Chemistry Reference
In-Depth Information
Ionic bonds are weaker than covalent (40-400 kJ/mol) [
47
], and therefore im-
mobilisation results in less deformation to the enzyme. As a result, activity loss is
typically less than that for covalent binding. In some cases, the process is also eas-
ily reversible under modified conditions, therefore allowing supports to be reused
if necessary [
2
].
Due to the weaker bonds, however, leaching of enzyme from the support can
be problematic, and this can lead to operational complexities (especially in highly
ionic solutions), as well as reduced enzyme reuse potential. The weaker bonds
also result in less stability than covalently bonded systems, and this therefore re-
sults in ionically bonded systems being less effective at more moderate processing
conditions.
With physical adsorption, the enzyme is physically adsorbed onto the surface of
the support through interactions such as van der Waals forces, hydrogen bonding
and hydrophobic interactions. Which interactions are involved largely depends on
the enzyme's conformation, as well as the support used. This method is viewed as
a relatively simple route, since no chemical changes to the enzyme or support are
necessary [
40
].
Supports used for covalent and ionic binding, which have strong adsorption ca-
pacities suitable for the enzyme in question, can be used. A key advantage of physi-
cal adsorption is that the supports generally require very little preparation. Resins,
polyacrylates, polypropylene, polystyrene, metal oxides, activated carbon and sili-
cas are common examples of suitable supports for a range of enzymes including
lipase, amidase and amylase. Celite (diatomaceous earth) is popular for lipase in
particular [
41
,
48
,
55
-
57
].
The physical interactions are the weakest of the attachments methods
(4-40 kJ/mol) [
47
], and as a result, this method typically results in least enzyme
deformation, thus high activities are maintained. As a consequence, a drawback of
these weak interactions is that as with ionic binding, leaching can be problematic,
especially since the interactions are easily reversed. This can therefore greatly re-
strict reuse potential of the enzyme. On the other hand, the supports can be easily re-
generated by removing the spent enzyme. Again, a lack of stability is also a problem
at more moderate processing conditions [
39
]. Due to its weaknesses, this method
is often combined with cross-linking methods which are subsequently mentioned.
Lipase has been successfully immobilised via ionic binding to PEI coated aga-
rose, with very high levels of activity being maintained (90 %). When immobilised
under optimal conditions, the immobilised material retained its optimal perfor-
mance, even when used in non-optimal conditions [
41
,
58
].
Physical adsorption of lipase onto a celite support has been shown to result in
very good product stability, with only negligible leaching. The preparation is also
simple, with the enzyme simply being precipitated with celite powder. Additives
such as sugar or albumin may be added to improve stability further [
41
,
59
].
Polystyrene resins have also been demonstrated as suitable supports for lipase
immobilisation via physical adsorption. Polystyrene resins are widely used to im-
mobilise lipase, due to their simple preparation and high adsorption capacity. CALB
also has a high affinity for the polystyrene, resulting in rapid immobilisation. Lipase
