1. POLYSACCHARIDE STRUCTURE

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. Often poorly defined (although some can form helices)
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
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

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

11. Ring formation / Ring structure
An aldose: Glucose
from

12. A ketose: Fructose
from

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

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 >3,4
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:

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:

24. (b) Buckled ribbon type conformation: Alginate
from:

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:

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
Alginates (video)
Pectin
Xanthan
Galactomannans
Cellulose
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

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:

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 ( residues) of mainly L-arabinose and D-galactose
from:

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:

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
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)

43. Guar gum - obtained from endosperm of Cyamopsis tetragonolobus
Locust bean gum - obtained from seeds of carob tree (Ceratonia siliqua)
from:

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:

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.)