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 I. Carbohydrates (Polysaccharides) II. Lipids III. Proteins IV. Nucleic Acids Monomers making up Polymer Example Monosaccharides Fatty acids and glycerol Amino acids Nucleotides Sugars, Starch, Cellulose Fats, steroids, cholesterol Enzymes, structural components DNA, RNA
Proteins:
• well defined • Coded precisely by genes, hence monodisperse • ~20 building block residues (amino acids) • Standard peptide link (apart from proline) • Normally tightly folded structures
Polysaccharides
• Often poorly defined (although some can form helices) • Synthesised by enzymes without template – polydisperse, and generally larger • Many homopolymers, and rarely >3,4 different residues • Various links a(11), a(12), a(1-4),a(16), b(13), b(14) 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.
• • • • •
(CH 2 O) 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
4 5 6 7 8
Nomenclature
Functional group
Ketone Aldehyde Tetrose Pentose Hexose Heptose Octose Tetrulose Pentulose Hexulose Heptulose 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
2 are possible 6 links most common but 1
Links may be
a
or
b
1 and 1 Link around glycosidic bond is fixed but anomeric forms on the other C-1 are still in equilibrium
Synthesis
Some Disaccharides
CH 2 OH CH 2 OH CH 2 OH H O OH H OH O H H O H OH O H OH maltose H CH 2 OH OH O H O OH H H cellobiose OH H CH 2 OH H H OH
(a
-D-glucosyl-(1->4)-
b
-D-glucopyranose) OH H OH
(b
-D-glucosyl-(1->4)-
b
-D-glucopyranose) CH 2 OH H H O OH H CH 2 OH H O OH OH H OH H OH O sucrose OH OH O H O H lactose H OH CH 2 OH O H OH H H H CH 2 OH H OH
(b
-D-galactosyl-(1->4)-
b
-D-glucopyranose) OH 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
H OH CH 2 OH H OH H O H OH 1 H O H 4 6 CH 2 OH 5 O H OH H H 3 2 OH 1 H O H CH 2 OH O H OH H H OH amylose H H O CH 2 OH O H OH H H OH H H O CH 2 OH O H OH H H 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
linkages, but it also has
branches
formed by
α
-(1 4) -(1 6) linkages. Branches are generally longer than shown above.
Structure of Amylopectin Fraction of Starch
H OH H OH CH 2 OH O H OH H H CH 2 OH O H OH H H O OH H H OH H O CH 2 OH O H OH H H H H CH 2 OH O H OH H OH H O OH H 4 6 5 H O CH H 1 OH H 3 2 amylopectin O H 2 OH 1 H O H 4 H CH 2 OH O H OH H OH H O H CH 2 OH O H OH H H OH H OH
Amylopectin
• •
Amylopectin causes red-violet colour a change on reaction with iodine.
This change is usually masked by the much darker reaction amylose to iodine.
of 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 iodine with
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
allow the bond to form. This produces a
“ heads-tails-heads ”
• The glucose units are linked into
straight chains
to sequence.
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)-L rhamnosyl-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 Ca 2+ (‘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 chromatography Mass 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 End group titration Elevation of boiling point Depression of freezing point Vapour pressure Osmometry Membrane Osmometry Light scattering Centrifugation sedimentation velocity Centrifugation sedimentation equilibrium Molecular weight range < 3
×
10 4 < 3
×
10 4 < 3
×
10 4 < 3
×
10 4 3
×
10 4 —1.5
×
10 6 1
×
10 4 —1
×
10 7 1
×
10 4 —1
×
10 7 1
×
10 4 —1
×
10 6 Intrinsic viscosity measurement High performance gel-permeation chromatography 1 1
× ×
10 10 4 2 —1 —1
× ×
10 10 7 7
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