Organic syntheses from CH4−N2 atmospheres: Implications for Titan

Numerous experiments have already been performed, simulating the evolution of gaseous mixtures containing CH₄ when submitted to energy flux. From their results, it appears that a variety of organic compounds, including unsaturated hydrocarbons and nitriles such as HCN, can be synthesized into noticeable amounts from CH₄–N₂ mixtures. In particular, systematic studies of the influence of the composition of the mixture on the nature and amount of synthesized compounds show that organic volatile nitriles, and particularly cyanoacetylene and cyanogen, are formed only in media rich in nitrogen. Those nitriles have been identified very recently in the atmosphere of Titan, and thus, data from such laboratory experiments may provide important indirect information on the organic chemistry occuring at the periphery of this satellite of Saturn. However, during these experiments, there is a continuous formation and accumulation of molecular hydrogen, which does not occur in the atmosphere of Titan, because of H₂ escape. In order to reassess the data already available from this type of laboratory studies, experiments on CH₄–N₂ atmospheres, with and without H2 escape, have been recently performed. The influence of this parameter on the chemical evolution of the atmosphere and on the nature and relative quantities of organic compounds has been studied.After reviewing these experiments, implications of the obtained results on the organic chemistry at the periphery of Titan are discussed.Paper presented at the 6th College Park Colloquium, October 1981.     

Carbon and energy yields in prebiotic syntheses using atmospheres containing CH4, CO and CO2

Yields based on carbon are usually reported in prebiotic experiments, while energy yields (moles cal^–1) are more useful in estimating the yields of products that would have been obtained from the primitive atmosphere of the earth. Energy yields for the synthesis of HCN and H₂CO from a spark discharge were determined for various mixtures of CH₄, CO, CO₂, H₂, H₂O, N₂ and NH₃. The maximum yields of HCN and H₂CO from CH₄, CO, and CO₂ as carbon sources are about 4×10^–8 moles cal^–1.     

Protein modeling and molecular dynamics simulation of the two novel surfactant proteins SP-G and SP-H

The prospects of a control for a novel gallium nitride pseudo-halide vapor phase epitaxy (PHVPE) with HCN were thoroughly analyzed for hydrocarbons–NH₃–Ga gas phase on the basis of quantum chemical investigation with DFT (B3LYP, B3LYP with D3 empirical correction on dispersion interaction) and ab-initio (CASSCF, coupled clusters, and multireference configuration interaction including MRCI+Q) methods. The computational screening of reactions for different hydrocarbons (CH₄, C₂H₆, C₃H₈, C₂H₄, and C₂H₂) as readily available carbon precursors for HCN formation, potential chemical transport agents, and for controlled carbon doping of deposited GaN was carried out with the B3LYP method in conjunction with basis sets up to aug-cc-pVTZ. The gas phase intermediates for the reactions in the Ga-hydrocarbon systems were predicted at different theory levels. The located π-complexes Ga…C₂H₂ and Ga…C₂H₄ were studied to determine a probable catalytic activity in reactions with NH₃. A limited influence of the carbon-containing atmosphere was exhibited for the carbon doping of GaN crystal in the conventional GaN chemical vapor deposition (CVD) process with hydrocarbons injected in the gas phase. Our results provide a basis for experimental studies of GaN crystal growth with C₂H₄ and C₂H₂ as auxiliary carbon reagents for the Ga-NH₃ and Ga-C-NH₃ CVD systems and prerequisites for reactor design to enhance and control the PHVPE process through the HCN synthesis.     

Energy yields for hydrogen cyanide and formaldehyde syntheses: The hcn and amino acid concentrations in the primitive ocean

Prebiotic electric discharge and ultraviolet light experiments are usually reported in terms of carbon yields and involve a large input of energy to maximize yields. Experiments using lower energy inputs are more realistic prebiotic models and give energy yields which can be used to estimate the relative importance of the different energy sources on the primitive earth. Simulated prebiotic atmospheres containing either CH₄, CO or CO₂ with N₂, H₂O and variable amounts of H₂ were subjected to the spark from a high frequency Tesla coil. The energy yields for the synthesis of HCN and H₂CO were estimated. CH₄ mixtures give the highest yields of HCN while H₂CO is most efficiently produced with the CO mixtures. These results are a model for atmospheric corona discharges, which are more abundant than lightning and different in character. Preliminary experiments using artificial lightning are also reported. The energy yields from these experiments combined with the corona discharge available on the earth, allows a yearly production rate to be estimated. These are compared with other experiments and model calculations. From these production rates of HCN (e.g. 100 nmoles cm⁻² yr⁻¹) and the experimental hydrolysis rates, the steady state concentration in the primitive ocean can be calculated (e.g., 4 × 10⁻⁶ M at pH 8 and 0°). A steady state amino acid concentration of 3 × 10⁻⁴ M is estimated from the HCN production rate and the rate of decomposition of the amino acids by passage through the submarine vents.     

