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Biomolecules

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Biomolecules PDF Notes, Important Questions and Formulas

Biomolecules & Polymers

CARBOHYDRATES

   1.  Introduction:

Carbohydrates received their name because of their general formula Cx(H2O)y, according to which they appear to be hydrates of carbon.

begin mathsize 12px style table attributes columnalign left end attributes row cell xCO subscript 2 text  + yH end text subscript 2 straight O text   end text rightwards arrow with Sunlight comma chlorphyll on top text   C end text subscript straight x left parenthesis straight H subscript 2 straight O right parenthesis subscript straight y text  +  xO end text subscript 2 end cell row cell text                                                             carbohydrate end text end cell end table end style

  • Photosynthesis:
    

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  • Cellular Respiration:

    begin mathsize 12px style table attributes columnalign left end attributes row cell text C end text subscript 6 straight H subscript 12 straight O subscript 6 text  +  6O end text subscript 2 text   end text rightwards arrow with Enzymes on top text    end text 6 CO subscript 2 text  + 6H end text subscript 2 straight O plus 38 text  ATP end text end cell row cell text                                                end text space space space space space space left parenthesis 36 text  ATP net gain end text right parenthesis end cell end table end style

    2.  Classification and structure of Carbohydrates:

Carbohydrates are polyhydroxy aldehydes and ketones and substances which hydrolyse to polyhydroxy aldehydes and ketones.
The simplest carbohydrates are called sugars or saccharides, (Latin: Saccharum, sugar). Carbohydrates can be classified as monosaccharides, oligosaccharides and polysacchaides.

Carbohydrates are polyhydroxy aldehydes and ketones and substances which hydrolyse to polyhydroxy aldehydes and ketones.

    3.  General Characteristic of Monosaccharides:

The important characteristics of monosaccharides as follows:

  1.  All monosaccahrides are water soluble due to the presence of hydrogen bonding between the different OH groups and surrounding water molecules.
  2. Monosaccharides have sweet taste and upon heating they get charred and give the smell of burning sugar.
  3. Monosaccharides are optically active in nature due to the presence of chiral carbon atoms.
  4. The chemical characteristics of monosaccharides are due to OH groups and carbonyl group which may be either aldehydic or ketonic group.

Glyceraldehyde contains one asymmetric carbon atom (marked by an astrisk) and can thus exist in two optically active forms, called the D-form and the L-form. Clearly, the two forms are mirror images that cannot be superimposed, that is they are enantiomers.






All four isomers have been prepared synthetically. The D-and L-erythrose are mirror images, that is, they areenantiomers. They have exactly the same degree of rotation but in opposite directions. Equal amounts of the two would constitute a racemic mixture, that is a mixture that would allow a plane-polarised light to pass through the solution unchanged.


Supplying hydrogen atoms to the five carbon atoms to satisfy their tetravalency, following structure (open chain) may be assigned to glucose: (* indicates assymetric carbon atom).

 

    4.  Configuration* of Glucose:

Since the above structure possesses four asymmetric carbon atoms (shown by asterisks), it an exist in 24 = 16 optically active forms, i.e., eight pairs of enantiomers. All these are known and correspond to the D- and L- forms of glucose, mannose, galactose, allose, glucose, idose and talose. The naturally occuring dextrorotatory glucose (+)- glucose is only one of the 16-stereoisomers.



Notations D- and L- for denoting configuration were given by Rosanof; according to this convention any compound whose bottom asymmetric carbon atoms has the configuration similar to the configuration of dextrorotatory glyceraldehyde (drawn above, i.e. the bottom carbon atom has –OH to the left and H to the right is given L-configuration. Remember that the symbols D- and L- have no relation with the specific rotation value, i.e., with (+) or (–) value. For example, the natural (–) fructose belongs to D-series, i.e., it is D(–)-fructose)

    5.  Objections to open-chain structure of glucose:

Even through open chain structure of (+) glucose explains most of its reactions, it fails to explain the following facts about it.

