Transcript Polysaccharides

Carbohydrates and Structural
Analysis of Polysaccharides
Di Wu
2012-11-05
Contents:
-
Introdution of carbohydrates
Monosaccharides
Oligosaccharides
Polysaccharides
Structure analysis of polysaccharides
Introdution of
carbohydrates
Ah! sweet mystery of life . . .
—Rida Johnson Young (lyrics) and Victor Herbert (music)
“Ah! Sweet Mystery of Life,” 1910
I would feel more optimistic about a bright future
for man if he spent less time proving that he can
outwit Nature and more time tasting her sweetness
and respecting her seniority.
—E. B. White, “Coon Tree,” 1977
Four Major Types of Biological Macromolecules
Type of Polymer
Monomers making up
Polymer
Example
I. Carbohydrates
(Polysaccharides)
Monosaccharides
Sugars, Starch,
Cellulose
II. Lipids
Fatty acids and glycerol
III. Proteins
Amino acids
Fats, steroids,
cholesterol
Enzymes,
structural
components
IV. Nucleic Acids
Nucleotides
DNA, RNA
Proteins:
Polysaccharides
• Often poorly defined (although some
• well defined
can form helices)
• Coded precisely by genes,
• Synthesised by enzymes without
hence monodisperse
template – polydisperse, and
• ~20 building block residues
generally larger
(amino acids)
• Many homopolymers, and rarely
• Standard peptide link (apart from
>3,4 different residues
proline)
• Various links a(11), a(12),
• Normally tightly folded structures
a(1-4),a(16), b(13), b(14)etc
• Range of structures (rodcoil
Carbohydrates：
Polyhydroxy aldehydes or ketones, or substances that yield
such compounds on hydrolysis. some also contain nitrogen,
phosphorus, or sulfur.
•
•
•
•
•
(CH2O)n
70-80% human energy needs (US~50%)
>90% dry matter of plants
Monomers and polymers
Functional properties
– Sweetness
– Chemical reactivity
– Polymer functionality
There are three major size classes of carbohydrates:
• Monosaccharides – carbohydrates that cannot be hydrolyzed
to simpler carbohydrates; eg. Glucose or fructose.
• Oligosaccharides – carbohydrates that can be hydrolyzed into
a few monosaccharide units; eg. Sucrose or lactose
• Polysaccharides – carbohydrates that are polymeric sugars; eg
Starch or cellulose
Monosaccharides
• 3-9 carbon atom sugars
-(pentoses 5, hexoses 6 most common in plants)
• have to be obtained by chemical reactions
• only a few are free in plant
-many as polysaccharides
The structure and classification of some monosaccharides
Nomenclature
Number of carbons
Functional group
Ketone
Aldehyde
4
Tetrose
Tetrulose
5
Pentose
Pentulose
6
Hexose
Hexulose
7
Heptose
Heptulose
8
Octose
Octulose
Oligosaccharides
• Composed of a few monosaccharide units by glycosidic
link from C-1 of one unit and -OH of second unit
• 13, 14, 1  6 links most common but 1  1 and 1
 2 are possible
• Links may be a or b
• Link around glycosidic bond is fixed but anomeric
forms on the other C-1 are still in equilibrium
Synthesis
Some Disaccharides
C H 2O H
C H 2O H
H
C H 2O H
O H
OH
H
H
O
OH
OH
H
H
maltose
OH
H
O
O
OH
OH
O OH
C H 2O H
H
O
H
H
cellobiose
H
H
H
OH
H
OH
(a-D-glucosyl-(1->4)-b-D-glucopyranose)
OH
H
OH
H
OH
(b-D-glucosyl-(1->4)-b-D-glucopyranose)
C H 2O H
H
O
OH
C H 2O H
H
H
O OH
C H 2O H
H
OH
sucrose
OH
H
OH
OH
OH
O
H
lactose
O
H
H
O
C H 2O H
O
H
H
H
OH
H
C H 2O H
H
OH
OH
H
OH
(b-D-galactosyl-(1->4)-b-D-glucopyranose)
H
(a-D-glucosyl-(1->2)-b-D-fructofuranose)
Higher Oligosaccharides
Polysaccharides
Polysaccharides are complex carbohydrates made up
linked monosaccharide units.
• Nomenclature:
Homopolysaccharide-a polysaccharide is made up of one type of
monosaccharide unit
Heteropolysaccharide-a polysaccharide is made up of more than
one type of monosaccharide unit
• Starch and glycogen are storage molecules
• Chitin and cellulose are structural molecules
• Cell surface polysaccharides are recognition molecules
Polisaccharides
Sources of Polysaccharides
• Microbial fermentation
• Higher plants
– seeds
– tree extrudates,
– marine plants,
• Chemical modification of other polymers
Some types of polysaccharides
1.Starch
• Starch is a storage compound in plants, and made of glucose
units
• It is a homopolysaccharide made up of two components:
amylose and amylopectin.
• Most starch is 10-30% amylose and 70-90% amylopectin
• Amylose – a straight chain structure formed by 1,4
glycosidic bonds between α-D-glucose molecules.
Structure of Amylose Fraction of Starch
6
CH 2 OH
O
H
H
OH
H
H
H
5
O
H
H
1
4 OH
O
OH
OH
H
O
H
H
H
OH
H
O
H
H
H
OH
H
OH
2
OH
H
OH
am ylose
H
OH
H
H
O
O
O
3
H
H
OH
1
H
O
H
CH 2 OH
CH 2 OH
CH 2 OH
CH 2 OH
H
OH
Amylose
• The amylose chain forms a
helix.