Formation of prebiochemical compounds in models of the primitive Earth's atmosphere

In order to understand the formation of organic compounds in the primitive atmosphere, the first steps of evolution in models of the primitive atmosphere were investigated. Mixtures containing C−H−N elements were subjected to a low pressure silent electric discharge for several seconds, and the resulting effluents were analysed mainly by gas chromatography, infrared spectrometry and chemical analysis. The formation of hydrocarbons (i.e. ethylene, acetylene, methylacetylene) and of nitrogen containing compounds (i.e. hydrogen cyanide, cyanogen, saturated nitriles, acrylonitrile, cyanoacetylene) is reported. The influence of the initial mixture composition on the amount of compounds formed was systematically studied. The nature of the nitrogen source (N₂ or NH₃) in the primitive atmosphere has a great influence on the amount and on the very nature of the synthesized products. It is shown that important precursors such as cyanogen and cyanoacetylene are formed only in very rich N₂ mediums. There results show the important role played by the nature of the primitive atmosphere in the determination of the chemical evolution pathways.     

Ion-molecule condensation reactions: A mechanism for organic synthesis in ionized reducing atmospheres

The CH₃ ⁺ ion, formed in ionized methane, undergoes consecutive eliminative condensation reactions with methane to form the carbonium ions C₂H₅ ⁺, i-C₃H₇ ⁺ and t-C₄H₉ ⁺. AtT<500°K, \(N_{CH_4 } \) ?10¹⁶ cm^?3 these ions react with NH₃ in competitive condensation-H⁺ transfer reactions, e.g. $$\begin{gathered} C_2 H_5 ^ + + NH_3 \xrightarrow{M} C_2 H_5 NH_3 ^ + \hfill \\ - - - \to NH_4 ^ + + C_2 H_4 \hfill \\ \end{gathered} $$ At particle densities of \(N_{CH_4 } \) <10¹⁶ cm^?3 proton transfer is the only significant reaction channel. At \(N_{CH_4 } \) >10¹⁷ cm^?3 condensation constitutes 5–20% of the overall reactions. The product of the condensation reaction further associates with CO₂ to form C₂H₅NH₃ ⁺·CO₂; the atomic composition of this cluster ion is identical with the protonated amino acid alanine. The carbonium ions i-C₃H₇ ⁺ and t-C₄H₉ ⁺ condense also with HCN to yield protonated isocyanides. HCNH⁺ also appears to condense with HCN atT>570°K, and form cluster ions with HCN at lower temperatures. The rate constants of the condensation reactions vary with temperature and pressure in a complex manner. Under conditions similar to those on Titan at an altitude of 100 km (T=100–150°K, \(N_{CH_4 } \) ≈10¹⁸ cm^?3), with a methane atmosphere containing 1% H₂ and traces of NH₃ and H₂O, ion-molecule condensation reactions followed by H⁺ transfer are expected to lead to the atmospheric synthesis of C₂H₆, C₃H₈, CH₃OH, C₂H₅OH and the terminal ions NH₄ ⁺, CH₃NH₃ ⁺ and C₂H₅NH₃ ⁺. At higher temperatures (250°K<T<400°K), the synthesis of i-C₄H₁₀, i-C₃H₇OH and t-C₄H₉OH and of the ions i-C₃H₇NH₃ ⁺ and t-C₄H₉NH₃ ⁺ is also expected. Electron recombination of the terminal ions may yield amines, imines and nitriles. Cycles of protonation and dissociative recombination of the alkanes and alcohols produced in condensation reactions will also produce unsaturated hydrocarbons, ketones and aldehydes in the ionized atmosphere.     

