BIOMOLECULES

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BIOMOLECULES
Carbohydrates
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What are Carbohydrates?

• The term is derived from French hydrate de
  carbone
• Made of Carbon, Hydrogen and Oxygen.
• The general emperical formula is Cx(H2O)y.
• All carbohydrates do not follow formula.
• E.g. deoxysugars, aminosugars.
• All compounds which follow this formula are not
  necessarily carbohydrates. E.g. acetic acid
  C2(H2O)2
• Definition Optically active polyhydroxy
  aldehydes or ketones or compounds which produce
  such units on hydrolysis.
• Sweet in taste, therefore called sugars.
• Greek sakcharon means sugar. Therefore,
  saccharide also used for carbohydrates.

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Their Functions

• Sources of energy
• Intermediates in biosynthesis of biochemical
  entities (fats and protiens).
• Associated with entities like glycosides,
  vitamins and antibodies.
• Form structural tissues (cellulose, lignin,
  murein, chitin)
• Participate in biological transport, cell-cell
  recognition, activation of growth factors,
  modulation of the immune system etc.

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Classification

• On the basis of products on hydrolysis.
• Monosaccharides
• cannot be hydrolysed to simpler units.
• Oligosaccharides
• Gives 2-10 simple units on hydrolysis.
• Depending on number of units, further classified
  as di-, tri-, tetra-saccharides (till 9 or 10)
• Polysaccharides
• Yield large number of units on hydrolysis.
• Non-sweet, they are also called non-sugars.

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Classification

• On the basis of their ability to reduce Fehlings
  solution and Tollens reagent
• Reducing sugars Can reduce the two solutions,
  due to free functional ( gtCO) groups. All
  monosaccharides are reducing sugars.
• Non-reducing sugars Functional group is bonded
  and cannot reduce the two solutions. E.g.
  disaccharides like sucrose.

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Monosaccharides
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Types
Carbon atoms General term Aldehyde Ketone
3 Triose Aldotriose Ketotriose
4 Tetrose Aldotetrose Ketotetrose
5 Pentose Aldopentose Ketopentose
6 Hexose Aldohexose Ketohexose
7 Heptose Aldoheptose Ketoheptose
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Glucose

• One of the most important monosaccharides.
• Prepared by hydrolysis of sucrose and starch.
• An aldohexose a.k.a dextrose.
• Molecular formula C6H12O6.
• Monomer of larger saccharides (starch, cellulose
  etc.)
• Most abundant organic material in the world.

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Structure of Glucose

• Prolonged heating with HI yielded n-hexane.
  Confirms straight chain nature.
• With NH2OH (hydroxylamine) and HCN, forms oxime
  and cyanohydrin respectively. Shows presence of
  gtCO (carbonyl group).

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Structure of Glucose

• Gets oxidised to carboxylic acid (gluconic acid)
  on reaction with Br2(l). Indicating an aldehydic
  group.

• Acetylation with (CH3CO)2O gives glucose
  pentaacetate. Confirms presence of 5 OH groups.
  Since it is stable, each one is attached to a
  different carbon.

• Glucose Gluconic acid give a dicarboxylic acid
  (saccharic acid) on oxidation with HNO3. Shows
  presence of 1o OH group.

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Structure of Glucose
Using these facts and many other properties,
Fischer arrived at the exact spatial arrangement
of OH and other groups.
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Structure of Glucose
Similarly, structures of gluconic and saccharic
acid can be drawn.
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Structure of other monosaccharides
Based on what we saw for glucose, we can extend
the same to other aldoses and ketoses.
Aldoses
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Structure of other monosaccharides
Ketoses
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D- L- Nomenclature
For naming monosaccharides, we make comparisions
with the two enantiomeric forms of the simple
aldotriose, glyceraldehyde.
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D- L- Nomenclature

• Compounds which can be chemically corelated to
  ()-isomer of glyceraldehyde have
  D-configuration.
• Those which can be corelated to the ()-isomer
  have L-configuration.
• Structure is written with the most oxidised
  carbon on top.
• D- L- nomenclature has no relation with the
  optical activity. D does not refer to
  dextrorotatory and L does not refer to
  levorotatory.

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D- L- Nomenclature
D-()-mannose
D-()-glyceraldehyde
D-()-galactose
Notice that C4 and C5 have OH on same side for
all the structures, except galactose. This, we
will learn, lead to an exception in naming in
another structure of monosaccharides.
D-()-fructose
Notice that there is no relation between D ().
D-()-glucose
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Cyclic Structure of Glucose

• Glucose did not give 2,4-DNP test, Schiffs test
• Doesnt form hydrogensulphite addition production
  with NaHSO3.
• Pentaacetate does not react with NH2OH
• gtabsence of free CHO group.
• 2 crystalline forms of glucose isolated,
  i.e.?-form (m.p. 419K) and ?-form (m.p. 423K).
  ?-form obtained by crystallisation of conc.
  Glucose solution (303K). ?-form from hot
  saturated solution (371K).
• These were drawbacks of open chain structure.

