Carbohydrates General molecular formula Cn(H2O)n Appeared to be hydrates of carbon. not all carbohydrates have this empirical formula: deoxysugars, aminosugars Carbohydrate - polyhydroxy aldehyde, ketones.
General characteristics Most carbohydrates are found naturally in bound form rather than as simple sugars Polysaccharides (starch, cellulose, inulin, gums) Glycoproteins and proteoglycans (hormones, blood group substances, antibodies) Glycolipids (cerebrosides, gangliosides) Glycosides Nucleic acids
D-Glucose in Nature The most abundant carbohydrate is D-glucose. Cells of organisms oxidize glucose for energy: In animals excess glucose is converted to a polymer called glycogen.
Disaccharides On hydrolysis give two molecules of monosaccharides E.g Sucrose (Cane sugar) Lactose (milk sugar) Maltose (malt sugar)
Polysaccharides Starch, cellulose, glycogen On the hydrolysis of each of them, they yields large number of monosaccharides.
Monosaccharides also known as simple sugars classified by 1. the number of carbons and 2. whether aldoses or ketoses most (99%) are straight chain compounds D-glyceraldehyde is the simplest of the aldoses (aldotriose) all other sugars have the ending ose (glucose, galactose, ribose, lactose, etc…)
Monosaccharides• General formula (CH2O)n• Triose: n = 3 (e.g., glyceraldehyde)• Tetrose: n = 4• Pentose: n = 5 (e.g., ribose)• Hexose: n = 6 (e.g., glucose)• Heptose: n = 7
CONCEPTS OF ISOMERSTwo or more different compounds which contain thesame number and types of atoms and the samemolecular weights.
Stereoisomers: Enantiomers and Diastereomers Stereoisomers Are not constitutional isomers since they have the constituent atoms connected in the same sequence! They only differ in the arrangement of their atoms in space! Stereoisomers can be subdivided into two categories: Enantiomers: Are stereoisomers whose molecules are mirror images of each other. (These are like our hands). The molecules of enantiomers are not superimposeable
Diastereomers: Are stereoisomers that are not mirror images of each other as indicated in (Fig.).
MonosaccharidesRepresented by Fischer projections CHO H C O H C OH H C OH CH2OH CH2OHD-Glyceraldehyde Emil Fischer Nobel Prize 1902
D- and L- Notation Prior to determination of absolute configurations, the 19th century chemists assigned arbitrary designations to structures: HC O HC O H OH HO H CH2OH CH2OH (R)-(+)-glyceraldehyde (S)-(–)-glyceraldehyde D-glyceraldehyde L-glyceraldehyde
D- and L- Notation If the OH group attached to the bottom-most chirality center is on the right, it is a D-sugar: The D- or L- together with the common name of the monosaccharide completely describes the structure, since the relative configurations at all chirality centers is implicit in the common name.
Aldotetroses Aldotetroses have two chirality centers hence 22 = 4 stereoisomers:
Enantiomers and epimers H H H H C O C O C O C O HO C H HO C HH C OH OH C H HO C H HO C HH C OH OH C H H C OH HO C H CH2OH CH2OH H C OH H C OH these two aldotetroses are enantiomers. They are stereoisomers that are mirror CH2OH CH2OH images of each other these two aldohexoses are C-4 epimers. they differ only in the position of the hydroxyl group on one asymmetric carbon (carbon 4)
Epimers A pair of diastereomers that differ only in the configuration about a single carbon atom are said to be epimers. Epimers Epimers H O H O H O H OH H OH HO H HO H HO H HO H HO H H OH H OH H OH H OH H OH CH2OH CH2OH CH2OH D(+)-Galactose D(+)-Glucose D(+)-Mannose Diastereomers
POLARIMETERDextrorotatory -plane polarized light rotated to clockwise (or to theright)Levoratatory - plane polarized light rotated to counterclockwise.
POLARIMETRY Measurement of optical activity in chiral orasymmetric molecules using plane polarized light Molecules may be chiral because of certain atomsor because of chiral axes or chiral planes Measurement uses an instrument called apolarimeter
polarimetry Magnitude of rotation depends upon: 1. the nature of the compound 2. the length of the tube (cell or sample container) usually expressed in decimeters (dm) 3. the wavelength of the light source employed; usually either sodium D line at 589.3 nm or mercury vapor lamp at 546.1 nm 4. temperature of sample 5. concentration of analyte in grams per 100 ml
observed x 100  D T = lxcD = Na D lineT = temperature oC obs : observed rotation in degree (specify solvent)l = length of tube in decimeterc = concentration in grams/100ml = specific rotation
Glucose (dextrose)Most common monosaccharide.Commercially from starch.Mutarotation ---The optical changes of glucose in watersolution to constant value 20D = +520 - D - glucose -> D - glucose <- b - D - glucose 20D = 113 20D = 52 20D = = 19At equilibrium = 35% of - form and 65% of b - form.