Mechanism for the Coupled Photochemistry of Ammonia and Acetylene: Implications for Giant Planets,Comets and Interstellar Organic Synthesis

Laboratory studies provide a fundamental understanding of photochemical processes in planetary atmospheres. Photochemical reactions taking place on giant planets like Jupiter and possibly comets and the interstellar medium are the subject of this research. Reaction pathways are proposed for the coupled photochemistry of NH₃ (ammonia) and C₂H₂ (acetylene) within the context Jupiter’s atmosphere. We then extend the discussion to the Great Red Spot, Extra-Solar Giant Planets, Comets and Interstellar Organic Synthesis. Reaction rates in the form of quantum yields were measured for the decomposition of reactants and the formation of products and stable intermediates: HCN (hydrogen cyanide), CH₃CN (acetonitrile), CH₃CH = N-N = CHCH₃ (acetaldazine), CH₃CH = N-NH₂ (acetaldehyde hydrazone), C₂H₅NH₂ (ethylamine), CH₃NH₂ (methylamine) and C₂H₄ (ethene) in the photolysis of NH₃/C₂H₂ mixtures. Some of these compounds, formed in our investigation of pathways for HCN synthesis, were not encountered previously in observational, theoretical or laboratory photochemical studies. The quantum yields obtained allowed for the formulation of a reaction mechanism that attempts to explain the observed results under varying experimental conditions. In general, the results of this work are consistent with the initial observations of Ferris and Ishikawa (1988). However, their proposed reaction pathway which centers on the photolysis of CH₃CH = N-N = CHCH₃ does not explain all of the results obtained in this study. The formation of CH₃CH = N-N = CHCH₃ by a radical combination reaction of CH₃CH = N? was shown in this work to be inconsistent with other experiments where the CH₃CH = N? radical is thought to form but where no CH₃CH = N-N = CHCH₃ was detected. The importance of the role of H atom abstraction reactions was demonstrated and an alternative pathway for CH₃CH = N-N = CHCH₃ formation involving nucleophilic reaction between N₂H₄ and CH₃CH = NH is advanced.     

Rates of fixation by lightning of carbon and nitrogen in possible primitive atmospheres

A thermochemical-hydrodynamic model of the production of trace species by electrical discharges has been used to estimate the rates of fixation of C and N by lightning in the primitive atmosphere. Calculations for various possible mixtures of CH₄, CO₂, N₂, H₂, and H₂O reveal that the prime species produced were probably HCN and NO and that the key parameter determining the rates of fixation was the ratio of C atoms to O atoms in the atmosphere. Atmospheres with C more abundant than O have large HCN fixation rates, in excess of 10¹⁷ molecules J^–1, but small NO yields. However, when O is more abundant than C, the NO fixation rate approaches 10¹⁷ molecules J^–1 while the HCN yield is small. The implications for the evolution of life are discussed.     

Study on the photochemical reaction of HCN and its polymer products relating to primary chemical evolution

The photochemical reaction of HCN at 184.9 nm is studied in the gas phase. (CN)₂, H₂, CH₄, NH₃, N₂H₄, C₂H₆, and CH₃NH₂ are identified as gas phase products, and a reaction mechanism is proposed. HCN polymers^** are also obtained as solid reaction products, and their structure is investigated by Infrared Spectroscopy, UV-Visible Spectroscopy, Mass Spectrometry, and Amino acid Analysis. The process and nature of the formation of the polymers are discussed.     

Gas-Phase Reactions in Extraterrestrial Environments: Laboratory Investigations by Crossed Molecular Beams

We have investigated gas-phase reactions of N(²D) with the most abundant hydrocarbons in the atmosphere of Titan by the crossed molecular beam technique. In all cases, molecular products containing a novel CN bond are formed, thus suggesting possible routes of formation of gas-phase nitriles in the atmosphere of Titan and primordial Earth. The same approach has been recently extended to the study of radical–radical reactions, such as the reaction of atomic oxygen with the CH₃ and C₃H₅ radicals. Products other than those already considered in the modeling of planetary atmospheres and interstellar medium have been identified. Presented at: National Workshop On Astrobiology: Search For Life In The Solar System, Capri, Italy, 26 to 28 October, 2005.     

