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.     

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.     

Project Cassini: A Saturn Orbiter/Titan Probe mission proposal

Titan is the only moon in the solar system with a substantial atmosphere. The organic chemistry of its N₂–CH₄ atmosphere may resemble that of the earth's primitive atmosphere before life arose. The investigation of the synthesis of prebiotic molecules in Titan's atmosphere and the atmospheric and surface environments of this planet-sized moon will be the focal point of the Cassini Project proposed to the European Space Agency for an international Saturn Orbiter/Titan Probe mission.     

Exobiology on Titan

With a dense N₂-CH₄ atmosphere rich in organics, both in gas and aerosol phases, and with the possible presence of hydrocarbons oceans on its surface, Titan, the largest satellite of Saturn, appears as a natural laboratory to study chemical evolution toward complex organic systems, in a planetary environment and over a long time scale. Thanks to many analogies with planet Earth, it provides a unique way to look at the various physical and chemical processes, and their couplings which may have been involved in terrestrial prebiotic chemistry. Indeed, analogies with the Earth have a limit since Titan's temperatures are much lower than on the Earth and since liquid water is totally absent. However, from that aspect, Titan also serves as a reference laboratory worth studying — indirectly — the role of liquid water in exobiology. The Cassini-Huygens mission currently developed by NASA and ESA will send an orbiter around Saturn and Titan and a probe in Titan's atmosphere. This mission which will be launched in 1997 for an expected arrival in 2004, offers a unique opportunity to study in detail extra-terrestrial, not life-controled, organic processes, and consequently it will have significant implications in the fields of exobiology and the origins of life.This paper is dedicated to the memory of Cyril PONNAMPERUMA who largely contributed to the development of the scientific fields of Chemical Evolution and Exobiology.     

A Mycobacterium strain with extended capacities for degradation of gasoline hydrocarbons

A bacterial strain (strain IFP 2173) was selected from a gasoline-polluted aquifer on the basis of its capacity to use 2,2, 4-trimethylpentane (isooctane) as a sole carbon and energy source. This isolate, the first isolate with this capacity to be characterized, was identified by 16S ribosomal DNA analysis, and 100% sequence identity with a reference strain of Mycobacterium austroafricanum was found. Mycobacterium sp. strain IFP 2173 used an unusually wide spectrum of hydrocarbons as growth substrates, including n-alkanes and multimethyl-substituted isoalkanes with chains ranging from 5 to 16 carbon atoms long, as well as substituted monoaromatic hydrocarbons. It also attacked ethers, such as methyl t-butyl ether. During growth on gasoline, it degraded 86% of the substrate. Our results indicated that strain IFP 2173 was capable of degrading 3-methyl groups, possibly by a carboxylation and deacetylation mechanism. Evidence that it attacked the quaternary carbon atom structure by an as-yet-undefined mechanism during growth on 2,2,4-trimethylpentane and 2,2-dimethylpentane was also obtained.     

Formation of organic compounds from simulated Titan atmosphere: perspectives of the Cassini mission.

Gas mixtures of methane and nitrogen were subjected to proton irradiation (PI), gamma irradiation (GI), UV irradiation (UV) or spark discharges (SD), and the products were analyzed to compare possible energy sources for synthesis of organics in Titan. SD mainly gave unsaturated hydrocarbons, while PI gave saturated hydrocarbons. N-containing organics were detected in PI, GI and SD, but not in UV. The formers yielded amino acids after acid-hydrolysis of solid phase products (tholin). Comparison of the present results with those by Cassini-Huygens [correction of Heygens] mission will make it possible to prove major energy sources for organic synthesis in Titan atmosphere.     

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.     

Ethene (ethylene) production in the marine macroalga Ulva (Enteromorpha) intestinalis L. (Chlorophyta, Ulvophyceae): effect of light-stress and co-production with dimethyl sulphide

Ethene (ethylene; H₂C = CH₂) is one of a range of non-methane hydrocarbons (NMHC) that affect atmospheric chemistry and global climate. Ethene acts as a hormone in higher plants and its role in plant biochemistry, physiology and ecology has been the subject of extensive research. Ethene is also found in seawater, but despite evidence that marine microalgae and seaweeds can produce ethene directly, its production is generally attributed to photochemical breakdown of dissolved organic matter. Here we confirmed ethene production in cultured samples of the macroalga Ulva (Enteromorpha) intestinalis. Ethene levels increased substantially when samples acclimatized to low light conditions were transferred to high light, and ethene addition reduced chlorophyll levels by 30%. A range of potential inhibitors and inducers of ethene biosynthesis were tested. Evidence was found for ethene synthesis via the 1-aminocylopropane-1-acrylic acid (ACC) pathway and ACC oxidase activity was confirmed for cell-free extracts. Addition of acrylate, a potential ethene precursor in algae that contain the compatible solute dimethylsulphoniopropionate, doubled the ethene produced but no acrylate decarboxylase activity was found. Nonetheless the data support active production of ethene and we suggest ethene may play a multifaceted role in algae as it does in higher plants.     

