Polysaccharides have been proposed as the first biopolymers
to have formed on Earth .
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.
Regular 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 interunit 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-(14)-linked
glucans, while dextrans, α-(16)
glucans, and cellulose, β-(14)-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
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.
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)
Rotation changes the energy of the structure
and this can be visualized as a potential energy map (as
shown for a β-(14)-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
Polysaccharide linkage through the methyl hydroxyl group
(for example, in α-(16)
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
a Carbohydrate web resources have
been collected .
b Strictly speaking
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
c Molecular dynamics
studies show cello-oligosaccharides have greater surrounding
water molecules (compared with malto-oligosaccharides) due
to their more extended structure .
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
using the AMBERS force field was used for the modeling presented on these