• This causes the blue colour
change on reaction with
iodine.
• Amylose is poorly soluble in
water, but forms micellar
suspensions
Amylopectin-a glucose polymer with mainly α -(14)
linkages, but it also has branches formed by α -(16)
linkages. Branches are generally longer than shown above.
Structure of Amylopectin Fraction of Starch
CH 2 OH
CH 2 OH
O
H
H
OH
H
H
H
H
OH
am ylo p e ctin
H
1
O
OH
H
CH 2 OH
H
O
H
H
OH
H
H
H
H
O
OH
O
4
5
O
H
OH
OH
H
OH
H
O
H
H
1
CH 2 OH
4
H
OH
H
2
OH
H
O
H
H
OH
H
O
3
H
CH 2 OH
6 CH 2
CH 2 OH
O
H
OH
H
OH
O
OH
H
O
H
H
O
H
OH
H
OH
H
OH
Amylopectin
• Amylopectin causes a
red-violet colour change
on reaction with iodine.
• This change is usually
masked by the much
darker
reaction
of
amylose to iodine.
Amylopectin
Starch therefore consists of amylose helices entangled on
branches of amylopectin.
2 Glycogen
• Storage polysaccharide in animals
• Glycogen constitutes up to 10% of liver mass and 1-2% of
muscle mass
• Glycogen is stored energy for the organism
• Similar in structure to amylopectin, only difference from
starch: number of branches
• Alpha(1,6) branches every 8-12 residues
• Like amylopectin, glycogen gives a red-violet color with
iodine
glycogen
3 Cellulose
• The β-glucose molecules are joined by condensation, i.e. the removal of
water, forming β-(1,4) glycosidic linkages.
• Note however that every second β -glucose molecule has to flip over to
allow the bond to form. This produces a “heads-tails-heads” sequence.
• The glucose units are linked into straight chains each 100-1000 units
long.
• Weak hydrogen bonds form between parallel chains binding them into
cellulose microfibrils.
• Cellulose microfibrils arrange themselves into thicker bundles called
microfibrils. (These are usually referred to as fibres.)
• The cellulose fibres are often “glued” together by other compounds such as
hemicelluloses and calcium pectate to form complex structures such as
plant cell walls.
Cellulose
4 pectin
Cell wall polysaccharide
‘smooth’ regions :Partial methylated or not methylated
poly-a-(14)-D-galacturonic acid residues;
‘hairy’ regions: due to presence of alternating a -(12)-Lrhamnosyl-a -(14)-D-galacturonosyl sections containing
branch-points with side chains (1 - 20 residues) of mainly
L-arabinose and D-galactose
Pectin Model
RG-II
• Source: Cell walls of higher plants (citrus rind)
• Structure: Largely a linear polymer of polygalacturonic acid with varying
degrees of methyl esterification. (Also some branches –HAIRY REGIONS)
– >50% esterified is a high methoxy (HM) pectin
– <50% esterified is a low methoxy (LM) pectin
• Functional Properties:
Main use as gelling agent (jams, jellies)
– dependent on degree of methylation
– high methoxyl pectins gel through H-bonding and in presence of sugar
and acid
– low methoxyl pectins gel in the presence of Ca2+ (‘egg-box’ model)
Thickeners
Water binders
Stabilizers
Other polysaccharides
•
Chitin (poly glucose amine), found in fungal cell walls and the
exoskeletons of insects.
•
Callose (poly 1-3 glucose), found in the walls of phloem tubes.
•
Dextran (poly 1-2, 1-3 and 1-4 glucose), the storage polysaccharide in
fungi and bacteria.
•
Inulin (poly fructose), a plant food store.
•
Agar (poly galactose sulphate), found in algae and used to make agar
plates.
•
Murein (a sugar-peptide polymer), found in bacterial cell walls.
•
Lignin (a complex polymer), found in the walls of xylem cells, is the
main component of wood.
Structure analysis of
polysaccharides
Information on polysaccharide structures
--Monosaccharide component
--Sugar linkage type
--Sugar sequence
--Monosaccharide configuration(αorβand D or L)
--Molecular weight
--Amount and position of substitute units
--Degree of branching
• Monosaccharide component
The polysaccharide samples are hydrolyzed by
HCl/MeOH and TFA, then analyzed by HPLC or GC
HPLC:
High pressure/performance
liquid chromatography
• Sugar linkage type
Chemical methods:
Periodate Oxidation and Smith degradation
Methylation analysis
GC-MS:
Gas chromatographyMass spectrometer
Physical methods:
NMR(Nuclear Magnetic Resonance)
• Sugar linkage type
• Monosaccharide configuration
• Substitute units
• Degree of branching
Physical methods:
FT-IR (Fourier transform infrared spectroscopy)
• Monosaccharide configuration
• Substitute units
Physical methods:
MS (Mass spectrometer)
• Sugar linkage type
• Monosaccharide configuration
• Substitute units
• Degree of branching
• Molecular weight
• Molecular weight
Determination methods
Molecular weight range
End group titration
< 3×104
Elevation of boiling point
< 3×104
Depression of freezing point
< 3×104
Vapour pressure Osmometry
< 3×104
Membrane Osmometry
3×104—1.5×106
Light scattering
1×104—1×107
Centrifugation sedimentation velocity
1×104—1×107
Centrifugation sedimentation equilibrium
1×104—1×106
Intrinsic viscosity measurement
1×104—1×107
High performance gel-permeation
chromatography
1×102—1×107
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