MONOSACCHARIDEHexoses1. Glucose (dextrose) --- rotate the polarized light to theright. H OH 6 C CH2 OH H C OH O 5 HO C H O 4 1 OH HO H C OH OH 3 2 H C OH CH2 OH
3. Fructose (levulsoe) --- Rotation in polarimeter is left CH2 OH CH2 OH CH2 OH OH C O HO C C HO C H HO C H HO C H O or O H C OH H C OH H C OH H C OH H C H C CH2 OH CH2 OH CH2 OH D-Fructose b-D-Fructose -D-Fructose
Fructose (levulsoe)b - D - Fructofuranose - D - Fructofuranose HOCH2 O OH HOCH2 O CH2 OH HO HO CH2 OH OH OH OH
Hexoses C6H12O6 CHO H C OH HO C H H C OH H C OH CH2OH D- Glucose
Oxidation reactions Aldoses may be oxidized to acids Aldonic acids: aldehyde group is converted to a carboxyl group ( glucose – gluconic acid) Saccharic acids (glycaric acids) – oxidation at both ends of monosaccharide) Glucose ---- saccharic acid Galactose --- mucic acid Mannose --- mannaric acid
Oxidation Reactions: 1- Nitric acid, HNO3: Nitric acid is a potent oxidizing reagent and will convert both aldehydes into carboxylic acids. This usually results in the conversion of an aldose into a dicarboxylic acid derivative:ose/aric
b- Conc. HNO3 CHO COOH H C OH H C OHHO C H conc. HNO3 HO C H H C OH H C OH H C OH H C OH CH2OH COOH glucose saccharic acid
CH2OH COOH C O H C OH CH2OHHO C H Conc. HNO3 + H C OH COOHH C OH COOH glycollic acidH C OH meso-tartaric acid CH2OH D- fructose
Bromine water Ose/onic H O HO O H OH H OHHO H Br2/H2O HO H H OH H OH H OH H OH CH2OH CH2OHD(+)-Glucose Gluconic acid
Monosaccharides are further classified as reducing or non-reducing sugars according to their behaviour toward Ag(I) (Tollens’ solution) or Cu(II) (Benedict’s solution). Both Tollens’ and Benedict’s tests are visual tests for aldehyde groups which, when oxidized to carboxylic acids, reduce the silver or copper ions yielding metallic silver metal or red copper(I)oxide precipitates (Fig.).
Effect of Ba(OH)2 or Ca(OH)2 The alkaline reaction conditions facilitate the a tautomeric equilibrium between the enol and keto forms of the open-chain structure. Enolization may result in the formation of glucose from fructose, mannose or vice versa.
H O H O H C OH H OH H C OH OH H OH H OH H OH H H OH H OH H OH H OH H OH H OH CH2OH CH2OH CH2OH 1,2-EnediolD-Glucose D-Mannose H H OH C O OH H H OH H OH CH2OH D-Fructose
Acylation of monosaccharides Reaction with acetic anhydride: the alcohol groups of sugars react with acetic anhydride to make ester derivatives.
