Biomedical Engineering Reference
In-Depth Information
be applied: Crude guar gum (10 g) is treated with 200 mL boiling, aqueous 80%
ethanol for 10 min. The obtained slurry is collected on a glass fi lter (no. 3) and
washed successively with ethanol, acetone, and diethyl ether. The solid is added
to 1 l of water, homogenized with a blender, and centrifuged at 2300 g for 15 min.
The supernatant is precipitated in two volumes of cold acetone, redissolved in hot
water, and ultracentrifuged at 82,000 g for 1.5 h at room temperature. Finally, the
supernatant is precipitated with two volumes of ethanol and precipitate is collected
on a glass fi lter (no. 4), washed with ethanol, acetone, and diethyl ether before
freeze - drying [125] .
Three kinds of bonds in guar gum are susceptible to enzymatic hydrolysis: the
endo - and exo -
- 1,6 linkage
between the mannose backbone and the galactose side chain. The enzymes that
cleave these bonds are, respectively, endo-
β
-1,4 linkages on the d - mannose backbone and the
α
and
exo -
β
- mannanase
and
α
galactosidase [126] . The 1,4 -
β
- d - mannosidic linkages in galactomannans can be
hydrolyzed by
- d - mannanases, a class of enzymes which is produced by plants,
bacteria, and fungi. The effi ciency of this reaction depends both on the degree
of polymerization and galactose substitution levels [127]. Galactomannans with
a galactose content of up to 32% are hydrolyzed with no signifi cant change of
the
K
m
or relative
V
max
values. However, if the galactose content approaches 34-
38%, the
K
m
values doubles, and the relative
V
max
values decreases by 10- 20%.
Typically, 6% of the mannosidic linkages in guar gum are hydrolyzed [128]. Since
galactomannans as guar gum are used in hydraulic fracturing of oil and gas wells,
it is necessary to employ thermostable enzymes for the enzymatic degradation.
Commonly applied enzymatic breakers are mixtures of hemicellulases produced
by
Aspergillus niger
. This enzyme preparation is moderately thermostable with
temperature optima of approximately 65 °C and it has shown to be effective for
galactomannan hydrolysis and viscosity reduction. McChutchen
et al.
isolated and
characterized a
β
-mannanase produced by the hyperther-
mophilic bacterium
Thermotoga neapolitana
5068. The purifi ed
α
- galactosidase and a
β
- galactosidase
had a temperature optimum of 100 - 105 ° C with a half - life of 130 min at 90 ° C
and 3 min at 100 ° C. The purifi ed
α
-mannanase was found to have a tempera-
ture optimum of 91 °C with a half-life of 13 h at that temperature and 35 min at
100 ° C [129] .
To yield derivatives with adjusted material properties, guar gum can be car-
boxymethylated by the reaction with the sodium salt of monochloroacetic acid in
presence of sodium hydroxide. Using homogenous reaction conditions, various
degrees of substitution can be synthesized. Aqueous solutions of carboxymethyl-
ated guar gum have higher viscosities compared to unmodifi ed guar gum [130].
Another important derivative is hydroxypropyl guar gum, a hydrophobic polymer
obtained from the native biopolymer via an irreversible nucleophilic substitution,
using propylene oxide in the presence of an alkaline catalyst. When guar gum is
modifi ed to hydroxypropyl guar, the added hydroxypropyl groups sterically block
the hydrogen bonding sites on the guar backbone and reduce the hydrogen-
bonding attractions between guar molecules. In comparison to guar gum, this
derivative has an improved viscosity [131].
β
