Introduction
to polysaccharides
Hydration
Alternatives for defining bound and unbound water
Polar effects, for example, α-D-galacturonic
acid
Weak hydrogen bonding, for example, α-L-arabinofuranose
Strong hydrogen bonding, for example, β-1-4-linked
D-xylose
Hydrophobic effects, for example, β-1-4-linked
D-xylose
Effects of other solutes: non-ionic
Effects of other solutes: ionic
Conclusions concerning polysaccharide hydration
Using a simplistic
approach to polysaccharide hydration, water can be divided into
'bound water', subcategorized as being capable of freezing or not,
and 'unbound water', subcategorized as being trapped or not, and with
'hydration' a s a general term concerning the amount of bound water. The term 'bound' is poorly understood, being very
difficult to explain (or investigate) exactly and has been defined
(perhaps in desperation) as 'non-bulk' water. 'Unbound'
water freezes at the same temperature as normal water (< 0 °C
dependent on cooling rate). However some water may take up to 24
hr to freeze. 'Bound freezable' water freezes at a lower temperature
than normal water, being easily supercooled. It also exhibits a
reduced enthalpy of fusion (melting). Although the inability to
freeze is often used to determine bound water, freezing may not
a good measure of hydration as it concerns the water content of
the glassy state; not aqueous hydration.
However alternative determinants of 'bound' water, such as the use
of NMR are also problematic as NMR determines binding dynamics (rates
of dissociation involving dissociation activation energy) rather
than thermodynamics (free energy changes). [Back to Top 
]
For alternative (1) the type of water is determined by freezing and melting calorimetry using the enthalpy of the phase change [1514] and thermogravimetry, measuring the weight changes due to absorption from set humidity atmospheres. Presence of sufficient freezing bound water is required for preferred biocompatibility. It is supposed that such a layer shields the inner non-freezing water layer from the hydration shell of other biological components [1700]. Non-freezing water may be trapped in a glassy state, lowering diffusion by several orders of magnitude and hindering crystal formation.
For the less precise alternative (2), which is used in the dietary fiber area, the firmly held water not removed by centrifugation, gives the water binding capacity (WBC) whereas the loosely associated water, which is not removed by filtration gives the water holding capacity (WHC).
Both schemes, however, only determine what they measure and this may be somewhat different from what they represent as measuring; that is, aqueous hydration. At the present time the literature provides little useful explanatory theory for polysaccharide hydration, with poor predictability.
In practical experience, the effects of water on polysaccharide and polysaccharide on water are complex and become even more complex in the presence of other materials, such as salts. Water competes for hydrogen bonding sites with intramolecular and intermolecular hydrogen bonding, certainly will determine the carbohydrate's flexibility and may determine the carbohydrate's preferred conformation(s) [254]. There is a high entropic cost (up to about 20.8 kJ mol-1 at 25 °C for a totally 'frozen' molecule) when water is bound and this must be reclaimed, for example, by the formation of stronger or extra hydrogen bonds [320]. There are several questions to be answered:
These questions can be answered by considering the following effects
on the polysaccharide hydration: polar, weak hydrogen bonding, strong
hydrogen bonding, hydrophobic and the presence of other non-ionic
and ionic solutes.a [Back to Top ]
D-galacturonic acid residues occur in pectins and tightly bind some water molecules. However, the amount bound strongly depends mostly on the cation counter-ion present rather than the sugar residue: thus, sodium galacturonate binds 4 molecules, and the K+, Ca2+ and Mg2+ salts bind 2, 12 and 13 water molecules respectively (water-ion binding greater than 13.0 kcal mol-1 [250]). Both acid oxygen atoms of the carboxylate group prefer to accept two or three hydrogen bonds, dependent on steric factors, each provided from different water molecules due to steric and anti-cooperative effects and as shown by an ab initio study [316].
As the distance between the two carboxylate oxygen atoms (O6A····O6B) is smaller (at about 2.2 Å) than H2O····HOH hydrogen bonded O····O distance (at about 2.8 Å) these water molecules will only fit in a locally collapsed, weaker, water structure.
