Transcript Introduction to Polysaccharides

POLYSACCHARIDE
STRUCTURE
References
• Tombs, M.P. & Harding, S.E., An Introduction
to Polysaccharide Biotechnology, Taylor &
Francis, London, 1997
• D.A. Rees, Polysaccharide Shapes, Chapman & Hall, 1977
• E.R. Morris in ‘Polysaccharides in Food’, J.M.V.
Blanshard & J.R. Mitchell (eds.), Butterworths, London.
1979, Chapter 2
• The Polysaccharides, G.O. Aspinall (ed.), Academic Press,
London, 1985
• Carbohydrate Chemistry for Food Scientists, R.L.
Whistler, J.N. BeMiller, Eagan Press, St. Paul, USA, 1997
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
• {some proteins do not
possess folded structure –
gelatin – an “honorary
polysaccharide”}
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
• {some proteins do not
possess folded structure –
gelatin – an “honorary
polysaccharide”}
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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(11), a(12),
a(1-4),a(16), b(13), b(14)etc
• Range of structures (rodcoil)
• Poly(amino acid) ~ compares
with some linear polysaccharides
Monosaccharides
• Contain between 3
and 7 C atoms
• empirical formula
of simple
monosaccharides (CH2O)n
• aldehydes or
ketones
from http://ntri.tamuk.edu/cell/carbohydrates.html
SomeTerminology
• Asymmetric (Chiral) Carbon – has
covalent bonds to four different groups,
cannot be superimposed on its mirror
image
• Enantiomers - pair of isomers that are
(non-superimposable) mirror images
Chirality rules
1. Monosaccharides contain one or more asymmetric Catoms: get D- and L-forms, where D- and Ldesignate absolute configuration
2. D-form: -OH group is attached to the right of the
asymmetric carbon
3. L-form: -OH group is attached to the left of the
asymmetric carbon
4. If there is more than one chiral C-atom: absolute
configuration of chiral C furthest away from carbonyl
group determines whether D- or L-
3 examples of
chiral Carbon
atoms:
from
http://ntri.tamuk.edu/cell/carbo
hydrates.html)
Ring formation / Ring structure
An aldose: Glucose
from http://ntri.tamuk.edu/cell/carbohydrates.html
A ketose: Fructose
from http://ntri.tamuk.edu/cell/carbohydrates.html
Ring Structure
• Linear known as “Fischer” structure”
• Ring know as a “Haworth projection”
• Cyclization via intramolecular hemiacetal (hemiketal)
formation
• C-1 becomes chiral upon cyclization - anomeric
carbon
• Anomeric C contains -OH group which may be a or b
(mutarotation a  b)
• Chair conformation usual (as opposed to boat)
• Axial and equatorial bonds
Two different forms of b-D-Glucose
Two different forms of b-D-Glucose
Preferred
Formation of di- and polysaccharide bonds
Dehydration synthesis
of a sucrose molecule
formed from
condensation of a
glucose with a
fructose
Lactose:
Maltose:
from
http://ntri.tamuk.edu/ce
ll/carbohydrates.html
Disaccharides
• Composed of two 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
Polysaccharides
Primary Structure:
Sequence of residues
N.B.
Many are homopolymers. Those that
are heteropolymers rarely have >3,4
different residues
Secondary & Tertiary
Structure
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Rotational freedom
hydrogen bonding
oscillations
local (secondary) and overall
(tertiary) random coil, helical
conformations
Movement around bonds:
from:
http://www.sbu.ac.uk
/water/hydro.html
Tertiary structure - sterical/geometrical
conformations
• Rule-of-thumb: Overall shape of the chain is
determined by geometrical relationship within each
monosaccharide unit
b(14) - zig-zag - ribbon like
b(1 3) & a(14) - U-turn - hollow helix
b(1 2) - twisted - crumpled
(16) - no ordered conformation
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Ribbon type structures
(a) Flat ribbon type conformation: Cellulose
Chains can align and pack closely together. Also
get hydrogen bonding and interactive forces.
from: http://www.sbu.ac.uk/water/hydro.html
(b) Buckled ribbon type conformation: Alginate
from: http://www.sbu.ac.uk/water/hydro.html
Hollow helix type structures
• Tight helix - void can be filled by including
molecules of appropriate size and shape
• More extended helix - two or three chains may
twist around each other to form double or triple
helix
• Very extended helix - chains can nest, i.e., close
pack without twisting around each other
Amylose forms inclusion complexes with iodine, phenol,
n-butanol, etc.
from: http://www.sbu.ac.uk/water/hydro.html
The liganded amyloseiodine complex: rows of
iodine atoms (shown in
black) neatly fit into the
core of the amylose helix.