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Cyclic Structure of Glucose

• It was proposed that one of the OH groups may
  add to the CHO group to form cyclic hemiacetal.
• Thus, forms a 6-membered ring. OH group at C5
  involved in ring formation.
• Explains absence of CHO group and existence of
  two forms ? ?.

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Later found that glucose cyclizes in 2
ways 6-membered (pyranose) 5-membered (furanose).
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Haworth Structures

• Cyclic structures in previuos slide called
  Haworth structure or Haworth form.
• Pyranose furanose for the 6- 5- membered
  ring respectively come from pyran and furan.

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Epimers and Anomers

• Epimers are diastereomers that differ in
  configuration of only one stereogenic center.
• Diastereomers (stereoisomers) non-superposable,
  non-mirror images, unlike enantiomers
  non-superposable mirror images.
• E.g. D-galactose C4 epimer of D-glucose.
• C1 epimers given special name anomers.
• ?- and ?- forms of glucose are anomers epimers.
  D-galactose and D-glucose are not anomers.
• Anomerism occurs only in closed chain structures
  (no OH group on carbonyl carbon in open chain).
• In closed chain, carbon (originally the sp2
  carbon in open chain) is called anomeric carbon.

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Drawing Haworth Structures

• Draw a six or 5-membered ring including oxygen as
  one atom
• Number the ring clockwise starting next to the
  oxygen
• If substituent is on right in Fisher projection,
  it will be down in Haworth (Down-Right Rule)
• For D-, highest numbered carbon is drawn up. For
  L-, it is drawn down
• For D-, OH group at anomeric position is down
  for ?, up for ?. For L-, ? is up and ? is down

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Drawing Haworth Structures

• In L-, OH at C4 is up. In D-, it is down.
• Exception galactose.
• For most sugars, OH on C4 is on same side as
  bridge Oxygen. Only for galactose, it is on other
  side.
• For D-glucose, C4s OH is down, but D-galactose,
  the C4 OH is up.
• Nomenclature 1st write anomeric form 2nd D or
  L, 3rd rotatory nature ()/(), 4th name of
  sugar. Replace se by suffix pyranose/furanose
  depending on ring size.

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Drawing Haworth Structures
C2 epimer of glucose
?-D-()-galactopyranose
?-D-()-mannopyranose
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Fructose

• Molecular formula C6H12O6.
• Straight chain compound, keto group on C2
  position.
• Levorotatory, specific rotation 92.4o

• Cyclic form obtained by addition of C5 OH at C2
  carbonyl carbon gives 5-membered furanose ring.

D-()-fructose
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Ribose and Deoxyribose

• Ribofuranose and 2-deoxyribofuranose are present
  in the sugar part of RNA and DNA molecules
  respectively.

?-D-ribofuranose
?-D-2-deoxyribofuranose
Notice that the oxygen is removed from the OH at
C2, there it is 2-deoxy.
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Questions for practice

• Draw the structures of
• ?-D-galactopyranose
• ?-L-fructofuranose
• ?-D-mannopyranose

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Disaccharides
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About disaccharides

• Two monosaccharides joined together by oxide
  linkage, formed by loss of water molecule.
• Thus the two units can be obtained by adding
  water, here, splitting by water or hydrolysis.
• Linkage between monosaccharide units through the
  O atom is called glycosidic linkage.

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Sucrose

• Also called cane sugar or table sugar.
• Made up of ?-D-glucopyranose ?-D-fructofuranose.
  
• IUPAC name
• ß-D-fructofuranosyl-(2?1)-a-D-glucopyranoside
• Commercially obtained from sugar cane or sugar
  beet.
• Used pharmaceutically to make syrups, troches
  etc.
• Non-reducing as functional groups are involved in
  glycosidic bond.
• Upon hydrolysis, it is given a special name of
  invert sugar.

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Sucrose
?-(1,2)-glycosidic linkage
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Specific rotations at 20o CInvert Sugar

• D-glucose 52.7
• D-fructose -92.4
• D-galactose 80.2
• L-arabinose 104.5
• D-mannose 14.2
• D-arabinose -105.0
• D-xylose 18.8
• Lactose 55.4
• Sucrose 66.5
• Maltose 130.4
• Invert sugar -19.8
• Dextrin 195

See that sucrose has ive rotation. On
hydrolysis, shows ive rotation because its
components have ive and ive rotation, the
latter being higher in magnitude. Thus net ive
rotation due to the solution, sucrose is thus
called invert sugar after hydrolysis.
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Maltose

• Also known as malt sugar.
• Made up of two ?-D-()-glucopyranose units.
• IUPAC name a-D-Glucopyranosyl-(1?4)-D-glucose
• Production of maltose from germinating cereals,
  like barley, part of brewing process.
• Common ingredient in confectionery.
• Reducing as a CHO group can be produced at C1 of
  second glucose in solution.

