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POLYSACCHARIDE STRUCTURE

POLYSACCHARIDE STRUCTURE. References. Tombs, M .P. & Harding, S.E., An Introduction to Pol ysaccharide Biotechnology, Taylor & Francis, London, 1997 D.A. Rees, Polysaccharide Shapes, Chapman & Hall, 1977

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POLYSACCHARIDE STRUCTURE

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  1. POLYSACCHARIDE STRUCTURE

  2. 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

  3. 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”}

  4. 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”} 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

  5. 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

  6. 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

  7. 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-

  8. 3 examples of chiral Carbon atoms: from http://ntri.tamuk.edu/cell/carbohydrates.html)

  9. Ring formation / Ring structure An aldose: Glucose from http://ntri.tamuk.edu/cell/carbohydrates.html

  10. A ketose: Fructose from http://ntri.tamuk.edu/cell/carbohydrates.html

  11. 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

  12. Two different forms of b-D-Glucose

  13. Two different forms of b-D-Glucose Preferred

  14. Formation of di- and polysaccharide bonds Dehydration synthesis of a sucrose molecule formed from condensation of a glucose with a fructose

  15. Lactose: Maltose: from http://ntri.tamuk.edu/cell/carbohydrates.html

  16. 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

  17. Polysaccharides Primary Structure: Sequence of residues N.B. Many are homopolymers. Those that are heteropolymers rarely have >3,4 different residues

  18. Secondary & Tertiary Structure • Rotational freedom • hydrogen bonding • oscillations • local (secondary) and overall (tertiary) random coil, helical conformations

  19. Movement around bonds: from: http://www.sbu.ac.uk/water/hydro.html

  20. 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

  21. 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

  22. (b) Buckled ribbon type conformation: Alginate from: http://www.sbu.ac.uk/water/hydro.html

  23. 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

  24. Amylose forms inclusion complexes with iodine, phenol, n-butanol, etc. from: http://www.sbu.ac.uk/water/hydro.html

  25. 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

  26. 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

  27. 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

  28. Polysaccharides – 6 case studies • Alginates (video) • Pectin • Xanthan • Galactomannans • Cellulose • Starch (Dr. Sandra Hill)

  29. 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

  30. from: http://www.sbu.ac.uk/water/hydro.html

  31. 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

  32. 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

  33. 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)

  34. 2. Pectin (E440) • Cell wall polysaccharide in fruit and vegetables • Main source - citrus peel

  35. 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

  36. 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

  37. 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

  38. Properties and applications • double helical conformation • pseudoplastic • shear-thinning • thickener • stabilizer • emulsifier • foaming agent • forms synergistic gels with galactomannans

  39. 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)

  40. 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)

  41. 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

  42. 5. Cellulose b-(14) glucopyranose from: http://www.sbu.ac.uk/water/hydro.html

  43. 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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