As hydrogen bonding (through donation) is weakened if one of the donor hydrogen bonds of water is hydrogen bonded to a stronger base than water, carboxylates are expected to give rise to a particularly weak hydrogen bond in next shell, so encouraging a local collapse in the hydrogen bonded network. Near polyelectrolytes the osmolarity is high and water activity and chemical potential are low. The potential of water is partially increased by collapsing the hydrogen bond network. If the surface is highly charged, the high-density water (HDW) zone may reach out to several nanometers and the local density of the first hydration shell may be greater than 1.1 g cm-3. The zone is weakly hydrogen-bonded, fluid and reactive, and accumulates small cations, multivalent anions and hydrophobic solutes. In order to keep the potential of the water constant the water surrounding this low potential HDW zone is reduced in potential to match so producing a zone of lower density water (LDW), which may be extensive enough as to lower the overall density of the medium [407]. These two zones (HDW) and (LDW) are unlikely to be sharply distinguishable or perfectly formed, but the chemical potential of the water will be similar throughout. This may be the case as found in the domain structure of solutions [1148].
The polysaccharide chain rigidity will depend on whether the counter ions are close (leading to possible intermolecular attraction but reducing the intramolecular charge repulsion) or far away (for example, in the LDW). The rules for the formation of ion pairs are followed with the carboxylate group considered to be poorly hydrated, so strongly hydrated ions (for example, Li+, Ca2+, Mg2+) will adsorb to the ionic surfaces less readily than poorly hydrated ions (for example, Cs+, K+, Na+).
Some poly-ionic polysaccharides (for example, pectins with more than 57% unmethylated carboxylate groups) can form junction
zones by means of divalent cations (for example, Ca2+)
so long as a minimum of 14 residues can cooperate. It may well be
that two carboxylate groups have to cooperate in prizing the bound
water away from the calcium ions to form the salt links that make
up the junction zones. The low potential of the water associated
with the ion pairs will affect nearby water to encourage a leveling
of the water potentials. This causes surrounding water to become
more expanded (lower in density). [Back to Top ]
α-L-arabinofuranose occurs in arabinoxylans and cannot form intra-residue hydrogen-bonding due to the position of its alcohol groups and the fluid nature of the furanose ring, which is constantly changing its conformation due to the low potential energy barriers between the large number of conformers with similar potential energies. However, it does form many single hydrogen bonds to water molecules. This hydrogen bonding is weak and destructuring with every carbohydrate hydroxyl groups acting as a donor, and preferably also as a double acceptor.
As well as these three water molecules, there is likely to also
be 1-2 non-bonded nearest neighbors. Similarly, ring oxygen atoms
will have 1-2 water molecules associated with them, plus 0-1 non-bonded
nearest neighbors and glycosidic oxygen atoms have one water molecule
associated with them, plus 0-1 non-bonded nearest neighbor. [Back to Top ]
Strong hydrogen bonding requires two hydrogen bonds from the polysaccharide to the same water molecule, one as donor and one as acceptor for maximum cooperativity. Strong (double) hydrogen-bonded water links often appear to displace single intra-residue hydrogen bonds as they have the similar requirements for suitably oriented vicinal hydroxyls and their presence can reduce the stereochemical demands. Such hydrogen bonding is likely to be stronger if the hydroxyl groups have less flexibility; thus hydrogen bonding strength will generally follow the trend: neighboring on pyran (for example, as shown opposite) > neighboring on furan > between hydroxyl groups on neighboring residues > between ring hydroxyl and methylhydroxyl. Polysaccharides are more hydrophobic if they have intra-molecular hydrogen bonds.