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Maltose
Glucose unit
Glucose unit
?-(1,4)-glycosidic linkage
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Lactose

• Called milk sugar, found in milk.
• Made up of ?-D-galactopyranose and
  D-glucopyranose units, ?-glucose for ?-lactose
  and same goes for ?.
• IUPAC name ß-D-galactopyranosyl-(1?4)-D-glucose
• Extracted from sweet or sour whey.
• Milk contains the ? and ?-anomers in a 23 ratio
• ?-lactose is sweeter and more soluble than
  ordinary ?-lactose
• Used in infant formulations, medium for
  penicillin production and as a diluent in
  pharmaceuticals
• Reducing as C1 of glucose can produce a CHO
  group.

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Lactose
?-lactose
?-lactose
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Relative sweetness of sugars and sweetners
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Polysaccharides
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About Polysaccharides

• Also called glycans.
• Two types
• Homopolysaccharides/Homoglycans
• e.g. starch, cellulose, glycogen, inulin
• Heteropolysaccharides/Heteroglycans
• e.g. gums, mucopolysaccharides
• Most commonly encountered carbohydrates.
• Act as the food storage or structural materials.
• Non-sugars, as they are not sweet in taste.

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Starch

• Most common storage polysaccharide in plants,
  most important dietary source for humans.
• High contents found in cereals, roots, tubers
  etc.
• Polymer of ?-D-()-glucopyranose.
• Made up of Amylose (10-30) and Amylopectin
  (70-90) depending on the source.
• Amylose, the water soluble component, is a long
  unbranched chain with 200-1000 monomer units held
  by ?-(1,4)-glycosidic linkage.
• Amylopectin, the insoluble component, is a
  branched chain polymer. Bonding is ?-(1,4) in
  chain and ?-(1,6) in branching.
• Molecular mass varies from few 1000 to ½ a
  million.

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Starch

• Branching in amylopectin occurs at every 12-30
  units.

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Starch

• Suspensions of amylose in water adopt a helical
  conformation
• Iodine ( I2 ) can insert in the middle of the
  amylose helix to give a blue color that is
  characteristic and diagnostic for starch.

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Cellulose

• Occurs exclusively in plants, most abundant
  organic substance in plants.
• Cotton flax 97-99 cellulose
• Wood 50 cellulose
• Makes up cell wall of plants, and also fungi
  (with chitin).
• Straight chain polymer of ?-D-glucopyranose.
• Linkage is ?-(1,4)-glycosidic linkage.
• Partial hydrolysis yields cellobiose.
• Gives no colour with I2.
• Held together with lignin in woody plant tissues.

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Structure of Cellulose
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Products obtained from cellulose

• Microcrystalline cellulose used as
  binder-disintegrant in tablets
• Methylcellulose suspending agent and bulk
  laxative
• Oxidized cellulose hemostat
• Sodium carboxymethyl cellulose laxative
• Cellulose acetate rayon photographic film
  plastics
• Cellulose acetate phthalate enteric coating
• Nitrocellulose explosives collodion (pyroxylin)

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Glycogen

• Also known as animal starch, responsible for
  storage in animal bodies.
• Structure is similar to amylopectin, more highly
  branched (at every 8-12 units).
• Present in liver, muscle and brain.
• When body needs glucose, glycogen is broken down.
• Also found in yeast and fungi.
• Bonds are same as in amylopectin.
• With I2, it gives a red-violet colour.

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Chitin

• 2nd most abundant carbohydrate polymer
• Present in the cell wall of fungi and in the
  exoskeletons of crustaceans, insects and spiders
• Used commercially in coatings (extends the shelf
  life of fruits and meats)

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Linear structurescellulose and chitin
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Inulin
Jerusalem artichokes

• Linear chain polymer
• Made up of ?-(1,2) linked fructofuranoses.
• Lower molecular weight than starch
• Gives yellow colour with I2
• Sources include onions, garlic, dandelions and
  jerusalem artichokes
• Used as diagnostic agent for the evaluation of
  glomerular filtration rate (renal function test)

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Importance of Carbohydrates

• Constitute a major portion of our diet.
• Honey has been used as instant source of energy
  for a long time by vaids in ayurveda.
• Starch and glycogen are storage molecules in
  plants and animals respectively.
• Cell wall of plant, bacteria and fungi have
  cellulose.
• Cellulose used in furniture wood and for clothing
  in the form of cotton fibre.
• Raw materials for industries like textile paper
  lacquers and breweries.
• Ribose and deoxyribose sugars make up genetic
  material.