Biomedical Engineering Reference
In-Depth Information
H
H
O
NC
N
CH
CH
3
(CH
2
)
17
C
OCH
3
O
(CH
2
)
4
N
HO
C
(CH
2
)
10
CH
3
40
CH
3
(CH
2
)
n
-1
X
H
CN
O
N
(CH
2
)
10
CH
3
C
42:
n = 4, X = none
43:
n = 7, X = O
N
C
(CH
2
)
10
CH
3
O
H
CN
CH
3
(CH
2
)
4
41
O(CH
2
)
9
CH
3
44
O(CH
2
)
9
CH
3
13
1.3
Stimulation of Gelation by Perturbations Other Than Temperature
1.3.1
Enzymatic
In situ
Formation of Gelators and Gels - Potential Biological Applications
A promising and recently expanding method of inducing gel formation utilizes
enzyme-mediated biochemical modification to convert non-gelating materials into
gels. Such an approach can take advantage of the high degree of selectivity
offered by biology that is rarely (if ever) matched by non-biological processes.
Many different types of enzymes have been utilized in this arena, including
phosphatases, kinases, proteases,
-lactamases, and esterases. The coupling of
fiber self-assembly/disassembly to biologically relevant molecules points toward a
broad range of potential biomedical applications including targeted drug delivery,
wound healing, biosensing, tissue growth, and sequestration of toxins. Several
recent reviews on this topic are available [100-102].
Figure 1.28 outlines two general approaches, each converting non-gelling species
into gelators, either through enzyme-mediated bond cleavage or bond formation.
In the first approach (pioneered by the Xu group [103]), an enzyme is used to
cleave a solubilizing group from a pre-gelator, thus converting it into a less soluble
derivative and inducing self-assembly. Alternatively, a gelator can be produced
in
situ
via enzyme-catalyzed bond formation between two soluble precursors.
The earliest report of
in situ
enzyme-mediated supramolecular gelation was
in 2004, when the Xu group reported the use of an alkaline phosphatase to
dephosphorylate an Fmoc-protected tyrosine derivative,
45
(Figure 1.29) [103a].
β
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