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Presentation on theme: "POLYSACCHARIDE STRUCTURE"— Presentation 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 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:

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

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:

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

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.

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:

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

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.)


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