What are Carbohydrates? The term is derived from French: hydrate de carbone Made of Carbon, Hydrogen and Oxygen. The general emperical formula is C.

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.

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.

ClassificationOn 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.

ClassificationOn the basis of their ability to reduce Fehling’s solution and Tollen’s reagent

•Reducing sugars : Can reduce the two solutions, due to free functional ( >C=O) groups. All monosaccharides are reducing sugars.

•Non-reducing sugars : Functional group is bonded and cannot reduce the two solutions. E.g. disaccharides like sucrose.

Types…CarboCarbon n atomsatoms

GeneraGeneral terml term

AldehydeAldehyde KetoneKetone

3 Triose Aldotriose Ketotriose

4 Tetrose Aldotetrose Ketotetrose

5 Pentose Aldopentose Ketopentose

6 Hexose Aldohexose Ketohexose

7 Heptose Aldoheptose Ketoheptose

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.

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 >C=O (carbonyl group).

322223/

6126 CHCHCHCHCHCHOHC HI

OHNCOC OHNH 2

Structure of Glucose•Gets oxidised to carboxylic acid (gluconic acid) on

reaction with Br2(l). Indicating an aldehydic group.

COOHCHO aqBr )(2

•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.

3)( )(23 CHCOOCHOHCH OCOCH

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

COOHOHCH HNO 3

2

Structure of GlucoseUsing these facts and many other properties,

Fischer arrived at the exact spatial arrangement of –OH and other groups.

Structure of GlucoseSimilarly, structures of gluconic and saccharic

acid can be drawn.

Structure of other monosaccharidesBased on what we saw for glucose, we can extend the same to other aldoses and ketoses.

C

C

CH2OH

OH)n(H

O

H

Aldose

C

C

CH2OH

OHH

O

H

Aldotriosen = 1

C

CH2OH

OHH

C O

H

C OHH

Aldotetrosen = 2

C

CH2OH

OHH

C O

H

C OHH

C OHH

Aldopentose n = 3

C O

H

C OHH

C OHH

CH OH

C

CH2OH

OHH

Aldohexose n = 4

Structure of other monosaccharides

C

C

CH2OH

OH)n(H

O

CH2OH

Ketose

CH2OH

C O

CH2OH

Ketotriose n = 0

CH2OH

C O

C OHH

CH2OH

Ketotetrose n = 1

C OHH

CH2OH

CH2OH

C O

C OHH

Ketopentose n = 2

C OHH

CH2OH

CH2OH

C O

C OHH

OHH

Ketohexose n = 3

D- & L- NomenclatureFor naming monosaccharides, we make

comparisions with the two enantiomeric forms of the simple aldotriose, glyceraldehyde.

(+)(-)

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.

D- & L- Nomenclature

D-(+)-glyceraldehyde

D-(+)-galactose

D-(–)-fructose

Notice that there is no relation between D & (–).

D-(+)-mannose

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-(+)-glucose

Cyclic Structure of Glucose

•Glucose did not give 2,4-DNP test, Schiff’s test

•Doesn’t form hydrogensulphite addition production with NaHSO3.

•Pentaacetate does not react with NH2OH

=>absence 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.

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 & .

Later found that glucose cyclizes in 2 ways:

6-membered (pyranose)

5-membered (furanose).

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’.

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.

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

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, C4’s –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.

Drawing Haworth Structures

-D-(+)-mannopyranose

-D-(+)-galactopyranose

*C2 epimer of

glucose

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

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’.

Questions for practice

Draw the structures of

1. -D-galactopyranose

2. -L-fructofuranose

3. -D-mannopyranose

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.

Sucrose•Also called cane sugar or table sugar.

•Made up of -D-glucopyranose & -D-fructofuranose.

•IUPAC name:

β-D-fructofuranosyl-(2→1)-α-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.

Sucrose

-(1,2)-glycosidic

linkage

Specific rotations at 20o C

Invert 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.

Maltose•Also known as malt sugar.

•Made up of two -D-(+)-glucopyranose units.

•IUPAC name: α-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.

Maltose

Glucose unit Glucose unit-(1,4)-glycosidic

linkage

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: β-D-galactopyranosyl-(1→4)-D-glucose

•Extracted from sweet or sour whey.•Milk contains the and -anomers in a 2:3 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.

Lactose

-lactose

-lactose

Relative sweetness of sugars and sweetners

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.

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.

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

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.

Starch

Cellulose•Occurs exclusively in plants, most abundant organic substance in plants.

Cotton flax: 97-99% celluloseWood: ~ 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.

Structure of Cellulose

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)

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.

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)

Linear structurescellulose and

chitin

Inulin

• 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)

Jerusalem artichokes

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.