Prebiotic processes in planetary atmospheres

Within 40 years of experimental studies in prebiotic chemistry, most of the building blocks of the living systems have been synthesized in plausible conditions of the primitive Earth. The starting ingredients correspond to two complementary classes: volatile organics, and their non volatile oligomers. They may have been formed in the atmosphere on the primitive Earth and/or imported by extra-terrestrial sources. Organic chemistry is involved in meteorites, comets, in the giant planets and several of their satellites. Again this chemistry presents the two complementary aspects. In particular, with a dense reduced atmosphere rich in organic compounds in gas and aerosol phases, Titan appears as a natural laboratory for studying prebiotic chemistry at a planetary scale.     

Prebiotic chemistry in clouds

Summary In the traditional concept for the origin of life as proposed by Oparin and Haldane in the 1920s, prebiotic reactants became slowly concentrated in the primordial oceans and life evolved slowly from a series of highly protracted chemical reactions during the first billion years of Earth's history. However, chemical evolution may not have occurred continuously because planetesimals and asterioids impacted the Earth many times during the first billion years, may have sterilized the Earth, and required the process to start over. A rapid process of chemical evolution may have been required in order that life appeared at or before 3.5 billion years ago. Thus, a setting favoring rapid chemical evolution may be required. A chemical evolution hypothesis set forth by Woese in 1979 accomplished prebiotic reactions rapidly in droplets in giant atmospheric reflux columns. However, in 1985 Scherer raised a number of objections to Woese's hypothesis and concluded that it was not valid. We propose a mechanism for prebiotic chemistry in clouds that satisfies Scherer's concerns regarding the Woese hypothesis and includes advantageous droplet chemistry.Prebiotic reactants were supplied to the atmosphere by comets, meteorites, and interplanetary dust or synthesized in the atmosphere from simple compounds using energy sources such as ultraviolet light, corona discharge, or lightning. These prebiotic monomers would have first encountered moisture in cloud drops and precipitation. We propose that rapid prebiotic chemical evolution was facilitated on the primordial Earth by cycles of condensation and evaporation of cloud drops containing clay condensation nuclei and nonvolatile monomers. For example, amino acids supplied by, or synthesized during entry of, meteorites, comets, and interplanetary dust would have been scavenged by cloud drops containing clay condensation nuclei. Polymerization would have occurred within cloud systems during cycles of condensation, freezing, melting, and evaporation of cloud drops. We suggest that polymerization reactions occurred in the atmosphere as in the Woese hypothesis, but life originated in the ocean as in the Oparin-Haldane hypothesis. The rapidity with which chemical evolution could have occurred within clouds accommodates the time constraints suggested by recent astrophysical theories.     

Biodegradation of Nitriles in Shale Oil

Enrichment cultures were obtained, after prolonged incubation on a shale oil as the sole source of nitrogen, that selectively degraded nitriles. Capillary gas chromatographic analyses showed that the mixed microbial populations in the enrichments degraded the homologous series of aliphatic nitriles but not the aliphatic hydrocarbons, aromatic hydrocarbons, or heterocyclic-nitrogen compounds found in this oil. Time course studies showed that lighter nitriles were removed more rapidly than higher-molecular-weight nitriles. A Pseudomonas fluorescens strain isolated from an enrichment, which was able to completely utilize the individual nitriles undecyl cyanide and undecanenitrile as sole sources of carbon and nitrogen, was unable to attack stearonitrile when provided alone as the growth substrate. A P. aeruginosa strain, also isolated from one of the enrichments, used nitriles but not aliphatic or aromatic hydrocarbons when the oil was used as a sole nitrogen source. However, when the shale oil was used as the sole source of carbon, aliphatic hydrocarbons in addition to nitriles were degraded but aromatic hydrocarbons were still not attacked by this P. aeruginosa strain.     

