Biomedical Engineering Reference
In-Depth Information
distribution is required. Ogawa et al.(
2009
) recently obtained three gellan samples
converted to Na
+
salt form and with varying molar masses (7.1
12.7 × 10
3
g mol
−
1
).
-
In this work they found that the temperatures for coil
helix transition and gel formation
were almost independent of molar mass in Na
+
,K
+
and Ca
2+
-
solutions, but differed
signi
cantly in pure water, where the coil
-
helix temperature was up to 20°C higher.
5.5
Gelation of alginates and pectins
Alginate is a collective term for a family of exopolysaccharides produced from brown
seaweeds and some bacteria, while pectins occur quite commonly in plant cell walls and
most commercial pectins are extracted from apple pulp or citrus peel. Both have been
extracted and used in a variety of products, alginates for example in foods, but also in
advanced biomedical treatments. Suitable alginate solutions, usually prepared as the Na
+
salt form, gel very readily when mixed with a solution of Ca
2+
. Pectins can be gelled by
ions, but also by lowering water activity, for example by adding low molecular mass
sugars
the traditional route in jam and preserve production. Unlike the carrageenans,
the gels formed from alginates and pectins are not generally regarded as thermorever-
sible, although such gels do tend to weaken on heating. In this section we consider only
pectin gelation with ions, and the structural parallels with alginate gelation.
-
5.5.1
Alginates
Chemically, alginates are linear copolymers of (1
4)-linked
β
-d-mannuronate (M) and
→
α
-l-guluronate (G) residues. Alginates have unique ion-exchange properties: most
monovalent counterions (except Ag
+
) form soluble alginate salts, whereas divalent and
multivalent cations (except Mg
2+
) form gels or precipitates. The af
nity has been found
to follow the order Mn
2+
<Zn
2+
,Co
2+
,Ni
2+
<Ca
2+
<Sr
2+
<Ba
2+
<Cd
2+
<Cu
2+
<Pb
2+
(Rinaudo,
2008
).
The residues are arranged in a blockwise pattern with G-blocks and M-blocks inter-
spersed with MG alternating blocks. Based on X-ray
fibre diffraction of polyguluronate
and polymannuronate sodium salts, it has been shown that the former form a two-fold
buckled helical shape, via diaxial 1, 4 linkages, which has large cavities between adjacent
paired chains, while the latter adopt an extended ribbon-like conformation. Using a
variety of methods including circular dichroism, electron micrography, dialysis experi-
ments and model building, Grant et al.(
1973
) proposed a so-called
model for
the gelation of alginates, where two or more chains are involved in cooperative binding,
forming the structure. The buckled chain is shown as a two-dimensional analogue of a
corrugated egg-box, with interstices in which the cations may pack and be coordinated
like the eggs in a real egg-box (
Figure 5.16
).
For example, G-block sequences of alginate show a speci
'
egg-box
'
nity for Sr
+
, which is
not shared by M-blocks. The origin of this behaviour appears to lie in the steric require-
ments for effective packing of cations within the egg-box junction zones which cross-link
alginate gels.
caf
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