Chapter 15
Prebiotic Organic Synthesis in Neutral Planetary Atmospheres 1
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H. J. Cleaves , J. H. Chalmers , A. Lazcano , S. L. Miller , and J. L. Bada
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Scripps Institution of Oceanography, University of California at San Diego, La Jolla,CA92093-0212 Facultad de Ciencias, UNAM, 04510 Mexico D.F., Mexico Department of Chemistry and Biochemistry, University of California at San Diego, La Jolla,CA92093-0314 *Corresponding author:
[email protected] 2
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It is now widely held that the early Earth's atmosphere was likely neutral, dominated by N andCO .The synthesis of organic compounds by the action of electric discharges on neutral gas mixtures has been shown to be much less efficient than with reducing gas mixtures. We show here that contrary to previous findings, significant amounts of amino acids are produced under these conditions. The low yields found previously were likely the result of oxidation of the organic compounds during hydrolytic workup by nitrite and nitrate produced in the reactions. Addition of oxidation inhibitors prior to hydrolysis results in the recovery of several hundred times more amino acids than reported previously. Organic synthesis from neutral atmospheres may thus have depended as much on oceanic conditions as on the characteristics of the primitive atmosphere itself. These findings imply the need for a critical re-evaluation of the importance of such syntheses on the primitive Earth and other planetary bodies that, like Mars, may have been endowed with CO and N -rich atmospheres throughout most of their history. 2
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© 2008 American Chemical Society In Chemical Evolution across Space & Time; Zaikowski, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.
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Introduction Today, organic compounds are so pervasive on the Earth's surface that it is hard to imagine the Earth devoid of organic material. However, during the period immediately after the Earth first formed some 4.5 Ga (billion years ago), there would have likely been no reservoir of organic compounds present. This was because soon after accretion, the decay of radioactive elements heated the interior of the young Earth to the melting point of rocks. Volcanic eruptions expelled molten rock and hot scorching gases out of the juvenile Earth's interior creating a global inferno. In addition, the early Earth was also being peppered by mountain-sized planetesimals, the debris left over after the accretion of the planets. Massive volcanic convulsions, coupled with the intense bombardment from space, generated surface temperatures so hot that the Earth at this point could very well have had an "ocean" of molten rock, i. e., a "magma ocean". Although temperatures would have slowly decreased as the infall of objects from space and the intensity of volcanic eruptions declined, elevated temperatures likely persisted for perhaps a hundred million years or longer after the formation of the Earth. During this period, temperatures would have probably been too hot for organic compounds to survive. Without organic compounds, life as we know could not have existed. However, based on data from ancient zircons, by approximately 4 Ga (or perhaps even earlier) the Earth's surface must have cooled down to the point that liquid water could exist and global oceans began to form (/). It was during this period that organic compounds would have first started to accumulate on the Earth's surface, as long as there were abiotic processes by which they could be synthesized or delivered intact to the Earth. This in turn would have marked an important transition in the chemistry of the early Earth since a reservoir of organic compounds is considered to be necessary for the origin of life. At the turn of the 20th century, some scientists began to seriously tackle the seemingly intractable problem of how life started on Earth and where the necessary organic compounds may have come from. In the 1920s, publications of Oparin and Haldane (2), as well as others suggested that life on Earth arose from an abiotic mixture of organic compounds produced by natural geochemical reactions. Based on an evolutionary analysis of metabolism and the then fledgling knowledge of the primitive Earth and planetary atmospheres, a detailed stepwise sequence of the events leading from the synthesis and accumulation of organic compounds to primordial life-forms whose maintenance and reproduction depended on external sources of reduced carbon. According to the modern version of the prebiotic soup theory for the origin of life (5) organic compounds derived from "home grown" chemical synthetic reactions directly on the Earth and the infall of organic rich material from space accumulated in the primordial oceans. These compounds then underwent further reactions in the primal broth producing ones with increasing molecular complexity. Some of these reactions took place at interfaces of mineral deposits
In Chemical Evolution across Space & Time; Zaikowski, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.
