Amino acids play a unique and vital role in biological systems. They are the monomeric building blocks of polypeptides and proteins which can serve multitudes of different functions in the cell, ranging from catalysis to transport (Lodish et al., 2004). On Earth, 20 key "biological" amino acids (as well as a few lesser-known modified versions) are used to build the larger polypeptides and proteins (Lehninger et al., 2004). Various amino acids have been found on meteorites, such as the famous Murchison and Murray meteorites (Cronin and Chang, 1993; Cronin et al., 1995; Cronin, 1998).
Chemically, amino acids have fascinating, if not contradictory structures. The very name "amino acid" refers to the presence of both a basic "amino" group and an acidic "carboxyl" group. These compounds are often referred to as "zwitterions" (Lehninger et al., 2004). The etymology of the word strongly reflects the nature of these compounds, with the root word, "zwitter" meaning "hermaphrodite" in German (Zwitter definition, 2007). The presence of functional groups on the side chains of the amino acids, such as hydroxyl or amino groups, further increases the possibility of interaction with silicates.
Work by Coradin and co-workers (Coradin and Livage, 2001; Coradin et al., 2002) showed that polymerization of sodium silicate occurs upon the addition of amino acids and peptides. We have also observed and described similar polymerization with the amino acids in our model system (Kolb and Liesch, 2006). Upon the addition of the "biological" amino acids, the sodium silicate solution instantly polymerizes into a hard or rubbery whitish gel. After gel formation and a standing period, our gels were isolated, washed, pulverized, and dried as described in our recent paper (Kolb and Liesch, 2006). We ultimately obtained a whitish material consisting of small chunks and powder from these dried gels.
Upon standing (and before processing the samples), some dissolution of the gels was observed. We refer to this transition from soluble (initial sodium silicate) to solid gel and back to a soluble portion as the sol-gel-sol transformation. The sol-gel-sol transformation could feasibly play a key role in the transport and protection of organics in the astrobiological context - particularly Mars. Recent discoveries have indicated the presence of both water and silicates on Mars (NASA, 2007b). If organics such as amino acids had been in aqueous solution (sol) on Mars at some point, they could have been preserved within silicates (gel) and then released again when water became abundant once again (sol).
We have also studied meteoritic amino acids (those found on meteorites, but not in known proteins). Some of these acids, for example sarcosine, behaved very much like the biological amino acids, while some other, such as 5-aminovaleric acid, behaved entirely differently. The case of 5-aminovaleric acid was unique among the amino acids because this was the only one we tested that formed a water-soluble gel. It was not until we worked with some of the sugars and alcohols that water-soluble gels were seen again in our laboratory.
Our IR analysis of the amino acid gels indicated the general absence of organics except in a few cases (Kolb and Liesch, 2006). Overall, it appears that amino acids merely catalyze the polymerization of sodium silicate. The trace organics indicate that entombment may be occurring to some minimal extent. In addition, it is possible that the washing and pulverization of the samples may remove some entombed, water-soluble organic materials.
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