Chemistry Reference
In-Depth Information
1
1
µ
µ
m
m
1
1
µ
µ
m
m
Figure 25
. A generalized structure of the polyamines and scanning electron microscope images of silica
structures formed in the presence of low molecular weight polyamines (left hand side) and higher
molecular weight polyamines (right hand side). From Kröger et al. 2000 and used with permission of
the PNAS.
particles that have defined shape and size (e.g., Perry 1989; Perry and Fraser 1991;
Wetherbee et al. 2000) and in some instances, degree of surface hydration (Perry et al.
1990). Fundamental particles are of the order of 3
5 nm in diameter and for diatoms,
spherical objects of the order of ca. 40 nm (exact size ranges are species-specific) have
been detected during the formation of the diatom cell wall by scanning electron microscopy
and can still be identified in mature walls using atomic force microscopy. Figure 11 shows
examples of silica particles visible by these two different methods. For sponges,
precipitation occurs around a proteinaceous filament within a membrane bounded
compartment and for diatoms, precipitation again occurs in a membrane bounded
compartment in the presence of polypeptide/polyamine/proteins. Investigations of
silicification in higher plants have shown that here too, silica deposits are closely associated
with proteinaceous material (Harrison 1996; Perry and Keeling-Tucker 1998, 2000, 2003).
In some organisms, the precise location of isolated proteinaceous material has been
identified (see section on pleuralins) and a functional role for these molecules proposed
that is not directly involved with silica deposition. For all other biosilica proteinaceous
materials, the precise role that the isolates play in silicification is not known
in vivo
. For
this reason
, in vitro
studies of silicification have been performed using the biosilica
extracts in an attempt to understand the role of the isolated biomolecules.
In the model studies of silicification silicon sources have included alkoxysilanes
(Zhou et al. 1999; Cha et al. 1999; Perry and Keeling-Tucker 2000), functionalized
alkoxysilanes (Cha et al. 1999), metastable solutions of silicic acid (e.g., Kröger et al.
1999), solutions of sodium silicate (Swift and Wheeler 1992) and solutions of a
catecholato complex of silicon (Perry and Keeling-Tucker 1998, 2003
)
Experimental
−
