HO OH OH OH
HO OH OH
Figure 3. Models for biosilicification. Ethanol entombed in silica gel (inside, left), or reacted to make an organo-silicate (inside, right).
preservation of sugars, alcohols, and other bio-organic materials that have an OH group. These two mechanisms are shown in Fig. 3, using ethanol as an example.
The experimental support for entombment comes from the work by Coradin and co-workers (Coradin and Livage, 2001; Coradin et al., 2002). These authors studied the interaction of amino acids and peptides with sodium silicate. They have found that various amino acids and especially peptides promote polymerization of silicic acid to give silica gels. The experiments were performed with a dilute solution of sodium silicate at various pH values in the presence of appropriate buffers. The surface of these gels was studied via SEM (scanning electron microscopy), and the bulk of the gels was investigated by the IR spectroscopy. The IR bands in the regions for the amide, carboxylate and alkyl groups were observed, but the Si-O-C bond was not identified. These results are compatible with the entombment mechanism.
The most recent support for the second mechanism, which is relevant to astrobiology, came from studies of Lambert and co-workers (Lambert et al., 2004) and Kastele and co-workers (Kastele et al., 2005). These studies show in an unambiguous manner that covalent Si-O-C bonds form between sugars, sugar alcohols, or nucleosides and silicates. Si-O-C bond formation was proposed to stabilize sugars and to preserve them during transport in space and on meteorites (Lambert et al., 2004). The experimental demonstration of the Si-O-C bonds was made via Si-29 NMR (nuclear magnetic resonance) spectroscopy (Lambert et al., 2004; Kastele et al., 2005) and X-ray crystallographic studies (Kastele et al., 2005). Unfortunately, Si-29 NMR and X-ray diffraction instruments have not yet been miniaturized and made robotic, and are thus not feasible for an in situ search for Si-O-C bonds on Mars.
Our experimental models include the interaction between sodium silicate and the following groups of organic compounds: Amino acids, biological and meteoritic; Maillard products (Kolb and Liesch, 2006); Metal-Maillard complexes (Liesch and Kolb, 2007a); sugars, alcohols and amino alcohols (Liesch and Kolb, 2007a); acid halides; and biomolecules, such as urea, AMP, ATP, and DNA (Kolb and Liesch, 2007a). With the exception of acid halides, which we have studied as a model for very reactive molecules, all other compounds are of biological and astrobiological importance.
We became interested in developing a variation of the model of Coradin and co-workers (Coradin and Livage, 2001; Coradin et al., 2002). They used dilute sodium silicate and buffers, in order to mimic biogenic media of diatoms. In our model we use concentrated sodium silicate solution without buffers. Our model would be more applicable to basic niches on Earth, such as soda lakes. Upon the addition of the various organic compounds mentioned above, the sodium silicate solution polymerized into a viscous gel in nearly every case. In some cases, such as with the amino acids, polymerization similar to that described by Coradin and co-workers (Coradin and Livage, 2001; Coradin et al., 2002) had been observed. In other cases, the interaction with silicates had been hinted at (Azrak and Angell, 1973; Sweryda-Krawiec et al., 1999) but our studies led to the discovery of novel physical and spectral properties of these polymerized gels (Liesch and Kolb, 2007a). The various organic compounds we have investigated interact with sodium silicate solution in a variety of different ways and possess unique characteristics in terms of physical appearance, sol-gel-sol transformations, and the properties of their IR spectra. We also set as a goal to identify the Si-O-C band in the IR spectra, and had some literature guidance on this subject (Hino and Sato, 1971; Bellamy, 1975; Benning et al., 2004).
4. Results and Discussion
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