N.B. Unliganded amylose
normally exists as a coil
rather than a helix in
solution
Tertiary Structure:
Conformation Zones
Zone A: Extra-rigid rod:
schizophyllan
Zone B: Rigid Rod:
xanthan
Zone C: Semi-flexible coil:
pectin
Zone D: Random coil:
dextran, pullulan
Zone E: Highly branched:
amylopectin, glycogen
Quarternary structure aggregation of ordered structures
Aggregate and gel formation:
• May involve
• other molecules such as Ca2+ or sucrose
• Other polysaccharides (mixed gels)
…this will be covered in the lecture from
Professor Mitchell
Polysaccharides – 6 case studies
1.
2.
3.
4.
5.
6.
Alginates (video)
Pectin
Xanthan
Galactomannans
Cellulose
Starch (Dr. Sandra Hill)
1. Alginate (E400-E404)
Source: Brown seaweeds (Phaeophyceae, mainly
Laminaria)
Linear unbranched polymers containing b(14)-linked D-mannuronic acid (M) and a(14)-linked L-guluronic acid (G) residues
Not random copolymers but consist of blocks of
either MMM or GGG or MGMGMG
from: http://www.sbu.ac.uk/water/hydro.html
Calcium poly-a-L-guluronate left-handed helix view down axis
view along axis, showing the hydrogen bonding and
calcium binding sites
from: http://www.sbu.ac.uk/water/hydro.html
Different types of alginates different properties e.g. gel strength
Polyguluronate: - gelation through addition of Ca2+
ions – egg-box
Polymannuronate – less strong gels, interactions
with Ca2+ weaker, ribbon-type conformation
Alternating sequences – disordered structure, no
gelation
Properties and Applications
• High water absorption
• Low viscosity emulsifiers and shear-thinning
thickeners
• Stabilize phase separation in low fat fat-substitutes
e.g. as alginate/caseinate blends in starch threephase systems
• Used in pet food chunks, onion rings, stuffed
olives and pie fillings, wound healing agents,
printing industry (largest use)
2. Pectin
(E440)
• Cell wall
polysaccharide in
fruit and
vegetables
• Main source citrus peel
Partial methylated poly-a-(14)-D-galacturonic acid residues
(‘smooth’ regions), ‘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
from: http://www.sbu.ac.uk/water/hydro.html
Properties and applications
• 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
3. Xanthan (E415)
Extracellular polysaccharide from Xanthomonas campestris
b-(14)-D-glucopyranose backbone with side chains of -(31)a-linked D-mannopyranose-(21)-b-D-glucuronic acid-(41)b-D-mannopyranose on alternating residues
from: http://www.sbu.ac.uk/water/hydro.html
Properties and applications
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double helical conformation
pseudoplastic
shear-thinning
thickener
stabilizer
emulsifier
foaming agent
forms synergistic gels with galactomannans
4. Galactomannans
b-(14) mannose (M) backbone with a(16) galactose (G) side chains
• Ratio of M to G depends on source
– M:G=1:1 - fenugreek gum
– M:G=2:1 - guar gum (E412)
– M:G=3:1 - tara gum
– M:G=4:1 - locust bean gum (E410)
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Guar gum - obtained from endosperm of Cyamopsis tetragonolobus
Locust bean gum - obtained from seeds of carob tree (Ceratonia siliqua)
from: http://www.sbu.ac.uk/water/hydro.html)
Properties and applications
• non-ionic
• solubility decreases with decreasing galactose
content
• thickeners and viscosifiers
• used in sauces, ice creams
• LBG can form very weak gels
5. Cellulose
b-(14) glucopyranose
from: http://www.sbu.ac.uk/water/hydro.html
Properties and applications
• found in plants as microfibrils
• very large molecule, insoluble in aqueous and most
other solvents
• flat ribbon type structure allows for very close
packing and formation of intermolecular H-bonds
• two crystalline forms (Cellulose I and II)
• derivatisation increases solubility (hydroxy-propyl
methyl cellulose, carboxymethyl cellulose, etc.)