The spark discharge synthesis of amino acids from various hydrocarbons

The spark discharge synthesis of amino acids using an atmosphere of CH₄ + N₂ + H₂O + NH₃ has been investigated with variable pNH₃. The amino acids produced using higher hydrocarbons (ethane, ethylene, acetylene, propane, butane, and isobutane) instead of CH₄ were also investigated. There was considerable range in the absolute yields of amino acids, but the yields relative to glycine (or -amino-n-butyric acid) were more uniform. The relative yields of the C₃ to C₆ aliphatic -amino acids are nearly the same (with a few exceptions) with all the hydrocarbons. The glycine yields are more variable. The precursors to the C₃-C₆ aliphatic amino acids seem to be produced in the same process, which is separate from the synthesis of glycine precursors. It may be possible to use these relative yields as a signature for a spark discharge synthesis provided corrections can be made for subsequent decomposition events (e.g. in the Murchison meteorite).     

Kinetic Studies of Ammonia Monooxygenase Inhibition in Nitrosomonas europaea by Hydrocarbons and Halogenated Hydrocarbons in an Optimized Whole-Cell Assay

The inhibitory effects of 15 hydrocarbons and halogenated hydrocarbons on NH₃ oxidation by ammonia monooxygenase (AMO) in intact cells of the nitrifying bacterium Nitrosomonas europaea were determined. Determination of AMO activity, measured as NO₂^- production, required coupling of hydroxylamine oxidoreductase (HAO) activity with NH₃-dependent NH₂OH production by AMO. Hydrazine, an alternate substrate for HAO, was added to the reaction mixtures as a source of reductant for AMO. Most inhibitors exhibited competitive or noncompetitive inhibition patterns. The competitive character generally decreased (K_iE/K_iES increased) as the molecular size of the inhibitors increased. For example, CH₄ and C₂H₄ were competitive inhibitors of NH₃ oxidation, whereas the remaining alkanes (up to C₄) and monohalogenated (Cl, Br, I) alkanes were noncompetitive. Oxidation of C₂H₅Br (noncompetitive) increased as the NH₄⁺ concentration increased up to 40 mM, whereas oxidations of inhibitors with competitive character (K_iE K_iES) were diminished at 40 mM NH₄⁺. Multichlorinated compounds produced nonlinear Lineweaver-Burk plots. Iodinated alkanes (CH₃I, C₂H₅I) and C₂Cl₄ were potent inhibitors of NH₃ oxidation. Maximum rates of NH₃, C₂H₄, and C₂H₆ oxidations were approximately equivalent, suggesting a common rate-determining step. These data support an active-site model for AMO consisting of an NH₃-binding site and a second site that binds noncompetitive inhibitors, with oxidation occurring at either site.     

A Reassessment of Prebiotic Organic Synthesis in Neutral Planetary Atmospheres

The action of an electric discharge on reduced gas mixtures such as H₂O, CH₄ and NH₃ (or N₂) results in the production of several biologically important organic compounds including amino acids. However, it is now generally held that the early Earth’s atmosphere was likely not reducing, but was dominated by N₂ and CO₂. The synthesis of organic compounds by the action of electric discharges on neutral gas mixtures has been shown to be much less efficient. We show here that contrary to previous reports, significant amounts of amino acids are produced from neutral gas mixtures. The low yields previously reported appear to be the outcome of oxidation of the organic compounds during hydrolytic workup by nitrite and nitrate produced in the reactions. The yield of amino acids is greatly increased when oxidation inhibitors, such as ferrous iron, are added prior to hydrolysis. Organic synthesis from neutral atmospheres may have depended on the oceanic availability of oxidation inhibitors as well as on the nature of the primitive atmosphere itself. The results reported here suggest that endogenous synthesis from neutral atmospheres may be more important than previously thought. Stanley L. Miller died May 20, 2007.     