However, the fixed orientation of the doubly-linked
water molecules may increase the hydrophobic extent of the carbohydrate's
top and bottom surfaces and so aid the formation of junction zones. This is a cooperative process that may take some
time. If the doubly-bound water is replaced by two singly bound
water molecules, the polysaccharide will become less hydrophobic,
there will be a further entry of water and a partial break-up of
any junction zones. However, as the formation of junction zones
is a cooperative process, it will be difficult to reverse. Also
singly linked water molecules involve greater (energetically unfavorable)
rearrangements in the surrounding water. If all other factors are
equal, the preferences being: intramolecular hydrogen bond >
doubly hydrogen bond linked water > two singly hydrogen bond
linked water molecules. [Back to Top ]
Several carbohydrates (for example, β-1-4-linked-D-xylose in arabinoxylans, β-1-4-linked-D-glucose in cellulose and β-glucans, β-1-3-linked-D-glucosein β-glucans and β-1-3-linked-D-xylose) may be considered as having two hydrophobic surfaces with a hydrogen-bonding edge. The "hydrophobic effect" is primarily a consequence of changes in the clustering in the surrounding water rather than water-solute interactions.
Where low-density water (LDW) overlays hydrophobic surfaces, there
will be a tendency for surface minimization by surfaces interacting
and excluding water, causing the formation of junction zones. It
is the incompatibility between the LDW and the hydrophobic surface that drives the structure formation.
The formation of low-density water next to hydrophobic surfaces,
and concomitant junction zone formation, is encouraged if this low-density
water is also associated with high-density water created near to
polar groups (see above). Alternatively, local
strong hydrogen bonding may be able to create low-density water
without assistance. It is likely however that local weak hydrogen
bonding will discourage junction zone formation. It is noteworthy
that the hydrophobic effect decreases with increased pressure (or
density) as it is dependent on the presence of tetrahedrally-placed
water molecules (as in LDW), which reduce in number under the distorting
influence of pressure [626]. [Back to Top ]
Kosmotropes are very soluble, well hydrated molecules, having no net charge and enforcing extensive hydrogen bonding. They may 'compensate' for the disrupting effects of high ionic concentrations in some natural microorganisms. Kosmotropes are molecules that stabilize the structure of macromolecules in solution. They stabilize polysaccharide junction zone formation in the same way as they are preferentially excluded from their surfaces. This exclusion entropically 'drives' junction formation. Low molecular weight sugars can cause hydrogen-bonding links between polysaccharides by dehydrating the surface.
The effects of dissolved gases are often ignored. However,
they are usually present (even in distilled and de-ionized
water), and may have important and varying effects [711].
Some gases are essentially structuring (for example, O2,
N2, Ar) whereas other are destructuring (for example, CO2). Structuring gases,
even with their low solubility, may accumulate at hydrophobic
surfaces (including the formation of tiny gas bubbles [459]
that may grow, if sufficient gas is available in solution,
to form gas filled capillaries [578])
where they are more soluble (the water here possessing effectively
very low dielectric constant) and so increase their structuring
effects. Such dissolved gas has a major effect on emulsion
stabilization and flocculation (degassing stabilizing the
emulsions by removing the (strange) long-range hydrophobic
attraction [711]) and confusing
any Hofmeister effects [671].
They may also be responsible for other strange effects such
as free radical production in electromagnetic
fields. [Back to Top ]
Ions generally have an effect on the structure of water like increased temperature or pressure. Small ions are strongly hydrated, creating local order but destroying the natural hydrogen bonded network. The Hofmeister series shows the promotion of hydrophobic associations, such that ions favoring low-density water promote amylose retrogradation and starch gelatinization. Ions in polysaccharide solutions may behave differently from when in solution by themselves, as the polysaccharides are capable of producing relatively stable low-density or high-density aqueous microenvironments. At a charged interface if LDW accumulates solute then it will relax back to normal density, as will the associated HDW.
Structure making salts concentrate in less structured
aqueous phase and decrease the compatibility between hydrophilic
polymers, whilst structure breaking salts concentrate in more structured
environments and increase such compatibility. [Back to Top ]
[Back to Top ]
a AA recent modeling study has characterized the hydration around a number of mannose and glucose oligosaccharides [1600]. [Back]
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This page was last updated by Martin Chaplin on 22 November, 2012
