Biology – Quantitative Biology
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..887s&link_type=abstract
European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a
Biology
Quantitative Biology
Scientific paper
The Prebiotic Hot-Volcanic-Coast Scenario It has been suggested that in the Hadean eon (4.5-3.8 Ga before present) no permanent continents but volcanic islands and short-lived protocontinents protruded from the first ocean [1, 2]. As the geothermal heat production was considerably higher than today, it is reasonable to assume that hot volcanic coasts were much more abundant. The salinity of the ocean was probably up to two times higher than the modern value [3]. Under these conditions, the evaporation of seawater at active volcanic coasts must have produced sea salt crusts - a process that can still be observed today [4]. On the hot lava rock, the salt crusts can subsequently experience temperatures up to some hundred degrees Celsius. The seawater probably contained abiotically formed organic molecules such as amino acids, which were inevitably embedded into the sea salt crusts. Different prebiotic sources of amino acids have been discussed: (i) comets and meteorites [5], electrical discharges in the atmosphere [6, 7], and deep-sea hydrothermal vents [8]. We undertook a systematic study of solid salt-amino acid mixtures, especially of their formation and thermal behavior under simulated conditions of the hotvolcanic- coast scenario. Laboratory Experiments Amino acids@salts Artificial Hadean seawater was prepared by dissolving NaCl (705 mmol), MgCl2 (80 mmol), KCl (15 mmol), CaCl2 (15 mmol), and an α-amino acid (5-10 mmol) or a mixture of α-amino acids. In order to model the first step of the hot-volcanic-coast scenario, the solutions were evaporated to dryness. Vibrational spectroscopy (IR, Raman) and X-ray powder diffraction showed that the resulting solid residues were not heterogeneous mixtures of salt and amino acid crystals. Instead the amino acid molecules were coordinated in calcium or magnesium complexes. We have studied the rac-alanine ( + H3NCH(CH3)COO -, Hala) system in more detail and found that the complex that is present in the mixture has the formula (CaCl2)3(Hala)2 · 6H2O. The coordination has a significant influence on the thermal behavior of the amino acids. It prevents them from subliming and forces them to undergo chemical reactions when heated. In fact, the hot-volcanic-coast scenario comprises not only the formation of solid sea salt-amino acid mixtures but also their subsequent exposure to higher temperatures. Chemical evolution I: from amino acids to pyrroles On heating to 350 °C in a slow stream of nitrogen gas, the solid salt-amino acid mixtures described above produced several methylated and ethylated pyrroles 1 (amino acid = rac-alanine, rac-valine, α-aminoisobutyric acid and rac-isovaline, respectively; see Fig. 1). The quantitative importance of this reaction in a prebiotic environment can be estimated as follows. During a volcanic eruption, more than 108 m3 of lava can enter the sea and evaporate the same volume of seawater [4]. Assuming that (i) the amino acid concentration in the upper ocean layer was 0.3 mmol/L [9], and (ii) only 10 mol-% of the amino acids formed pyrroles, we calculate from our experimental chemical yield that ca. 103 kg of pyrroles could have been formed in a single volcanic eruption. Thus, over long periods of time, large amounts of alkylpyrroles may have accumulated. Alkylpyrroles are thermally quite stable and easily evaporate at higher temperatures. Therefore they must have escaped from the very hot places of their formation and condensed at cooler ones. Chemical evolution II: oligomerization of pyrroles Another interesting aspect of the hot-volcanic-coast scenario is the formation of hydrogen chloride (HCl) when lava flows into the ocean and decomposes the sea salt component MgCl2 · 6H2O [4]. It is known that HCl catalyzes the reaction between pyrrole and aldehydes, for example formaldehyde, in water. Formaldehyde is regarded as a prebiotic molecule [10]. We therefore studied the reaction of kryptopyrrole 2 with formaldehyde in HCl-containing artificial seawater (salt concentration ca. 4 %; Fig. 1). After one hour of reflux, a water insoluble dark green residue was iso- EPSC Abstracts, Vol. 3, EPSC2008-A-00081, 2008 European Planetary Science Congress, Author(s) 2008 lated and analyzed by gas chromatography-mass spectrometry. Comparison with an authentic sample proved that the dipyrromethene 3 had been formed. Conclusions We have, for the first time, demonstrated that conditions favorable for the transformation of amino acids into oligopyrroles may have been present on the young Earth. If the hot-volcanic-coast scenario is true, the first oligopyrrole-type photoreceptor and electrontransfer molecules were available quite early in Earth's history. Future experiments will focus on (i) the inclusion of additional relevant materials such as lava rock and prebiotic oxidation reagents for the formation of dipyrromethenes (e. g. nitrite and nitrate [7]), (ii) the formation of oligopyrroles larger than dipyrromethenes, and (iii) metal complexes of oligopyrroles. References [1] Martin, H., Albarède, F., Claeys, P., Gargaud, M., Marty, B., Morbidelli, A. and Pinti, D. L. (2006) Earth Moon Planets, 98, 97-151. [2] Russell, M. J. and Arndt, N. T. (2005) Biogeosciences, 2, 97-111. [3] Knauth, L. P. (1998) Nature, 395, 554-555. [4] Edmonds, M. and Gerlach, T. M. (2006) Earth Planet. Sci. Lett., 244, 83-96. [5] Pizzarello, S. (2004) Orig. Life Evol. Biosph., 34, 25-34. [6] Plankensteiner, K., Reiner, H. and Rode, B. M. (2006) Mol. Diversity, 10, 3-7. [7] Cleaves, H. J., Chalmers, J. H., Lazcano, A., Miller, S. L. and Bada, J. L. (2008) Orig. Life Evol. Biosph., 38, 105-115. [8] Huber, C. and Wächtershäuser, G. (2006) Science, 314, 630-632. [9] Miller, S. L. (1987) in Cold Spring Harbor Symposia on Quantitative Biology, Vol. 52, Cold Spring Harbor Laboratory, Cold Spring Harbor, 17-27. [10] Blair, S. K., Magnani, L., Brand, J., Wouterloot, J. G. (2008) Astrobiology, 8, 59-73.
Fox Sean
Strasdeit H.
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