This lecture explains the structure of carbohydrates

1. Carbohydrates
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2. Introduction
• Carbohydrates are the most abundant organic molecules in nature
• They are primarily composed of the elements carbon, hydrogen, and
oxygen.
• The name carbohydrate means 'hydrates of carbon'
• Some carbohydrates have the empirical formula (CH2O)n, where n> 3,
but not all carbohydrates follow this formula
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3. Introduction
• Carbohydrates are defined as
polyhydroxyaldehydes or ketones,
or compounds that produce them
on hydrolysis
• The term 'sugar' is applied to
carbohydrates that are soluble in
water and sweet to taste
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4. Functions of carbohydrates
―Abundant dietary source of energy
• 4 Cal/g
―Precursors for many organic compounds
• For example: fats, amino acids.
―Participate in cell membrane structure and cellular functions
• Glycoproteins and glycolipids participate in the membrane structure
• Growth, adhesion of cells
―Structural components of many organisms
• For example: The fiber (cellulose) of plants, exoskeleton of some insects and the cell
wall of microorganisms.
―Storage form of energy (glycogen)
• Eg: glycogen
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5. Classification of Carbohydrates
• Carbohydrates are also known as saccharides, meaning sugar
• They are classified into three groups based on the number of sugar
units:
1. Monosaccharides: contain one sugar unit.
2. Disaccharides: contain two sugar units.
3. Oligosaccharides: contain 3 - 10 sugar units
4. Polysaccharides: contain more than 10 sugar units
• Monosaccharides, disaccharides, and oligosaccharides are sweet,
crystalline, and water-soluble, hence called sugars
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6. Monosaccharides
• Monosaccharides are the simplest group of carbohydrates.
• They have the general formula Cn(H2O)n.
• They cannot be further hydrolyzed.
• Monosaccharides are divided into different categories based on the
functional group and the number of carbon atoms.
• Aldoses: the functional group is an aldehyde
• e.g. glyceraldehyde, glucose.
• Ketoses : the functional group is a keto
• e.g. dihydroxyacetone, fructose
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7. Monosaccharides…
• Monosaccharides are divided into categories based on functional
groups and number of carbon atoms.
• Trioses have 3 carbons
• Tetroses have 4 carbons
• Pentoses have 5 carbons
• Hexoses have 6 carbons
• Heptoses have 7 carbons
• Glucose is an aldohexose while fructose is a ketohexose.
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8. Monosaccharides…
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9. Monosaccharides— Structural Aspects
• Stereoisomerism is an important character of monosaccharides.
• Stereoisomers are the compounds that have the same structural formulae but
differ in their spatial configuration.
• A carbon is said to be asymmetric when it is attached to four different
atoms or groups.
• The number of asymmetric carbon atoms (n) determines the possible
isomers of a given compound which is equal to 2n.
• Glucose contains 4 asymmetric carbon atoms, hence it has 24 = 16 possible
stereoisomers.
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10. Glyceraldehyde —the reference carbohydrate
• Glyceraldehyde is the simplest monosaccharide.
• It has one asymmetric carbon atom.
• It exists as two stereoisomers.
• It has been chosen as the reference carbohydrate to represent the
structure of all other carbohydrates.
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11. D- and L-isomers
• D and L isomers are mirror images of each other
• The spatial orientation of H and OH groups on the carbon atom
adjacent to the terminal primary alcohol carbon (C5 for glucose)
determines whether the sugar is D- or L-isomer
• If the OH group is on the right side, the sugar is of D-series, and if on
the left side, it belongs to L-series
• The structures of D- and L-glucose are based on the reference
monosaccharide, D- and L-glyceraldehyde (glycerose)
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12. D- and L-isomers
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13. Optical activity of sugars
• Optical activity is a feature of compounds with an asymmetric carbon
atom.
• Dextrorotatory (d+) and levorotatory (l–) are terms used to describe
compounds that rotate the plane of polarized light to the right or left,
respectively.