  1.  Glucose does not restore Schiff’s reagent colour.
  2.  Glucose does not form a bisulphite and aldehydeammonia compound.
  3. Glucose forms two isomeric penta-acetates neither of which reacts with carbonyl reagents.
  4. The existence of the two isomeric glucoses and the change in specific rotation (mutarotation) is not explained by an open-chain formula.
  5. Glucose reacts with methanol in presence of dry HCl gas to form two isomeric glucosides.

Since glucose is less soluble in ethanol, it separates out on cooling the reaction mixture. Commercially, it is obtained by the hydrolysis of starch which is available from relatively inexpensive source such as maize, potatoes and rice.

Constitution of Glucose:

1.  Molecular Formula: By the usual analytical methods, the molecular formula glucose is found to be C6H12O6.

2.  Straight Chain of six carbon atoms:

  1. Reduction of glucose with conc. HI and phosphorus gives 2-iodohexane and n-hexane. This indicates that six carbon atoms in glucose are present in a straight chain

    begin mathsize 12px style table attributes columnalign left end attributes row cell straight C subscript 6 straight H subscript 12 straight O subscript 6 text   end text rightwards arrow from Prolonged text  heating end text to HI divided by straight P of CH subscript 3 text  -  end text left parenthesis CH subscript 2 right parenthesis subscript 4 minus CH subscript 3 end cell row cell Glucose text                                    n-hexane end text end cell end table end style
  2.  Glucose when oxidized with bromine water gives gluconic acid which when reduced with excess of HI gives n-hexanoic acid, CH3.(CH2)4.COOH confirming the presence of a straigth chain of six carbon atoms in glucose.

3.  Presence of five hydroxyl groups:

When treates with acetic anhydride, glucose forms penta-acetate indicating the presence of 5 – OH groups and since glucose is a stable compound, the five –OH groups must be attached to 5 different carbon atoms.

4.  Presence of an aldehydic groups:

  1. Glucose forms a cyanohdrin with hydrogen cyaide and a mono-oxime with hydroxylamine suggesting the presence of a carbonyl group.

  2. Glucose reduces Fehling solution and Tollen’s reagent indicating that the carbonyl group is aldehydic in nature.
  3. The presence of aldehydic group in glucose is confirmed by its oxidation to gluconic acid having the same number of carbon atoms.

    begin mathsize 12px style table attributes columnalign left end attributes row cell straight C subscript 5 straight H subscript 11 text .CHO +  end text left parenthesis straight O right parenthesis text   end text rightwards arrow from blank to Br subscript 2 of straight C subscript 5 straight H subscript 11 straight O subscript 5. COOH end cell row cell text      end text Glucose text                         Gluconic acid end text end cell end table end style
    Now since aldehydic group is monovalent, it must be present on the end of the chain.

5.  Open chain structure:On the basis of the above points, glucose may be assigned following prt structure orientation shown in the α anomer has the –OH trans to the –CH2OH group and the β anomer has the –OH cis to the –CH2OH group.


STRUCTURE FORMULAS FOR MONOSACCHARIDES:

Although many of the properties of D(+)-glucose can be explained in terms of an open-chain structure (1, 2, or 3), a considerable body of evidence indicates that the open-chain structure exists, primarily, in equilibrium with two cyclic forms. These can be represented by structures 4 and 5 or 6 and 7. The cyclic forms of D(+)-glucose are hemiacetals formed by an intramolecular reaction of the –OH group at C5 with the aldehyde group. Cycliation creates a new stereogenic centre at C1, and this stereogenic centre explains how two cyclic forms are possible. These two cyclic forms are diastereomers that different only in the configuration of C1. In carbohydrate chemistry diastereomers of this type of called anomers, and the hemiacetal carbon atom is called the anomeric carbon atom.

 

Structures 4 and 5 for the glucose anomers are called Haworth formulas and, although they do not give an accuate picture of the shap of the six-membered ring, they have many practical uses. Demonstrates how the representation of each stereogenic centre of the openchain form can be correlated with its representation in the Haworth formula.

Each glucose anomer is designated as an α anomer or a β anomer depending on the location of the –OH group of Cl. When we draw the cyclic forms of a D sugar in the

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