Miller-Urey reaction

It appears that conditions in the solar nebula were appropriate for the FTT but not the Miller-Urey reaction. Kinetic calculations (Lewis and Prinn, 1980) as well as observations on comets (Delsemme, 1977) show that CO and COj, not CH, were the principal forms of carbon. And the dust-laden solar nebula was opaque to UV, precluding any photochemical reactions. It seems best, however, to approach the problem empirically, by examining the meteoritic organic compounds themselves for clues to their formation. We shall review these compounds class by class, looking for the signatures of the FTT or Miller-Urey reactions. 

Actually, much of the experimental work on chemical evolution (cf. Lemmon, 19701 utilizes such unstable compoumls, e.g. HCN, HCHO, HC=CCN, H2NCN, etc., on the grounds that they can be made by Miller-Urey reactions. But they can also be made by spontaneous reactions of CO, NH3, and Hj (Anders et al., 1974). Hence this class of reactions provides some common ground between the two main types of abiotic synthesis. 

This resemblance is highly significant if one considers that 10,359 structural isomers exist for saturated hydrocarbons with 16 C atoms (Lederberg, 1972). Apparently the meteoritic hydrocarbons were made by FTT reactions, or some other process of the same extraordinary selectivity. The Miller-Urey reaction, incidentally, shows no such selectivity. Gas chromatograms of hydrocarbons made by electric discharges in methane show no structure whatsoever in the region around Cjg (Ponnamperuma et al., 1969). Apparently all 10 possible isomers are made in comparable yield, as expected for random recombination of free radicals. 

The Miller-Urey reaction fails qualitatively rather than quantitatively. It produces no detectable normal acids above Cg, even in the presence of an alkaline aqueous phase that is known to favor growth of linear chains by formation of a monolayer (Allen and Ponnamperuma, 1967). Given the fundamentally random nature of the Miller-Urey reaction, there is little hope that it will ever achieve the needed selectivity for normal isomers. 

The Miller-Urey reaction has been notably less successful in producing N-hetero-cyclics. Only adenine has thus far been made, by electron irradiation of CH, NHj, HjO, and Hj (Ponnamperuma et al., 1963). The yield was only 0.01 %. Better success was achieved by reactions involving unstable reactants, such as HCN (Lemmon, 1970), but the reactions, being spontaneous, actually are related no less to FTT than to Miller-Urey reactions (Sec. 3.3). 

The Miller-Urey reaction has been quite successful in duplicating these results. All 20 amino acids identified in meteorites, and 12 others, were produced by electric discharges on CH4-NH3-H2O-H2 mixtures, in the presence of an aqueous phase (Ring et al., 1972 Wolman et al., 1972). Even the proportions of the various amino acids resemble those in Murchison to within 1-2 orders of magnitude. 

Polymeric materials also form in Miller-Urey reactions, by both spark discharge (Miller, 1955) and UV irradiation (Sagan and Khare, 1979). These materials have not been studied in detail, but the elemental" analyses show high N contents (36% and 11%, vs. 2.4% for Murchison and 1.23% for FTT polymer). The H/C ratios also are higher (1.28 and 1.23, vs. 0.70 and 0.78), suggesting a predominantly aliphatic and/or alicyclic, rather than aromatic, structure. 

No attempt has yet been reported to produce carbynes by the Miller-Urey reaction. This should not be held against it, since carbynes have only very recently been discovered in meteorites. At least acetylene and some of its simpler derivatives have been made in the Miller-Urey synthesis (Friedman et al., 1971). 

The Miller-Urey reaction gives a fractionation of only —0.4 + 0.2%o (Lancet, 1972). 

On the other hand, one cannot rule out the possibility that D-rich, ionic species formed at some stage in the solar nebula, and reacted with previously-produced, polymeric material. This is essentially a Miller-Urey reaction with a built-in, isotopic tracer. Perhaps these two alternatives can be distinguished by isotopic analysis of carefully separated fractions of the organic material. 

More recently, Lewis and Prinn (1980) have systematically examined the reduction kinetics of CO and Nj in the solar nebula. Taking into account gas-phase reactions as well as surface-catalyzed reactions (for the rather inefficient catalysts pr nt above 400 K), they conclude that reaction rates were so slow relative to the rates of radial mixing or nebular evolution that no more than 1 % of the and CO would have been reduced to NHj and CH over the lifetime of the nebula. Methane, the starting material of the Miller-Urey reaction, apparently was only a minor constituent of the solar nebula. 

