldehyde; D-Glyceraldehyde
Cont.
 A sugar may be Dextrorotatory or levorotatory
irrespective of its D or L configuration.
 Anomeric carbon : is a new asymmetric carbon that
is created by cyclization at the carbon bound to
oxygen in hemiacetal formation.
 If the hydroxyl on the anomeric carbon is below the
plane of the ring ,it is said to be in the alpha position,
if it is above the plane of the ring , it is said to be beta
position
Cont.
Cont.
 In solution alpha and beta slowly change into an
equilibrium mixture of both. This process is known
as mutarotation.
Glycosidic links
 Acetals : Hemiacetals reacts with alcohols to form
acetals.
 Glycosides: a sugar react with an alcohols to form an
acetal known as glycoside.
 If the sugar residue is glucose, the derivative is a
glucoside, if fructose, it is fructoside , if galactose , it
is a galactoside.
 When R is another sugar , the gylcoside is a
disaccharaide.
cont.
 Glycosidic linkages : Reading from left to right.
Therefore , sucrose has an alpha 1,2 glycosidic
linkage.
 The disaccharides maltose possess an unattached
anomeric carbon atom, which may have either the
alpha or beta configuration. The glycosidic linkage in
maltose must be in the alpha 1,4 glycosidic
configuration.
maltose
Polysaccharides
 Amylose is a linear unbranched polymer of alpha
units in a repeating sequence of α 1,4 – glycosidic
linkages.
 Amylopectin is a branched polymer of a α D glucose
with α 1,4 glycosidic linkages and with α 1,6
branching points that occur at intervals of
approximately 25 to 30 α D glucose residues.
 Glycogen is a major storage form of carbohydrates in
animals found mostly in liver and muscles . It is
highly branched form of amylopectin .
Cont.
 Cellulose is composed of chain of d- glucose units
joined by Beta 1,4 linkages. The chains are
exclusively linear (i.e unbranched).
 1. Cellulose is a structural polysaccharides of plants
cells.
 2.Although cellulose forms a part of the human
diet(i.e diet in vegetables and fruits).it is not
hydrolyzed by human enzymes systems.
Carbohydrates derivatives
 A. Phosphoric acid esters of monosaccharides
 1. Phosphorylation is the initial step in the
metabolism of sugars.
 2. phosphorylation sugars such as d – glucose 1phosphate are metabolic intermediates.
Phosphorylation
Cont.
 Amino sugars . In these a , hydroxyl group is
replaced by an amino or an acetylamino group.
D acetylglucosamine
cont.
 1. Glucosamine is the product of hydrolysis of chitin,
the major polysaccharides of the shells of insects and
crustaceans(arthopods).
 2. Galactosamine is found in the polysaccharides of
cartilage, chondroitin sulphate(oso3-).
Cont.
 Sugars acid : are produced by oxidation of the
aldehydic carbon , the hydroxyl carbon , or both.
Ascorbic acid (vitamin C ) is a sugar acid.
 Deoxy sugars include 2- deoxyribose , found in DNA
Sugar alcohols
 Monosaccharides , both aldoses and ketones may be
reduced at the carbonyl carbon to the corresponding
polyhydroxy alcohols(sugar alcohols).
 Aldoses yield the corresponding alcohols , while
ketoses forms two alcohols because a new
asymmetric carbon is found in the process.
Cont.
 Commonly occuring aldoses and ketoses forms the
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following sugar alcohols:
A. D-Glucose yields D sorbitol.
B. D Mannose yields D mannitol.
C. D Galactose yields dulcitol.
D. D.Fructose,and ketose yields D mannitol and D
sorbitol.
Cont.
 There are enzyme sysytem in humans tissues that
can reduce monosaccharides to their corresponding
alcohols.e.g
 The formation of D-Fructose from D glucose via Dsorbitol in seminal vesicle.
 In cases of galactokinase deficency ,excess galactose
may be reduced to dulcitol in the lens of the eye.
Cont.
 Sugar alcohols are also normal constituents of cell
components as for e.g ribitol which is the
carbohydrates moiety of riboflavin, found in the
electron carriers riboflavin phosphate and flavin –
adenine dinucleotide.