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

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”}

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

• Contain between 3 and 7 C atoms
• empirical formula of simple monosaccharides -
(CH
2
O) n
• aldehydes or ketones from http://ntri.tamuk.edu/cell/carbohydrates.html

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

Chirality rules
1.
Monosaccharides contain one or more asymmetric Catoms: get D- and L-forms , where D- and Ldesignate absolute configuration
2.
D-form : -OH group is attached to the right of the asymmetric carbon
3.
L-form : -OH group is attached to the left of the asymmetric carbon
4.
If there is more than one chiral C-atom : absolute configuration of chiral C furthest away from carbonyl group determines whether D- or L-

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

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

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

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

Two different forms of b -D-Glucose

Two different forms of b -D-Glucose
Preferred

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

Lactose:
Maltose: from http://ntri.tamuk.edu/ce ll/carbohydrates.html

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

Secondary & Tertiary
Structure

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

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
(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

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

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

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

The liganded amyloseiodine 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

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
Aggregate and gel formation:
• May involve
• other molecules such as Ca 2+ or sucrose
• Other polysaccharides (mixed gels)
…this will be covered in the lecture from
Professor Mitchell

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

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

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

Calcium polya
-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

Different types of alginates different properties e.g. gel strength
Polyguluronate: - gelation through addition of Ca 2+ ions – egg-box
Polymannuronate – less strong gels, interactions with Ca 2+ weaker, ribbon-type conformation
Alternating sequences – disordered structure, no gelation

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 threephase systems
• Used in pet food chunks, onion rings, stuffed olives and pie fillings, wound healing agents, printing industry (largest use)

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

Partial methylated polya
-(1

4)-D-galacturonic acid residues
(‘smooth’ regions), ‘hairy’ regions due to presence of alternating a
-(1

2)-L-rhamnosyla
-(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 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
Ca 2+ (‘egg-box’ model)
• Thickeners
• Water binders
• Stabilizers

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

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

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.)