POLYSACCHARIDE STRUCTURE. References Tombs, M.P. & Harding, S.E., An Introduction to Polysaccharide Biotechnology, Taylor & Francis, London, 1997

POLYSACCHARIDE STRUCTURE

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 etc

• Range of structures (rodcoil)• Poly(amino acid) ~ compares

with some linear polysaccharides

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

• 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

1. Monosaccharides contain one or more asymmetric C-atoms: get D- and L-forms, where D- and L- designate 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-

Chirality rules

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

3 examples of chiral Carbon atoms:

Ring formation / Ring structure


An aldose: Glucose


A ketose: Fructose

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 or mutarotation • Chair conformation usual (as opposed to boat)• Axial and equatorial bonds

Two different forms of -D-Glucose

Two different forms of -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:


Disaccharides

• Composed of two monosaccharide units by glycosidic link from C-1 of one unit and -OH of second unit

• 13, 14, 1 6 links most common but 1 1 and 1 2 are possible

• Links may be or • 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 >3,4different residues

Secondary & Tertiary Structure

• Rotational freedom• hydrogen bonding• oscillations• local (secondary) and overall

(tertiary) random coil, helical conformations

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

14) - zig-zag - ribbon like 1 3) & 4) - U-turn - hollow helix 1 2) - twisted - crumpled (16) - no ordered conformation

Ribbon type structures

Chains can align and pack closely together. Also get hydrogen bonding and interactive forces.


(a) Flat ribbon type conformation: Cellulose


(b) Buckled ribbon type conformation: Alginate

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.


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

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 Ca2+ or sucrose• Other polysaccharides (mixed gels)

…this will be covered in the lecture from Professor Mitchell

Polysaccharides – 6 case studies

1. Alginates (video)2. Pectin3. Xanthan4. Galactomannans5. Cellulose6. Starch (Dr. Sandra Hill)

1. Alginate (E400-E404)

Source: Brown seaweeds (Phaeophyceae, mainly Laminaria)

Linear unbranched polymers containing -(14)-linked D-mannuronic acid (M) and -(14)-linked L-guluronic acid (G) residues

Not random copolymers but consist of blocks of either MMM or GGG or MGMGMG


Calcium poly--L-guluronate left-handed helix view down axis

view along axis, showing the hydrogen bonding and calcium binding sites


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

• 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)

• Cell wall polysaccharide in fruit and vegetables

• Main source - citrus peel

Partial methylated poly--(14)-D-galacturonic acid residues (‘smooth’ regions), ‘hairy’ regions due to presence of alternating -(12)-L-rhamnosyl- -(14)-D-galacturonosyl sections containing branch-points with side chains (1 - 20 residues) of mainly L-arabinose and D-galactose


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)

Extracellular polysaccharide from Xanthomonas campestris

-(14)-D-glucopyranose backbone with side chains of -(31)--linked D-mannopyranose-(21)--D-glucuronic acid-(41)--D-mannopyranose on alternating residues



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

4. Galactomannans

-(14) mannose (M) backbone with -(16) 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)


• non-ionic• solubility decreases with decreasing galactose

content• thickeners and viscosifiers• used in sauces, ice creams• LBG can form very weak gels

5. Cellulose

-(14) glucopyranose


• 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.)


Documents

POLYSACCHARIDE STRUCTURE. References Tombs, M.P. & Harding, S.E., An Introduction to Polysaccharide Biotechnology, Taylor & Francis, London, 1997