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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
from http://ntri.tamuk.edu/cell/carbohydrates.html
An aldose: Glucose
from http://ntri.tamuk.edu/cell/carbohydrates.html
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:
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
• 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.
from: http://www.sbu.ac.uk/water/hydro.html
(a) Flat ribbon type conformation: Cellulose
from: http://www.sbu.ac.uk/water/hydro.html
(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.
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
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
from: http://www.sbu.ac.uk/water/hydro.html
Calcium poly--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--(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
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
-(14)-D-glucopyranose backbone with side chains of -(31)--linked D-mannopyranose-(21)--D-glucuronic acid-(41)--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
-(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)
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
-(14) glucopyranose
from: http://www.sbu.ac.uk/water/hydro.html
• 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.)
Properties and applications