Carb Chem Lecture-i

8/8/2019 Carb Chem Lecture-i

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CARBOHYDRATE CHEMISTRY

Dr. Mohammed Shakil Akhtar



Lecture Outlinesy Introduction and General Characteristics.

y

Function of Carbohydrates.y Classification of Carbohydrates.

y Properties of Monosaccharides.

y Sterioisomerism

y Optical Activity

y Cyclic Structures

y Anomers and anomeric carbon

y Reducing Sugars

y Glycosides

y Disaccharides

y Polysaccharides

y Glycoproteins

y Sugar Derivatives



Introduction and General Characteristics

y Carbohydrates are the single most abundant class

of or ganic molecules found in nature, constitutingone of the four major classes of biomolecules.

y They are µhydrates of carbon¶ having the general

molecular formula name (CH2O)n where n= number of

carbon atoms ( 3 or more). They are composed of C, Hand O.

y Most carbohydrates are found naturally in bound

form rather than as simple sugars e.g starch,

glycoproteins and glycolipids.

y Carbohydrates are µaldehyde¶ or µketone¶ compounds

with multiple hydroxyl (- OH ) groups.



y When the functional group is

an µaldehyde group¶ they areknown as µaldoses¶ e.g

glyceraldehyde, glucose .

yWhen the functional group

is a µketo group¶ they are

referred to as µketoses¶ e.g.

dihydroxyacetone, fructose

The aldehyde or ketone group of a carbohydrate isalso known as a µcarbonyl group¶ and the the carbon

atom of the aldehyde or ketone group is known as the

µcarbonyl carbon¶.



ALDOSES



KETOSES





Functions of Carbohydratesy Carbohydrates serve as ener gy stores and fuels.

Breakdown of carbohydrates provides the ener gy thatmaintains life. Glucose is main source of ener gy for

the brain.

y Carbohydrates participate in the structure of cell

membranes (glycoproteins and glycolipids).y Pentose sugars (ribose and deoxyribose) form the

structure of nucleic acids.

y Carbohydrates play important roles in recognition

between cell types. Recognition events are importantin normal cell growth, fertilization and transformation

of cells.

y Carbohydrates act as intermediates in the biosynthesisfats and proteins.



CL ASSIFICATION OF CARBOHYDRATES

They are classified according to the ³number of

structural units´ into:

y Monosaccharides

y Disaccharides

y Oligosaccharides and

y Polysaccharides



Monosaccharides

y

The smallest and the simplest ones, for which n = 3,are glyceraldehyde and dihydroxyacetone. They are

trioses.

y Glyceraldehyde is also an aldose because it contains

an aldehyde group; whereas dihydroxyacetone is aketose because it contains a keto group, (the suffix-

ose means sugar)

y Monosaccharides are the carbohydrates that cannotbe hydrolysed into simpler units (have one structural

unit). Also known as simple sugars.



Classification of Monosaccharides

They are classified according to :1. Number of carbon atoms in the molecule: They may

be subdivided into, trioses (3C), tetroses(4C),

pentoses(5C), hexoses(6C), heptoses(7C) or

octoses(8C).



2. Type of functional group as aldosugars and

ketosugars.

y Examples of aldoses are glyceraldehyde, erythrose,

ribose, glucose.

y Examples of ketoses are dihydroxyacetone,

Erythrulose, Ribulose, Fructose.





POL YSACCHARIDES

y They yield more than ten molecules of

monosaccharides on hydrolysis. They may be either

linear or branched polymers and may contain hundredsor even thousands of monosaccharide units . Examples

are starch, glycogen, dextran.



Properties of Monosaccharide





1. Monosaccharides are crystals and dissolve in water.

2. Monosaccharides have a Sweet taste.

3. Monosaccharides contain asymmetric carbon atom

(which is a carbon atom attached to four different

chemical groups) except dihydroxyacetone.

y Due to presence of asymmetric carbon atom,

monosaccharides have two main physical properties

which are:

y

Optical Isomerism or Stereoisomerismy Optical Activity

4. Cyclic structure of Monosaccharides



Asymmetric Carbon



No Asymmetric

Carbon

4 Asymmetric

Carbon

3 Asymmetric

Carbon



y Optical isomerism or stereoisomerism is the ability of substances to be present in more than one form. It is due

to the presence of asymmetric carbon atom. These forms

are named optical isomers.

Optical Isomerism or Stereoisomerism

y So optical isomers or stereoisomers or isomers are

compounds that have same structural formula but differ

in their configuration around the asymmetric carbon

atom.



y A monosaccharide containing one or more than one

asymmetric carbon atom can exist in a number of

isomers equal to 2n, where n equal to the number of

asymmetric carbon atoms.

e.g Glucose has 4 asymmetric carbon atoms, so

the number of optical isomers equal to 24

=2x2x2x2 = 16.



ENANTIOMERS

y Enantiomers are a type of isomers that are mirror

images of each other by differing in the configuration

around the asymmetric carbons. They are designated

as ³D´ and ³L´ isomers based on the position of ±OH

group before the last ( sub-terminal) carbon.



y In D isomer (D-glucose) the -OH group attached to

the asymmetric carbon ( sub-terminal C5 )is towards

the right while in L isomer ( L-glucose) -OH group ison the left.

³The majority of the sugars in humans are D-sugars´.

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Carb Chem Lecture-i