Reduction either done catalytically (hydrogen and a catalyst) or enzymatically the resultant product is a polyol or sugar alcohol (alditol) glucose form sorbitol (glucitol) mannose forms mannitol fructose forms a mixture of mannitol and sorbitol glyceraldehyde gives glycerol
Reaction of carbonyl groups CHO CH2OHH C OH H C OHHO C H HO C H Na/HgH C OH H C OHH C OH H C OH CH2OH CH2OH D- Glucose sorbitol
CH2OH CH2OH CH2OH C O H C OH HO C HHO C H HO C H HO C H + Na/HgH C OH H C OH H C OHH C OH H C OH H C OH CH2OH CH2OH CH2OH D- fructose sorbitol mannitol
Reaction with HCN give cyanohydrin CN CHO H OHH C OH H C OHHO C H HO C H HCNH C OH H C OHH C OH H C OH CH2OH CH2OH D- Glucose Glucose cyanide
Reaction with hydroxylamine CHO CH N OHH C OH H C OHHO C H HO C H NH2OH. HClH C OH H C OHH C OH H C OH CH2OH CH2OH D- Glucose Glucose oxime
Formation of osazones once used for the identification of sugars consists of reacting the monosaccharide with phenylhydrazine a crystalline compound with a sharp melting point will be obtained D-fructose and D-mannose give the same osazone as D-glucose seldom used for identification; we now use HPLC or mass spectrometry
Formation of Osazone CHO CH=NHNHPh CH=NHNHPhH C OH PhNHNH2 H C OH C O PhNHNH2 PhNHNH2 (CHOH)3 (CHOH)3 (CHOH)3 CH=NHNHPh CH2OH CH2OH CH2OH C NHNHPh D- Glucose (CHOH)3 CH2OH CH2OH CH2OH CHO C O PhNHNH2 C NHNHPh Osazone C NHNHPh (CHOH)3 (CHOH)3 PhNHNH2 (CHOH)3 CH2OH PhNHNH2 CH2OH CH2OH D- fructose
Formation (Glycosides). Acetal derivatives formed when a monosaccharide reacts with an alcohol in the presence of an acid catalyst are called glycosides. "ose" suffix of the sugar name is replaced by "oside", and the alcohol group name is placed first
Sucrose -D-glucopyranosido-b-D-fructofuranoside b-D-fructofuranosido--D-glucopyranoside sugar cane or sugar beet hydrolysis yield glucose and fructose (invert sugar) ( sucrose: +66.5o ; glucose +52.5o; fructose –92o) used pharmaceutically to make syrups, troches
Sucrose2-0--D-Glucopyranosyl b-D-Fructofuranoside CH2 OH 1 O CH2 OH O H OH 2 5 HO HO O 3 4 CH2 OH OH 6 OH
Lactose b-D-galactose joined to -D-glucose via b (1,4) linkage milk contains the and b-anomers in a 2:3 ratio b-lactose is sweeter and more soluble than ordinary - lactose used in infant formulations, medium for penicillin production and as a diluent in pharmaceuticals
LactosePrincipal sugar in milk CH2 OH CH2 OH OH O O O OH OH OH OH OH
Maltose 2-glucose molecules joined via (1,4) linkage known as malt sugar produced by the partial hydrolysis of starch (either salivary amylase or pancreatic amylase)
Disaccharides (anydrides of 2 monosaccharides): Maltose: 4-0--D-Glucopyranosyl (1->4) -D- Glucopyranose CH2 OH CH2 OH O O OH OH HO O OH OH OH
Cellobiose4-0-b-D-Glucopyranosyl (1->4)-b-D-Glucopyranose CH2 OH CH2 OH O O OH O OH OH HO OH OH
Polysaccharides homoglycans (starch, cellulose, glycogen, inulin) heteroglycans (gums, mucopolysaccharides) characteristics: polymers (MW from 200,000) White and amorphous products not sweet not reducing; do not give the typical aldose or ketose reactions) form colloidal solutions or suspensions
Starch most common storage polysaccharide in plants composed of 10 – 30% -amylose and 70-90% amylopectin depending on the source the chains are of varying length, having molecular weights from several thousands to half a million
STARCHThe reserve carbohydrate of plants. Occurs as granulesin the cell. Made of amylose and amylopectin.Amylose --- ploymer of -D- Glucose (1->4) linkage-straight-chain. CH2 OH CH2 OH CH2 OH CH2 OH O O O O OH OH OH OH O O O OH OH OH OH OH
Amylose and amylopectin are the 2 forms of starch. Amylopectinis a highly branched structure, with branches occurring every 12to 30 residues
suspensions of amylosein water adopt a helical conformationiodine (I2) can insert inthe middle of the amylosehelix to give a blue colorthat is characteristic anddiagnostic for starch
Cellulose Polymer of b-D-glucose attached by b(1,4) linkages Yields glucose upon complete hydrolysis Partial hydrolysis yields cellobiose Most abundant of all carbohydrates Cotton flax: 97-99% cellulose Wood: ~ 50% cellulose Gives no color with iodine Held together with lignin in woody plant tissues
POLYSACCHARIDECellulose --- polymer of b-D-Glucose (1, 4) linkage.Repeating cellobiose moiety. CH2 OH CH2 OH CH2 OH CH2 OH O O O O OH O OH O OH O OH OH OH OH OH OH n
Glycogen also known as animal starch stored in muscle and liver present in cells as granules (high MW) contains both (1,4) links and (1,6) branches at every 8 to 12 glucose unit complete hydrolysis yields glucose glycogen and iodine gives a red-violet color hydrolyzed by both and b-amylases and by glycogen phosphorylase