284 with primitive ocean water, while others occurred when the primitive ocean constituents were concentrated by various mechanisms such as evaporation in shallow water regions or the formation of eutectic brines produced during the freezing of parts of the oceans.
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Organic Compounds from Space Exogenous delivery of organic matter by asteroids, comets and interplanetary dust particles (IDPs) could have played a significant role in seeding the early Earth with the compounds considered to be necessary for the origin of life (4). This conclusion was based on knowledge of the organic composition of meteorites. Carbonaceous chondrites, a class of stony meteorites, have a high abundance of organic carbon, more than three percent by weight in some cases. Many of these types of meteorites have been extensively analyzed for organic compounds. The organic matter is dominated by a hot water insolublefraction.Although present in lesser amounts, the hot water soluble organic matter has been found to consist of polycyclic aromatic hydrocarbons (PAHs), aliphatic hydrocarbons, carboxylic acids, fiillerenes and amino/hydroxy acids (J). The purines adenine, guanine, xanthine and hypoxanthine have also been detected, as well as the pyrimidine uracil in concentrations of200 to 500 parts per billion (ppb). The importance of exogenous delivery of organic matter to the early Earth is critically dependent on the survivability of organic compounds during the delivery process. It is presently unclear exactly how much organic material would escape destruction during asteroid, comet and interplanetary dust particle infall to the Earth's surface.
"Home-Grown" Prebiotic Syntheses It was not until the advent of organic chemical syntheses that processes could be studied that may have been involved in the direct synthesis of organic compounds on the early Earth. Friedrich Wôhler's report in 1828 on the synthesis of urea from silver cyanate and ammonium chloride represented thefirstsynthesis of an organic compoundfrominorganic starting materials (J). Although it was not immediately recognized as such, a new era in chemical research had been begun: in 1850, Adolph Strecker achieved the laboratory synthesis of alanine from a mixture of acetaldehyde, ammonia and hydrogen cyanide (J). This was followed by the experiments of Alexandr M . Butlerov (3) showing that the treatment of formaldehyde with strong alkaline catalysts, such as sodium hydroxide (NaOH), leads to the synthesis of sugars. The laboratory synthesis of biochemical compounds was soon extended to include more complex experimental settings. By the end of the 19th century a large
In Chemical Evolution across Space & Time; Zaikowski, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.
285 amount of research on organic synthesis had been performed, and had led to the abiotic formation of fatty acids and sugars using electric discharges with various gas mixtures (5). This work was continued into the 20th century by Klages (5) and Ling and Nanji (5), who reported the formation of glycine from formaldehyde and potassium cyanide. In addition, Lob, Baudish, and others worked on the synthesis of amino acids by exposing wet formamide (CHONH ) to a silent electrical discharge (3) and to UV light (5). There is no indication that any of these researchers had any interest in the question of how life began on Earth, or in the synthesis of organic compounds under possible prebiotic conditions. This is not surprising. Since it was generally assumed that that the first living beings had been autotrophic, plant-like organisms, the abiotic synthesis of organic compounds did not appear to be a necessary prerequisite for the emergence of life. These organic syntheses were not conceived as laboratory simulations of Darwin's warm little pond, but rather as attempts to understand the autotrophic mechanisms of nitrogen assimilation and C 0 fixation in green plants. The situation changed at the start of the 1950s when several noted chemists were drawn towards the origin of life. Driven by his interest in evolutionaiy biology, Melvin Calvin attempted to simulate the synthesis of organic compounds under primitive Earth conditions using high-energy radiation sources. He and his group had limited success: the irradiation of solutions of C 0 with the Crocker Laboratory 60inch cyclotron led only to formic acid, albeit in fairly good yields (5). The biggest breakthrough took place in 1953 when Stanley Miller, working in the laboratory of Harold Urey,firstpresented convincing evidence for the synthesis of important biochemical compounds under what was considered at the time plausible early Earth conditions. In this now classic experiment, Miller using a gas mixture of H , C H and NH and a spark discharge as an energy source (to mimic lightning and coronal discharges on the early Earth) was able to demonstrate the transformation of almost 50% of the original carbon present as methane into a wide variety of organic compounds (
