dr.ehab carbohydrates-summary
TRANSCRIPT
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
Classification of
carbohydrates
Monosaccharides Trioses, tetroses, pentoses, hexoses
DisaccharidesMaltose, sucrose, lactose
Oligosaccharides 3 to 9
Polysaccharides or glycans Homopolysaccharides
Heteropolysaccharides
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 ISOMERS
Two or more different compounds which contain the
same number and types of atoms and the same
molecular 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.).
Monosaccharides
Represented by Fischer projections
Emil Fischer
Nobel Prize 1902
C
CHO
CH2OH
H OH
D-Glyceraldehyde
C
CH2OH
C OH
OHH
D- and L- Notation
Prior to determination of absolute configurations, the 19th century chemists assigned arbitrary designations to structures:
HC O
CH2OH
OHH
HC O
CH2OH
HHO
(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:
C
CH2OH
OHH
C O
H
C OHH
C
CH2OH
HOH
C O
H
C HOH
these two aldotetroses are enantiomers.
They are stereoisomers that are mirror
images of each other
C O
H
C HHO
C HHO
CH OH
C
CH2OH
OHH
C O
H
C HHO
C HHO
CHO H
C
CH2OH
OHH
these two aldohexoses are C-4 epimers.
they differ only in the position of the
hydroxyl group on one asymmetric carbon
(carbon 4)
Enantiomers and epimers
Epimers
A pair of diastereomers that differ only in the
configuration about a single carbon atom are said to be
epimers.
H OH
OH
HO H
HO H
H OH
CH2OH
H OH
OH
HO H
H OH
H OH
CH2OH
HO H
OH
HO H
H OH
H OH
CH2OH
D(+)-Galactose D(+)-MannoseD(+)-Glucose
EpimersEpimers
Diastereomers
POLARIMETER
Dextrorotatory -plane polarized light rotated to clockwise (or to the
right)
Levoratatory - plane polarized light rotated to counterclockwise.
POLARIMETRY
Measurement of optical activity in chiral or
asymmetric molecules using plane polarized light
Molecules may be chiral because of certain atoms
or because of chiral axes or chiral planes
Measurement uses an instrument called a
polarimeter
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
[]DT
l x c
observed x 100 =
D = Na D line
T = temperature oC
obs : observed rotation in degree (specify solvent)
l = length of tube in decimeter
c = concentration in grams/100ml
[] = specific rotation
Specific rotation of various
carbohydrates at 20oC
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
Most common monosaccharide.
Commercially from starch.
Mutarotation ---The optical changes of glucose in water solution to constant value
20D = +520
- D - glucose -> D - glucose <- b - D - glucose
20D = 113 20
D = 52 20D = = 19
At equilibrium = 35% of - form and 65% of b - form.
Glucose (dextrose)
MONOSACCHARIDE
Hexoses
1. Glucose (dextrose) --- rotate the polarized light to the
right.
OH
OH
CH
H C OH
C H
H
HO
H C
CH2OH
O
C
OH
O
OH
OHHO
CH2 OH
1
23
45
6
3. Fructose (levulsoe) --- Rotation in polarimeter is left
D-Fructose b-D-Fructose -D-Fructose
CH2OH
O
CH2OH
C
HO HC
OHCH
H C
OH
O
CH2OH
C
HO HC
OHCH
H C
CH2OHCH2OH
CH
HO
H C OH
C HHO
C
OH
CH2OH
O
or
b - D - Fructofuranose - D - Fructofuranose
O
HO
OH
CH2 OH
HOCH2 OH
O
HO
OH
HOCH2CH2OH
OH
Fructose (levulsoe)
Hexoses C6H12O6
C
C
C
CHO
C
CH2OH
OHH
H OH
HO H
H OH
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
C
C
C
CHO
C
CH2OH
OHH
H OH
HO H
H OH
glucose
conc. HNO3
C
C
C
COOH
C
COOH
OHH
H OH
HO H
H OH
saccharic acid
C
C
C
CH2OH
C
CH2OH
OHH
H OH
HO H
O
D- fructose
COOH
C
C
COOH
OHH
H OH
