Introduction to Polysaccharides

1
POLYSACCHARIDE STRUCTURE
2
(No Transcript)
3
(No Transcript)
4
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

5

• 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

6

• Polysaccharides
• Often poorly defined (although some can form
  helices)
• Synthesised by enzymes without template
  polydisperse, and generally larger
• Many homopolymers, and rarely gt3,4 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

• 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

7
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
8
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

9
Chirality rules

• Monosaccharides contain one or more asymmetric
  C-atoms get D- and L-forms, where D- and L-
  designate absolute configuration
• D-form -OH group is attached to the right of
  the asymmetric carbon
• L-form -OH group is attached to the left of the
  asymmetric carbon
• If there is more than one chiral C-atom absolute
  configuration of chiral C furthest away from
  carbonyl group determines whether D- or L-

10
3 examples of chiral Carbon atoms
from http//ntri.tamuk.edu/cell/carbohydrates.html
)
11
Ring formation / Ring structure
An aldose Glucose
from http//ntri.tamuk.edu/cell/carbohydrates.html
12
A ketose Fructose
from http//ntri.tamuk.edu/cell/carbohydrates.html
13
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

14
Two different forms of b-D-Glucose
15
Two different forms of b-D-Glucose
Preferred
16
Formation of di- and polysaccharide bonds
Dehydration synthesis of a sucrose molecule
formed from condensation of a glucose with a
fructose
17
Lactose Maltose
from http//ntri.tamuk.edu/cell/carbohydrates.html
18
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

19
Polysaccharides

• Primary Structure
• Sequence of residues
• N.B.
• Many are homopolymers. Those that
• are heteropolymers rarely have gt3,4
• different residues

20
Secondary Tertiary Structure

• Rotational freedom
• hydrogen bonding
• oscillations
• local (secondary) and overall (tertiary) random
  coil, helical conformations

21
Movement around bonds
from http//www.sbu.ac.uk/water/hydro.html
22
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

23
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
24
(b) Buckled ribbon type conformation Alginate
from http//www.sbu.ac.uk/water/hydro.html
25
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

26
Amylose forms inclusion complexes with iodine,
phenol, n-butanol, etc.
from http//www.sbu.ac.uk/water/hydro.html
27
The liganded amylose-iodine 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
28
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
29
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

30
Polysaccharides 6 case studies

1. Alginates (video)
2. Pectin
3. Xanthan
4. Galactomannans
5. Cellulose
6. Starch (Dr. Sandra Hill)

31
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
32
(No Transcript)
33
from http//www.sbu.ac.uk/water/hydro.html
34
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
35
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
36
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 three-phase systems
• Used in pet food chunks, onion rings, stuffed
  olives and pie fillings, wound healing agents,
  printing industry (largest use)

37
2. Pectin (E440)

• Cell wall polysaccharide in fruit and vegetables
• Main source - citrus peel

38
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
39
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

40
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
41
Properties and applications

• double helical conformation
• pseudoplastic
• shear-thinning
• thickener
• stabilizer
• emulsifier
• foaming agent
• forms synergistic gels with galactomannans

42
4. Galactomannans

• b-(1?4) mannose (M) backbone with a-(1?6)
  galactose (G) side chains
• Ratio of M to G depends on source
• MG11 - fenugreek gum
• MG21 - guar gum (E412)
• MG31 - tara gum
• MG41 - locust bean gum (E410)

43
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)
44
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

45
5. Cellulose
b-(1?4) glucopyranose
from http//www.sbu.ac.uk/water/hydro.html
46
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.)