Contribution of Methanotrophic and Nitrifying Bacteria to CH4 and NH4+ Oxidation in the Rhizosphere of Rice Plants as Determined by New Methods of Discrimination

Methanotrophic and nitrifying bacteria are both able to oxidize CH₄ as well as NH₄⁺. To date it is not possible to estimate the relative contribution of methanotrophs to nitrification and that of nitrifiers to CH₄ oxidation and thus to assess their roles in N and C cycling in soils and sediments. This study presents new options for discrimination between the activities of methanotrophs and nitrifiers, based on the competitive inhibitor CH₃F and on recovery after inhibition with C₂H₂. By using rice plant soil as a model system, it was possible to selectively inactivate methanotrophs in soil slurries at a CH₄/CH₃F/NH₄⁺ molar ratio of 0.1:1:18. This ratio of CH₃F to NH₄⁺ did not affect ammonia oxidation, but methane oxidation was inhibited completely. By using the same model system, it could be shown that after 24 h of exposure to C₂H₂ (1,000 parts per million volume), methanotrophs recovered within 24 h while nitrifiers stayed inactive for at least 3 days. This gave an “assay window” of 48 h when only methanotrophs were active. Applying both assays to model microcosms planted with rice plants demonstrated a major contribution of methanotrophs to nitrification in the rhizosphere, while the contribution of nitrifiers to CH₄ oxidation was insignificant.     

The atmosphere of Titan

Summary The discovery that Titan had an atmosphere was made by the identification of methane in the satellite's spectrum in 1944. But the abundance of this gas and the identification of other major constituents required the 1980 encounter by the Voyager 1 spacecraft. in the intervening years, traces of C₂H₂, C₂H₄, C₂H₆ and CH₃D had been posited to interpret emission bands in Titan's IR spectrum. The Voyager infrared Spectrometer confirmed that these gases were present and added seven more. The atmosphere is now known to be composed primarily of molecular nitrogen. But the derived mean molecular weight suggests the presence of a significant amount of some heavier gas, most probably argon. It is shown that this argon must be primordial, and that one can understand the evolution of Titan's atmosphere in terms of degassing of a mixed hydrate dominated by CH₄, N₂ and³⁶Ar. This model satisfactorily explains the absence of neon and makes no special requirements on the satellite's surface temperature. The organic chemistry taking place on Titan today invites comparision with chemical evolution on the primitive Earth prior to the origin of life.Adapted in commemoration of the many contributions of Harold Urey to the study of planetary atmospheres from an article in press in J Planet Sci (1982)     

Suppression of peatland methane emission by cumulative sulfate deposition in simulated acid rain

This field manipulation study tested the effect of weekly pulses of solutions of NH₄NO₃ and (NH₄)₂SO₄ salts on the evolution of CH₄ and N₂O from peatland soils. Methane and nitrous oxide emission from a nutrient-poor fen in northern Minnesota USA was measured over a full growing season from plots receiving weekly additions of NH₄NO₃ or (NH₄)₂SO₄. At this relatively pristine site, natural additions of N and S in precipitation occur at 8 and 5 kg ha^–1 y^–1, respectively. Nine weekly additions of the dissolved salts were made to increase this to a total deposition of 31 kg N ha^–1 y^–1 on the NH₄NO₃-amended plots and 30 and 29 kg ha^–1 y^–1 of N and S, respectively, in the (NH₄)₂SO₄-amended plots. Methane flux was measured weekly from treatment and control plots and all data comparisons are made on plots measured on the same day.After the onset of the treatments, and over the course of the growing season, CH₄ emission from the (NH₄)₂SO₄-amended plots averaged 163 mg CH₄ m^–2 d^–1, significantly lower than the same-day control plot mean of 259 mg CH₄ m^–2 d^–1 (repeated measures ANOVA). Total CH₄ flux from (NH₄)₂SO₄ treatment plots was one third lower than from control plots, at 11.7 and 17.1 g CH₄ m^–2, respectively. Methane emission from the NH₄NO₃-amended plots (mean of 256 mg CH₄ m^–2 d^–1) was not significantly different from that of controls measured on the same day (mean of 225 mg CH₄ m^–2 d^–1). Total CH₄ flux from NH₄NO₃ treatment plots and same-day controls was 16.9 and 15.1 g CH₄ m^–2, respectively. In general, stable, relatively warm and wet periods followed by environmental `triggers' such as rainfall or changes in water table or atmospheric pressure, which produced a CH<sub>4</sub> `pulse' in the other plots, produced no observable peak in CH₄ emission from the (NH₄)₂SO₄-amended plots. Nitrous oxide emission from all of the plots was below the detection limit over the course of the experiment.     

