Biomedical Engineering Reference
In-Depth Information
Random coil
Ca
++
Ca
++
“Half egg-box” binding
Ca
++
“Egg-box” dimer
Aggregation of dimers
Figure 5.16
Schematic representation of Ca chelation to poly(l-guluronate) sequences. Open circles represent
Ca
2+
ions. Dimerization involves calcium chelation only to the interior faces of the participating
chains (50% of the stoichiometric requirement). On more extensive aggregation, this ratio will tend
towards 100% with increasing aggregate size, while site binding of calcium to the exterior faces of
dimers (
binding), without further aggregation, raises the ratio to 150%. Reprinted
with permission from Morris et al.(
1978
) © 1978 Elsevier.
'
half egg-box
'
The alginate model of
Figure 5.16
was further examined by SAXS (
Chapter 2
) and the
cross-sectional radius of gyration R
c
was determined to be 0.31
-
0.56 nm (Stokke et al.,
2000
). The SAXS data suggested that dimerization of chain segments was the principal
association mode at low fractional Ca
2+
saturation of G. Increasing the Ca
2+
saturation of
guluronic acid, either by concentration or selection of alginate source, yielded coexisting
lateral association modes. These junction zone multiplicities occur because there is a
delicate balance between the block length distribution of the
-l-GulA residues, polymer
concentration and Ca
2+
. The relationship between rheological properties and the struc-
ture determined by SAXS was studied (Yuguchi et al.,
2000
). The storage modulus of
gels was found to increase as the bundles composed of associated alginate chains grow
(
Figure 5.17
). Gel elasticity was thought to be mainly sustained by single chains in the
alginate sample with a low fraction of
α
-l-GulA. A good correlation was found between
the storage modulus and the mean cross-sectional radius, and it was concluded that
alginates with a high fraction of
α
-l-GulA associate into thicker bundles, which join to
form a network. It was also suggested that gel elasticity is due to the
α
flexible joints
between bundles, since the fraction of single chains is extremely low.
The overall composition of M/G residues and their distribution patterns vary with
seaweed species. However, very recently the availability of C-5 epimerases, speci
c
enzymes which can convert MM- to MG-blocks in the polymer or can introduce
G-blocks of various lengths, has been pioneered by the group led by Skjåk-Bræk
(Campa et al.,
2004
; Holtan et al.,
2006
). This powerful approach has allowed many
special
ne structure and the
composition and distribution patterns of M/G-blocks (Donati et al.,
2005
). Monovalent
sodium and smaller divalent magnesium ions cannot induce the gelation of alginate, but
'
tailor-made
'
alginate samples to be prepared, by tuning the
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