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”}

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”}

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

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-

3 examples of chiral Carbon atoms: from http://ntri.tamuk.edu/cell/carbohydrates.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/cell/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 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 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 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 Alginates (video) Pectin Xanthan Galactomannans Cellulose 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 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

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-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 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 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.)