Prebiotic Synthesis of Hydrophobic and Protein Amino Acids

Abstract

The formation of amino acids by the action of electric discharges on a mixture of methane, nitrogen, and water with traces of ammonia was studied in detail. The presence of glycine, alanine, a-amino-n-butyric acid, a-aminoisobutyric acid, valine, norvaline, isovaline, leucine, isoleucine, alloisoleucine, norleucine, proline, aspartic acid, glutamic acid, serine, threonine, allothreonine, a-hydroxy---aminobutyric acid, and a,-y-diaminobutyric acid was confirmed by ion-exchange chromatography and gas chromatography-mass spectrometry. All of the primary a-amino acids found in the Murchison Meteorite have been synthesized by this electric discharge experiment.Most prebiotic syntheses that start with the primitive atmospheric constituents give substantial yields of glycine, alanine, and a-amino-n-butyric acid (1). Prebiotic syntheses of the higher aliphatic amino acids have been claimed, for example, by the action of electric discharges on CH4 + NH3 + H20 (2-6), by heating CH4 + NH3 + H20 to 900-1200°(7, 8), and by the action of shock waves on CH4, C2H6, NH3, and H20 (9). The amino acids were identified only by an amino-acid analyzer (2-4, 6, 7, 9), only by paper electrophoresis (8), or only by gas chromatography (5). However, these techniques are not sufficient by themselves to identify an amino acid.In the original synthesis of amino acids by electric discharges (10-12), only glycine, alanine, a-amino-t-butyric acid, a-aminoisobutyric acid, and ,B-alanine, of the simple aliphatic amino acids, were synthesized in sufficient yield to obtain identification by a melting point of a derivative. Recently developed techniques permit the identification of compounds found in lower yield by this synthesis.The synthesis under prebiotic conditions of aspartic and glutamic acid (2-8, 12-14), serine (2, 5, 6-8, 13), threonine (2, 3, 5-7), and proline (3, 4, 7, 8, 13) have been reported but they have not been properly identified [except for aspartic acid (14) ]. The synthesis of these amino acids (except proline) has also been reported from the polymerization of HCN (1), but again without proper identification. A prebiotic synthesis of threonine should also yield allothreonine, but this amino acid has never been reported. In addition, several investigators have reported the appearance of a large peak at the isoleucine position on the amino-acid analyzer (2, 4-6, 13). The identification of this peak as isoleucine has been questioned (4, 13). It is evident that this compound cannot be isoleucine, since a corresponding peak for alloisoleucine is not observed.Most electric-discharge experiments have been done with a large amount of ammonia present. Use of nitrogen instead of ammonia in such experiments does not change the major products, although the yield of amino acids is lower (12).The use of a higher concentration of ammonia in such experiments has been criticized (15), and it is now thought that the ammonia concentration in the prebiotic atmosphere was not likely to have been greater than 10-5 atm (16,17). Although this i...

Nonprotein Amino Acids from Spark Discharges and Their Comparison with the Murchison Meteorite Amino Acids

“…These include glycine, alanine, sarcosine, f3-alanine, N-methylalanine, a-aminon-butyric acid, and a-aminoisobutyric acid. We have recently repeated the electric discharge experiment (8) and have demonstrated, by modern techniques, the presence of aspartic acid, glutamic acid, valine, isovaline, norvaline, and proline, all of which are found in the meteorite. We have also demonstrated the electric discharge synthesis of leucine, isoleucine, alloisoleucine, norleucine, serine, threonine, allothreonine, a-hydroxy--y-aminobutyric acid, and a,7y-diaminobutyric acid, which have not yet been found in the meteorite, although some of them may be present [e.g., the leucines (3) ].…”

Astrobiology and the Possibility of Life on Earth and Elsewhere…

“…Since Miller's initial publication, many laboratories have replicated and extended his results, exploring variations on the synthesis using other types of apparatus designs, gas mixtures and energy sources (Groth and Weyssenhoff 1957;Menor-Salvan et al 2009;Miller 1957aMiller , 1957bMiyakawa et al 2002aMiyakawa et al , 2002bMiyakawa et al , 2002cParker et al 2010Parker et al , 2011Ring et al 1972;Miller 1983a, 1983b), demonstrating the synthesis of a much wider variety of organic compounds. There is a consensus of the types of products obtained using similar gases, though there are important differences in the C and energy yields of various intermediate species such as HCHO and HCN (Miller and Schlesinger 1984) depending on the energy source and gas mixture used (Heinrich et al 2007), factors independent of surface conditions.…”

Conducting Miller-Urey Experiments

“…Gas chromatography-mass spectrometry (GC-MS) analysis of a CH 4 /NH 3 mixture subjected to an electric discharge showed the synthesis of α-aminonitriles, which are amino acid precursors 7 . In 1972, using a simpler apparatus, first introduced by Oró 8 ( Figure 1B), Miller and colleagues demonstrated the synthesis of all of the coded α-amino acids 9 and nonprotein amino acids 10 that had been identified in the Murchison meteorite to date, by subjecting CH 4 , N 2 , and small amounts of NH 3 to an electric discharge. Later, using this same simplified experimental design, gas mixtures containing H 2 O, N 2 , and CH 4 , CO 2 , or CO were sparked to study the yield of hydrogen cyanide, formaldehyde, and amino acids as a function of the oxidation state of atmospheric carbon species 11 .…”

“…The method described here has been tailored to instruct researchers how to conduct spark discharge experiments that resemble the Miller-Urey experiment using a simplified 3 L reaction vessel, as described in Miller's 1972 publications 9,10 . Since this experiment involves a high voltage electric arc acting on inflammable gases, it is crucial to remove O 2 from the reaction flask to eliminate the risk of explosion, which can occur upon combustion of reduced carbon-bearing gases such as methane or carbon monoxide, or reaction of H 2 with oxygen.…”