• Optical isomers can be designated as D(+), D(–), L(+), and L(–) based
on their structural relation with glyceraldehyde.
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14. Optical activity of sugars
• A racemic mixture is a mixture of equal amounts of d- and l-isomers,
and do not exhibit any optical activity.
• In medical practice, the term dextrose is used for glucose in solution
because of its dextrorotatory nature.
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15. Configuration of D-aldoses
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16. Configuration of D-ketoses
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17. Epimers
• Epimers are two monosaccharides that differ
from each other in their configuration around a
single specific carbon atom (other than anomeric
atom)
• Glucose and galactose are C4-epimers,
• Glucose and mannose are C2-epimers
• The interconversion of epimers is called
epimerization and is catalyzed by epimerases
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18. Enantiomers
• Enantiomers are a type of stereoisomers that
are mirror images of each other.
• Enantiomers are designated as D- and L-sugars.
• Majority of the sugars in the higher animals
(including man) are of D-type
• Diastereomers are stereoisomers that are not
mirror images of each other.
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Diastereiomers of erythrose

19. STRUCTURE OF GLUCOSE
• For a better understanding of glucose structure, let us consider the
formation of hemiacetals and hemiketals
• These are produced when an aldehyde or a ketone reacts with alcohol
• The hydroxyl group of monosaccharides can react with its own
aldehyde or keto functional group to form hemiacetal and hemiketal
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20. STRUCTURE OF GLUCOSE
• In glucose, the aldehyde group at C1 reacts with alcohol group at C5
to form two types of cyclic hemiacetals namely α and β
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21. Pyranose and furanose structures
• Haworth projection formulae are depicted by
• A six-membered ring pyranose (based on pyran) or
• A five-membered ring furanose (based on furan).
• The cyclic forms of glucose are known as α-D-glucopyranose and α-D-
glucofuranose
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22. Anomers—mutarotation
• The α and β cyclic forms of D-glucose are known as anomers.
• They differ from each other in the configuration only around C1
known as anomeric carbon (hemiacetal carbon).
• α anomer, the OH group held by anomeric carbon is on the opposite side of
the group CH2OH of sugar ring.
• β-anomer the OH group held by anomeric carbon is on the same side of the
group CH2OH of sugar ring.
• The anomers differ in certain physical and chemical properties.
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23. Anomers—mutarotation
• Mutarotation is a gradual change of specific rotation of any optically
active substance having free aldehyde (-CHO) or ketone (C=O) group.
• α-Glucose freshly dissolved In water, has specific rotation of +112.
• β- Glucose when freshly dissolved in water, has specific rotation of
+19.
• When both anomers are left for sometimes, a and p sugars are
interconvert and slowly change Into an equilibrium mixture of a, p
and open chain glucose which has specific rotation of + 52.5.
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24. Anomers—mutarotation
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25. Properties of monosaccharides:
Physical properties
• All monosaccharides are soluble in water
• All monosaccharides show the property of optical activity
• All monosaccharides can exist in α and β forms.
• All monosaccharides can undergo mutarotation.
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26. REACTIONS OF MONOSACCHARIDES
• Oxidation: Oxidation of sugars gives acids e.g. gluconic acid
• Reduction: Reduction of carbonyl group gives the corresponding alcohol
e.g.
• Glucose gives sorbitol, ribose gives ribitol, galactose gives galacticol etc.
• Reducing sugars: Sugars containing free aldehyde or ketone group can
reduce other reagents e.g. cupric ions of Fehling's and Benedict's reagents
Into cuprous ions:
• These tests are one of the earliest tests for sugar detection in urine of
diabetics
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27. REACTIONS OF MONOSACCHARIDES
• Reactions of phosphoric: reaction of phosphoric acid with
monosaccharides gives phosphate esters
• Fermentation: action of bacterial or yeast enzymes on carbohydrate.
• Fermentation of sugars give ethyl alcohol and C02
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