Synthesis of hydrocarbons under presumed prebiotic conditions using high-frequency discharge

Summary Various hydrocarbons were synthesized by high-frequency discharge in a primordial terrestrial model atmosphere. The products were extracted by benzene or methanol and analyzed by GC-MS. The mean carbon chain length of the hydrocarbons formed by the discharge through pure CH₄ gas was less than 6. Benzene was also obtained. Some isomers were obtained for each of the hydrocarbons containing a given number of carbons. When a small amount of C₂H₂ was added to the CH₄, longer chain compounds were formed, as compared with discharge in CH₄ only. However, when the amount of C₂H₂ was increased, unextractable high molecular weight compounds were produced. The amounts of products decreased as the mixing ratio of CO₂ to CH₄ increased. No hydrocarbons were detected when the ratio of CO₂/CH₄ exceeded 1.     

Effect of H2S on the formation of organic compounds from C,N, H,S model atmospheres submitted to a glow discharge

H₂S has been often invoked as the initial source of sulfur in prebiotic evolution, and several sulfur-containing compounds has been proposed as intermediates in the primordial synthesis of biologically relevant sulfur-containing chemicals. The possibilities of synthesis of the principal key intermediates by glow discharges in CH₄−N₂−H₂S mixtures is studied. It is shown that synthesis of important intermediates such as HCN, (CN)₂, CHCCN and CH₃SH is possible from such mixtures if the amount of H₂S is not more than 10%. For higher amounts of H₂S, the syntheses are strongly inhibited.     

Abiotic synthesis of organic molecules from minerals containing traces of dissolved H2O,CO2 and N2 part II: Experimental data

Even though a given mineral, for instance olivine, may contain only traces of dissolved H₂O, CO₂ and N₂ the gases which evolve from its surface during heating comprise (a) highly reduced molecules such as H₂, CH₄, C_mH_n and more complex hydrocarbons, HCN and other N-bearing compounds (b) oxidized species in various degrees of oxidation from formaldehyde and CO to oxygen. These gases evolve sequentially besides H₂O, CO₂ and possibly N₂, their relative amounts being controlled by experimental parameters such as the rate of heating. Preliminary indications of amino acids have been obtained by liquid extraction. The chemical complexity is a consequence of radical reactions between different solute species in the surface and the bulk of the mineral grains. Data for synthetic MgO and for mantle-derived olivine are presented.     

Photorespiration in C3–C4 intermediate species of Alternanthera and Parthenium: Reduced ammonia production and increased capacity of CO2 refixation in the light

The pattern of photorespiratory ammonia (PR–NH₃) formation and its modulation by exogenous bicarbonate or glycine were investigated in C₃–C₄ intermediates of Alternanthera (A. ficoides and A. tenella) and Parthenium hysterophorus in comparison to those of C₃ or C₄ species. The average rates of PR–NH₃ accumulation in leaves of the intermediates were slightly less than (about 25% reduced) those in C₃ species, and were further low in C₄ plants (40% of that in C₃). The levels of PR–NH₃ in leaf discs decreased markedly when exogenous bicarbonate was present in the incubation medium. The inhibitory effect of bicarbonate on PR–NH₃ accumulation was pronounced in C₃ plants, very low in C₄ species and was moderate in the C₃–C₄ intermediates. Glycine, an intermediate of photorespiratory metabolism, raised the levels of PR–NH₃ in leaves of not only C₄ but also C₃–C₄ intermediates, bringing the rates close to those of C₃ species. The rate of mitochondrial glycine decarboxylation in darkness in C₃–C₄ intermediates was partially reduced (about 80% of that in C₃ species), corresponding to the activity-levels of glycine decarboxylase and serine hydroxymethyltransferase in leaves. The intermediates had a remarkable capacity of reassimilating photorespiratory CO₂ in vivo, as indicated by the apparent refixation of about 85% of the CO₂ released from exogenous glycine in the light. We suggest that the reduced photorespiration in the C₃–C₄ intermediate species of Alternanthera and Parthenium is due to both a limitation in the extent of glycine production/decarboxylation and an efficient refixation/recycling of internal CO₂.Abbreviations GDC glycine decarboxylase - GS glutamine synthetase - GOGAT glutamate-oxoglutarate aminotransferase - -HPMS -hydroxy-2-pyridinemethanesulfonic acid - INH isonicotinyl hydrazide - MSO L-methionine sulfoximine - PR–NH₃ photorespiratory-ammonia - SHMT serine hydroxymethyltransferase