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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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references
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
slide5
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”}
slide6
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
monosaccharides
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
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
chirali ty rules
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-
slide10

3 examples of chiral Carbon atoms:

from http://ntri.tamuk.edu/cell/carbohydrates.html)

ring formation ring structure
Ring formation / Ring structure

An aldose: Glucose

from http://ntri.tamuk.edu/cell/carbohydrates.html

slide12

A ketose: Fructose

from http://ntri.tamuk.edu/cell/carbohydrates.html

ring structure
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
formation of di and polysaccharide bonds
Formation of di- and polysaccharide bonds

Dehydration synthesis of a sucrose molecule formed from condensation of a glucose with a fructose

slide17

Lactose:

Maltose:

from http://ntri.tamuk.edu/cell/carbohydrates.html

disaccharides
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
Polysaccharides

Primary Structure:

Sequence of residues

N.B.

Many are homopolymers. Those that

are heteropolymers rarely have >3,4

different residues

secondary tertiary structure
Secondary & Tertiary Structure
  • Rotational freedom
  • hydrogen bonding
  • oscillations
  • local (secondary) and overall (tertiary) random coil, helical conformations
movement around bonds
Movement around bonds:

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

tertiary structure sterical geometrical conformations
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
ribbon type structures
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

slide24

(b) Buckled ribbon type conformation: Alginate

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

hollow helix type structures
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
slide26

Amylose forms inclusion complexes with iodine, phenol,

n-butanol, etc.

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

slide27

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

slide28

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
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
Polysaccharides – 6 case studies
  • Alginates (video)
  • Pectin
  • Xanthan
  • Galactomannans
  • Cellulose
  • Starch (Dr. Sandra Hill)
1 alginate e400 e404
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

slide34

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

slide35

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
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)
2 pectin e440
2. Pectin (E440)
  • Cell wall polysaccharide in fruit and vegetables
  • Main source - citrus peel
slide38

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 applications39
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
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 applications41
Properties and applications
  • double helical conformation
  • pseudoplastic
  • shear-thinning
  • thickener
  • stabilizer
  • emulsifier
  • foaming agent
  • forms synergistic gels with galactomannans
4 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)
slide43

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 applications44
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
5. Cellulose

b-(14) glucopyranose

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

slide46

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.)