Chemistry Reference
In-Depth Information
Long chain polyamines (LCPs), isolated from diatoms function in tandem with
silaffins and silacidins to direct the synthesis of bio-silica. The addition of LCPs
to silicic acid solutions induces silica formation via an electrostatic interaction
between the amines and silanols of silicic acid [
9
]. Sumper proposed a phase sepa-
ration model to account for the action of LCPs in buffered solution [
10
,
11
].
The other major group of silica precipitating organisms are marine sponges
which possess three related enzymes, silicatein α, β and γ, which are required
for spicule formation [
4
,
12
,
13
]. Silicateins are structurally homologous to the
caspase and papain families of cysteine proteases but instead of a cysteine residue,
a serine is used as the active nucleophile making these enzymes homologous to the
serine family of hydrolases [
4
]. Silicatein has been shown to precipitate amorphous
silica from buffered solutions using tetraethoxysilane as a silica precursor [
13
].
Enzymes that have not been traditionally associated with processing silica or
silica precursors have been used to perform or enhance the rate of silica chemistry;
most notably, trypsin, [
14
-
16
] α-chymotrypsin, [
16
] lysozyme, [
17
,
18
] lipases, [
19
,
20
] bovine serum albumin [
17
] and carbonic anhydrase [
21
].
3.2
Enzyme-Mediated Hydrolysis and Condensation
of Alkoxysilanes
Building upon early work by the Morse group, we embarked on a research avenue
focusing on the interaction between enzymes and silicon based substrates with a
focus on finding enzymes that had a capacity for catalytic activity at silicon . Trypsin
and α-chymotrypsin were originally thought to be suitable candidates as catalysts
that could be used under mild conditions (i.e., near neutral pH, low ionic strength, and
low temperature and pressure) [
22
]. When either of the enzymes was charged with
TEOS, amorphous monolithic silica gels were produced. The hydrolysis of TEOS
by trypsin was generally slow, requiring 24-48 h. α-Chymotrypsin also showed the
capacity to produce amorphous silica that was macroscopically indistinguishable
from that produced by other modes of catalysis. After approximately 24 h a clear and
colorless, viscous, monophasic sol appeared. Prolonged ageing at room temperature
resulted in the formation of a hard silica monolith. To ensure that silica formation
was the result of an enzymatic process the active site of trypsin and α-chymotrypsin
were each blocked by a soybean inhibitor [
23
]. Prolonged reaction times did not
promote silica formation with the inhibited enzymes; in separate experiments, the
soybean inhibitor did not demonstrate any catalytic activity towards TEOS. Solid-
state
29
Si NMR spectra of enzyme-produced silica gels suggested that catalyst choice
had only a marginal effect on the extent of siloxane condensation (Fig.
3.2
).
We expanded the scope of our investigation to include other enzymes such as
pepsin (carboxypeptidase), lipase from
Candida rugosa
(serine hydrolase), human
serum albumin, bromelain (cysteine protease from pineapple) and papain (cyste-
ine protease from papaya). In addition, the number of alkoxysilanes screened was
expanded to include methyl-, ethyl-, allyl- and phenyl-trimethoxysilane, as well as
a select few bis(triethoxysilyl) alkanes (Fig.
3.3
).