A few facts about the Miller-Urey experiments the now famous original apparatus was modified and improved by Miller himself, and by other groups, in order to improve product yields. In the reaction vessel, temperatures near the reaction zone were between 350 and 370 K, but as high as 870-920 K at the centre of the reaction. Experiments took between several hours and a whole week. The main products (starting with the highest yields) were formic acid, glycine, lactic acid and... 

These differences may reflect mainly the effort expended on the two methods, rather than their intrinsic merits. The FTT work was done before the meteorite results became available, and so many of the non-protein amino acids simply were not looked for. Also, both syntheses persumably involve the same two steps (Miller et al., 1976) formation of unstable intermediates at high T, and rapid quenching and hydrolysis of the reaction products. The standard Miller-Urey flask, with its small spark zone and large liquid phase, is an optimal configuration for this purpose, in contrast to the FTT flask, where the hot and cold zones are in reverse ratio. If the intermediates and reaction paths indeed are similar (Miller et al., 1976), then it should be possible to improve yields in the FTT synthesis merely by changing the configuration of the apparatus, to provide a larger cold zone and faster quench. 

It appears that FTT reactions can account reasonably well for most features of organic matter in meteorites. The only alternative process, the Miller-Urey synthesis, fails to account for the aliphatic and aromatic hydrocarbons, nitrogen heterocyclics, many oxygen compounds, the polymer, and carbon isotope fractionations, though it remains an alternative and perhaps superior source of amino acids and may, in an extended sense, be responsible for the deuterium enrichments. 

Ion-molecule reactions in interstellar clouds Radiation chemistry in interstellar grain mantles Condensation in stellar outflows Equilibrium reactions in the solar nebula Surface catalysis (Fischer-Tropsch) in the solar nebula Kinetically controlled reactions in the solar nebula Radiation chemistry (Miller-Urey) in the nebula Photochemistry in nebular surface regions Liquid-phase reactions on parent asteroid Surface catalysis (Fischer-Tropsch) on asteroid Radiation chemistry (Miller-Urey) in asteroid atmosphere... 

H FIGURE 1.4 An example of the Miller-Urey experiment. Water is heated in a closed system that also contains CH, NH, and An electric discharge is passed through the mixture of gases to simulate lighming. After the reaction has been allowed to take place for several days, organic molecules such as formaldehyde (HCHO) and hydrogen cyanide (HCN) accumulate. Amino acids are also frequently encountered as products of such reactions. 

Amino acid formation in the Urey-Miller experiment and almost certainly in the prebiotic environment is via the Stecker synthesis shown in Figure 8.3. This reaction mechanism shows that the amino acids were not formed in the discharge itself but by reactions in the condensed water reservoir. Both HCN and HCO are formed from the bond-breaking reactions of N2 and H2O in a plasma, which then react with NH3 in solution. The C=0 group in formaldehyde or other aldehydes is replaced by to form NH and this undergoes a reaction with HCN to form the cyano amino compound that hydrates to the acid. The Strecker synthesis does not provide stereo-control over the carbon centre and must result in racemic mixtures of amino acids. There is no room for homochirality in this pathway. 

Endogenous organic synthesis Urey-Miller experiments as a source of prebiotic molecules via the Strecker synthesis for amino acids, HCN polymerisation for purines and pyrimidines and the formose reaction for sugars... 

Other pathways are in fact possible, and under this heading the Urey-Miller-type (UMT) synthesis is frequently mentioned. In the UMT synthesis, the initial molecules are highly reduced (CH4, H20, NH3). In the case of hydrocarbons, the reaction is given by... 

Both pathways probably involved quite different conditions, the main difference being the absence of liquid water in the interstellar medium. Nevertheless the basic building blocks and chemical reactions should have been roughly similar, thus leading to important connections between these two routes. While a wide variety of amino acids are prebiotically relevant (as attested by either Urey-Miller experiments or meteorite analysis), we shall focus in this section on a-amino acids (as the most relevant to biochemistry) and closely related compounds. 

Hydrogen is discussed here in some detail, because Miller and Urey (1959) suggested that its presence in the primitive atmosphere led to the formation of methane on account of the reaction... 

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