meso-tartaric acid
Conc. HNO3
CH2OH
COOH+
glycollic acid
Bromine water
Ose/onic
H OH
OH
HO H
H OH
H OH
CH2OH
D(+)-Glucose
Br2/H2OH OH
OHO
HO H
H OH
H OH
CH2OH
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 OH
OH
OH H
H OH
H OH
CH2OH
OH H
OH
OH H
H OH
H OH
CH2OH
D-MannoseD-Glucose
C
CH OH
OH H
H OH
H OH
CH2OH
H OH
C O
OH H
H OH
H OH
CH2OH
H
H
OH
D-Fructose
1,2-Enediol
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
C
C
C
CHO
C
CH2OH
OHH
H OH
HO H
H OH
D- Glucose
C
C
C
CH2OH
C
CH2OH
OHH
H OH
HO H
H OH
sorbitol
Na/Hg
C
C
C
CH2OH
C
CH2OH
OHH
H OH
HO H
O
D- fructose
C
C
C
CH2OH
C
CH2OH
OHH
H OH
HO H
H OH
sorbitol
Na/Hg
C
C
C
CH2OH
C
CH2OH
OHH
H OH
HO H
HO H
+
mannitol
Reaction with HCN give cyanohydrin
C
C
C
CHO
C
CH2OH
OHH
H OH
HO H
H OH
D- Glucose
C
C
C
C
CH2OH
OHH
H OH
HO H
H OH
Glucose cyanide
HCN
CN
OHH
Reaction with hydroxylamine
C
C
C
CHO
C
CH2OH
OHH
H OH
HO H
H OH
D- Glucose
NH2OH. HCl
C
C
C
CH
C
CH2OH
OHH
H OH
HO H
H OH
Glucose oxime
N OH
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
C
(CHOH)3
CHO
H OH
D- Glucose
CH2OH
PhNHNH2C
(CHOH)3
CH=NHNHPh
H OH
CH2OH
PhNHNH2C
(CHOH)3
CH=NHNHPh
O
CH2OH
PhNHNH2
C
(CHOH)3
CH=NHNHPh
NHNHPh
CH2OH
OsazoneC
(CHOH)3
CH2OH
O
D- fructose
CH2OH
PhNHNH2C
(CHOH)3
CH2OH
NHNHPh
CH2OH
PhNHNH2
C
(CHOH)3
CHO
NHNHPh
CH2OH
PhNHNH2
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
Sugar cane
Sugar beet
Sucrose
2-0--D-Glucopyranosyl b-D-Fructofuranoside
O
OH
OHHO
CH2 OH
CH2OH
OCH2OH
O
HO
OH
H1
2
3 4
5
6
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
Lactose
Principal sugar in milk
O
OH
OH
CH2 OH
O
OH
OH
CH2 OH
O
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
O
OH
OHHO
CH2 OH
O
OH
OH
CH2 OH
OOH
Cellobiose
4-0-b-D-Glucopyranosyl (1->4)-b-D-Glucopyranose
O
OH
OHHO
CH2 OH
O
OH
OH
CH2 OH
O
OH
Sucralfate (Carafate)
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
The reserve carbohydrate of plants. Occurs as granules
in the cell. Made of amylose and amylopectin.
Amylose --- ploymer of -D- Glucose (1->4) linkage-
straight-chain.
STARCH
O
OH
OH
CH2 OH
OHO OO
O
OH
OH
CH2 OH
O
OH
OH
CH2 OH
O
OH
OH
CH2 OH
O
OH
OH
CH2 OH
O
OH
OH
CH2 OH
Amylose and amylopectin are the 2 forms of starch. Amylopectin
is a highly branched structure, with branches occurring every 12
to 30 residues
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
(in starch)
(in cellulose)
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
POLYSACCHARIDE
Cellulose --- polymer of b-D-Glucose (1, 4) linkage.
Repeating cellobiose moiety.
O
OH
OH
CH2 OH
OH
O OO
O
OH
OH
CH2 OH
O
OH
OH
CH2 OH
O
OH
OH
CH2 OH
O
OH
OH
CH2 OH
O
OH
OH
CH2 OH
O
OH
OH
CH2 OH
O
OH
OH
CH2 OH
n
Structure of cellulose
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
GLYCOGEN
Animal starch.
- (1 -> 4) linkage and - (1 -> 6) linkage
12 : 1
RELATIVE SWEETNESS OF DIFFERENT SUGARS
Sucrose 100
Glucose 74
Fructose 174
Lactose 16
Invert Sugar 126
Maltose 32
Galactose 32
CARBOHYDRATE DETERMINATION
1. Monosaccharides and Oligosaccharides
A. Enzymatic Method
1. Glucose oxidase
2. Hexokinase
B. Chromatography Method
1. Paper or thin layer chromatography
2. Gas chromatography
3. Liquid column chromatography
2. Polysaccharides
Glucose Oxidase System
Glucose Oxidase
D-Glucose + O2 Gluconic Acid + H2O2
Peroxidase
H2O2+ 0 - Dianisidine 2 H2O + Oxidized 0-Dianisidine
(Colorless) (Brown)
OCH3H3CO
H2N NH2
H3CO OCH3
HN NH
Synthetic Sweeteners