Chemistry Reference
In-Depth Information
Chapter 3
Protease-Mediated Hydrolysis and
Condensation of Tetra- and Trialkoxysilanes
Mark B. Frampton and Paul M. Zelisko
3.1
Introduction
Many marine organisms and terrestrial plants have evolved molecular machinery
to process silicic acid or sodium silicate from their local environments to form
elaborate shells, frustules, [
1
-
3
] spicules, [
4
] or other structural elements [
5
-
7
].
Organisms typically process silica under ambient temperatures, standard pressure,
and near neutral pH, where the external concentration of silica is generally in the
micromolar range. The ability of these organisms to form elaborate structures only
serves to highlight the important mechanisms that these organisms have evolved to
process silicon-based compounds.
3.1.1
Biosilica Synthesis
The synthesis of silica-derived materials usually requires extremes of temperature,
pressure and pH, while diatoms and marine sponges have evolved biochemical pro-
cesses to direct the templating of inorganic silica. In diatoms there have been two
major silica precipitating proteins that have been identified, the silaffins and sila-
cidins. The silaffins (silaffin-1A
1
, 1A
2
, 1B and 2) so named for their
sil
ica
affin
ity,
have been studied at the molecular level and their primary amino acid sequence
determined [
1
,
2
]. The amino acid sequence of silaffin−1A
1
and 1A
2
indicate a high
serine and lysine content. Many of the serine residues are phosphorylated and the
lysine residues are almost universally found with post-translational modifications
(Fig.
3.1
) [
2
,
8
].
