Mixed gels - Physical Gels from Biological and Synthetic Polymers - page 321

Xanthan-xyloglucans
Most of the work on xanthan xyloglucan co-gelation has been carried out with

xyloglucan from tamarind seeds (TSX), in which a mixture with xanthan or with gellan
gum forms a transparent gel. Kim et al.( 2006 ) showed that gelation occurs in a mixture
of TSX and xanthan gum and that the system exhibits thermoreversible gel-like
properties at low temperatures. A steep increase in both storage and loss moduli was
observed on cooling, and a conspicuous exothermic DSC peak appeared at the same
temperature, although these characteristic behaviours did not appear for the aqueous
solution of either component alone. The effect on this interaction of acetyl and pyruvate
groups in the xanthan side chain was also investigated. The mixture of xyloglucan and
acetate-free xanthan showed essentially the same interaction as that with native
xanthan, but the lack of a pyruvate group inhibits the interaction, indicating that the
pyruvate group substituted at the terminal mannose of the xanthan side chain plays a
key role. The linewidth from proton NMR showed a drastic restriction in the xanthan
side chain, not observed even in the helix state of xanthan alone. This suggests that, in
the mixture, the xanthan side chain adopts a new state different from either the coil or
ordered form.

-

Gels from other polysaccharide mixtures
Dea et al.( 1977 ) reported that only a small amount of locust bean gumwas suf

cient to
cause a low concentration of agarose to form a cohesive gel, while a higher concen-
tration of KGM was required. In a later paper, Dea ( 1981 ) reported that KGM induced
agarose to form a thermoreversible gel at an otherwise non-gelling concentration. He
pointed out that the temperature dependence of the optical rotation for KGM - agarose

mixtures differed signi locust bean gum mixtures,

which he attributed to the difference of the backbone conformations of KGM and
galactomannan.
Ridout et al.( 1998 ) used rheometry to establish interactions between KGM and the
bacterial polysaccharide acetan. They found that there was an enhancement of viscos-
ity in dilute solution, whereas at higher concentrations gelation occurred, even though
the interaction between KGM and acetan was weaker than that of LBG

cantly from that of gelling agarose -

acetan.
Deacetylation of acetan particularly enhanced the interaction between KGM and
acetan. They also used X-ray

-

fibre diffraction to study the interaction. Figure 10.19

shows the X-ray fibre diffraction pattern of a deacetylated acetan - KGMmixed gel. The

new structure corresponding to firmly estab-

lished the existence of intermolecular binding between these two polysaccharides. The
deacetylated acetan

' heterotypic ' junction zones in the gels

KGM mixture X-ray diffraction pattern resembles that of acetan
but the junction zones appear to be six-fold helices, whereas acetan alone forms

-

ve-
fold helices. Chandrasekaran et al.( 2003 ) extended this work and proposed a molec-
ular model for binary

' '

gelation systems. The model revealed that a double helix
in which one strand is acetan and the other is glucomannan is stereochemically feasible.
The authors suggested that this molecular model could be generalized to the other
binary systems.

weak