Chemistry Reference
In-Depth Information
100
100
4% alginate
4% alginate
10
10
1
1
2% alginate
2% alginate
0.1
0.1
0.1
100
100
0.01
1
10
0.1
1
10
Shear rate (per second)
Angular frequency (rad per second)
(a)
(b)
Fig. 6.8
Shear-thinning behaviour of alginate, highlighting Cox-Merz superimposition
of steady-state viscosity and complex viscosity in 2 and 4% alginate solutions using
(a) rotational measurements and (b) dynamic oscillatory measurements.
complex dynamic viscosity can be considered analogous to steady-state
viscosity. This is highlighted by comparing Fig. 6.8a with b.
6.5
ENZYMATICALLY TAILORED ALGINATE
In order to fully exploit the potential of alginates, several research
groups have managed to engineer specific MG sequences and sub-
sequently tailor physical properties. As mentioned above, alginate is
biosynthesised initially as poly-β-D-mannuronate, from which some
of the residues are converted to α-L-guluronate by enzymatic epimeri-
sation at C(5) by mannuronan C(5) epimerases, producing polymers
with various sequences. Recent genome elucidation of the alginate-
producing bacterium
Azobacter vinelandii
has shown that the bacteria
produce seven different mannuronan epimerases to facilitate alginate
biosynthesis. These enzymes have subsequently been produced in
E. coli
using recombinant enzyme technology and are termed AlgE1-AlgE7.
Although these enzymes perform the same function (i.e. C(5) epimerisa-
tion of mannuronan), their collective action can produce alginates with
vastly different sequences. For example, both AlgE1 and AlgE6 produce
polyguluronate blocks. However, AlgE1 initially forms only long ho-
mopolymeric G-blocks
50 residues, while AlgE6 gives shorter blocks
with a broader block size distribution (Holtan
et al.
, 2006). Alg4, on
the other hand, converts mannuronate to guluronate in an alternating
manner (Hartmann
et al.
, 2006).
In a recent study by Hartmann (Hartmann
et al.
, 2006), alginate sam-
ples from different sources were modified using recombinant AlgE4,
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