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Polysaccharide Structure

Polysaccharides are polymeric carbohydrates, composed of monosaccharides arranged in chains.

 

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< Polysaccharide hydration


Polysaccharides a have been proposed as the first biopolymers to have formed on Earth [584]. They are classified (see Nomenclature) on the basis of their main monosaccharide components and the sequences and linkages between them, as well as the anomeric configuration of linkages, the ring size (furanose or pyranose), the absolute configuration (D- or L-) and any other substituents present. Certain structural characteristics such as chain conformation and intermolecular associations will influence the physicochemical properties of polysaccharides. The most stable arrangement of atoms in a polysaccharide will be that which satisfies both the intra- and inter-molecular forces. Regularly ordered polysaccharides, in general, are capable of assuming only a limited number of conformations due to severe steric restrictions on the freedom of rotation of sugar units about the inter-unit glycosidic bonds. There is also a clear correlation between allowed conformations and linkage structure. The structural non-starch polysaccharides, such as cellulose and xylan, have preferred orientations that automatically support extended conformations. Storage polysaccharides such as the chains in amylopectin tend to adopt wide helical conformations. The degree of stiffness and regularity of polysaccharide chains is likely to affect the rate and extent of their fermentation. Pentose sugars such as arabinose and xylose can adopt one of two specific conformations, furanose rings (often formed by arabinose) that can oscillate and are more flexible, and pyranose rings (usually formed by xylose and glucose) which are less flexible. Cereal arabinoxylans are composed of β-linked xylan chains and are relatively stiff molecules with extended conformations. The flexibility of arabinoxylans is decreased with increasing arabinosylation, but the key parameter is likely to be the distribution of these side-chains along the backbone since this will have the most direct effect on conformation. Also, due to their extended conformation, arabinoxylans exhibit a very high viscosity in aqueous solution. Pectins, containing galacturonic acid residues, form more flexible extended conformations and also have regular "hairy" regions with pendant arabinogalactans. Carbohydrates, especially those containing large numbers of hydroxyl groups, are often thought of as being hydrophilic but they are also capable of generating apolar surfaces depending on the monomer ring conformation, the epimeric structure, and the stereochemistry of the glycosidic linkages. Apolarity has been shown for dextrin, α-(1->4)-linked glucans, while dextrans, α-(1->6) glucans, and cellulose, β-(1->4)-glucans, are much less hydrophobic (in solution) and unable to project an apolar surface. Hydrophobicity will also be affected by the degree of polysaccharide hydration, particularly the amount of intra-molecular hydrogen bonding. Hydrophobicity will affect their availability for fermentation in the gut, and their binding to bile acids.

 

Polysaccharides are more hydrophobic if they have a greater number of internal hydrogen bonds, and as their hydrophobicity increases there is less direct interaction with water. Carbohydrates contain hydroxyl (alcohol) groups that preferentially interact with two water molecules each if they are not interacting with other hydroxyl groups on the molecule. Interaction with hydroxyl groups on the same or neighboring residues will necessarily reduce the polysaccharide's hydration status. β-linkages to the 3- and 4- positions in mannose or glucose homopolymers allow strong inflexible inter-residue hydrogen bonding, so reducing polymer hydration, and giving rise to rigid inflexible structural polysaccharides whereas α-linkages to the 2-, 3- and 4- positions in mannose or glucose homopolymers give rise to greater aqueous hydration and more flexible linkages [791]. c

 

Sugar residues have a specific conformation, often the so-called 4C1 chair conformation. This is illustrated on the right below where the ring oxygen is at the back, the 4-carbon is 'up' and the 1-carbon is 'down'. Conversely, furanose rings can oscillate and have a more flexible structure than pyranose rings, which means that they are less likely to have a fixed interaction with a molecule of water as energy will be lost in this process.

rotations occurring in polysaccharide links

 

 

 

The flexibility of polysaccharide chains depends on the ease of rotation around the anomeric links (see terminology, the torsion angles phi (φH, H1C1OC4 or H1C1OC6), psi (ψH, C1OC4H4 or C1OC6C5) and omega (ωH, OC6C5H5) are shown) b.

potential energy landscape for the link in linear beta-xylans

Rotation changes the energy of the structure and this can be visualized as a potential energy map (as shown for a β-(1->4)-xylan). In this case there are two main potential energy minima (at A and B) and the molecule can be seen to be rather flexible, with a low-energy route (shown in red) between them. Such differences in conformation can lead to effects on  viscosity. d

 

Polysaccharide linkage through the methyl hydroxyl group (for example, in α-(1->6) linked dextrans) are more flexible due to the extra degree of freedom in the link (ω). Such molecules often prefer trans conformations, around this bond, relative to one of the three other bonds neighboring the linking carbon atom (for example, O6 trans to the H5 is the gauche, gauche (gg) conformation (ωH, OC6C5H5 ~ 180°); O6 trans to the O5 is the tg conformation (ωH ~ 60°); O6 trans to the C4 is the gt conformation (ωH ~ 300°)).

Interactions with the aqueous solvent may determine the preferred conformation by disrupting intramolecular hydrogen bonding [254].

 

rotamers around the C5-C6 bond


Footnotes

a Carbohydrate web resources have been collected [611]. [Back]

 

b Strictly speaking the torsions are defined as φ, OringC1OCi , ψ, C1OCiCi-1 and ω, OCiCi-1Ci-2 but the use of the hydrogen atoms as shown is easier and often used. [Back]

 

c Molecular dynamics studies show cello-oligosaccharides have greater surrounding water molecules (compared with malto-oligosaccharides) due to their more extended structure [864]. This does not correspond to the situation with the polymers where the cello-polysaccharides (e.g. cellulose) are insoluble and the malto-polysaccharides (e.g. amylose) are moderately soluble. [Back]

 

d Hyperchem, using the AMBERS force field was used for the modeling presented on these pages. [Back]

 

 

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This page was established in 2001 and last updated by Martin Chaplin on 8 August, 2017


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