Solvent as a competitive inhibitor for Candida antarctica lipase B

In enzyme-catalyzed reactions, the choice of solvent often has a marked effect on the reaction outcome. In this paper, it is shown that solvent effects could be explained by the ability of the solvent to act as a competitive inhibitor to the substrate. Experimentally, the effect of six solvents, 2-pentanone, 3-pentanone, 2-methyl-2-pentanol, 3-methyl-3-pentanol, 2-methylpentane and 3-methylpentane, was studied in a solid/gas reactor. As a model reaction, the CALB-catalyzed transacylation between methyl propanoate and 1-propanol, was studied. It was shown that both ketones inhibited the enzyme activity whereas the tertiary alcohols and the hydrocarbons did not. Alcohol inhibition constants, K(i)(I) were changed to "K(i)", determined in presence of 2-pentanone, 3-pentanone, and 3-methyl-3-pentanol, confirmed the marked inhibitory character of the ketones and an absence of inhibition of 3-methyl-3-pentanol. The molecular modeling study was performed on three solvents, 2-pentanone, 2-methyl-2-pentanol and 2-methyl pentane. It showed a clear inhibitory effect for the ketone and the tertiary alcohol, but no effect for the hydrocarbon. No change in enzyme conformation was seen during the simulations. The study led to the conclusion that the effect of added organic component on lipase catalyzed transacylation could be explained by the competitive inhibitory character of solvents towards the first binding substrate methyl propanoate.     

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)     

A Mycobacterium Strain with Extended Capacities for Degradation of Gasoline Hydrocarbons

A bacterial strain (strain IFP 2173) was selected from a gasoline-polluted aquifer on the basis of its capacity to use 2,2,4-trimethylpentane (isooctane) as a sole carbon and energy source. This isolate, the first isolate with this capacity to be characterized, was identified by 16S ribosomal DNA analysis, and 100% sequence identity with a reference strain of Mycobacterium austroafricanum was found. Mycobacterium sp. strain IFP 2173 used an unusually wide spectrum of hydrocarbons as growth substrates, including n-alkanes and multimethyl-substituted isoalkanes with chains ranging from 5 to 16 carbon atoms long, as well as substituted monoaromatic hydrocarbons. It also attacked ethers, such as methyl t-butyl ether. During growth on gasoline, it degraded 86% of the substrate. Our results indicated that strain IFP 2173 was capable of degrading 3-methyl groups, possibly by a carboxylation and deacetylation mechanism. Evidence that it attacked the quaternary carbon atom structure by an as-yet-undefined mechanism during growth on 2,2,4-trimethylpentane and 2,2-dimethylpentane was also obtained.     

Triplet fluoranthenes: Aromaticity versus unpaired electrons

Three fluoranthenes and one substituted fluoranthene, 2,2-dimethyl-2H-dibenzo[cd,k]fluoranthene, were investigated using the unrestricted symmetry-broken and complete active space methods. It was shown that four Kekuléan hydrocarbons are diradicals, implying that their ground state is a triplet. In the energetically less favorable singlet state these hydrocarbons exhibit pronounced diradical character. This occurance is explained with the tendency of the investigated molecules to delocalize their π-electrons. This leads to aromatic stabilization which is stronger than destabilization due to unpaired electrons. Our results for 2,2-dimethyl-2H-dibenzo[cd,k]fluoranthene are in excellent accord with experimental findings of McMaster et al. concerning this compound.     

Molybdenum in enzymatic and heterogeneous catalysis

The chemistry common to molybdenum at the active centers of molybdoenzymes and at the surface of heterogeneous catalysts is described. Oxomolybdenum(VI) compounds catalyze selective oxidation of unsaturated hydrocarbons, e.g., propene to acrolein. Similarly, oxomolybdenum species take part in reactions catalyzed by molybdoenzymes, e.g., xanthine oxidase, sulfite oxidase, nitrate reductase. In these reactions H+, O2- or HO-, and electrons transfer between substrate molecules and molybdenum atoms and groups at the active centres. The chemistry involved is the acid-base and redox chemistry of molybdenum. Molybdenum disulfide catalyzes hydrogenation of unsaturated hydrocarbons, e.g., acetylene. The active site is a coordinately unsaturated molybdenum atom in a sulfur-ligand environment. The enzyme nitrogenase, which is a protein-bound iron-molybdenum sulfide, is also an excellent hydrogenation catalyst. Both catalysts exploit the chemistry of lower-valent molybdenum coordinated by sulfur. The extent to which understanding of the catalysis can be transferred between the two types of catalyst is assessed.     

Metabolism of 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane and 1,1-dichloro-2,2-bis(p-chlorophenyl)ethane in the mouse

The metabolites of 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT) and 1,1-dichloro-2,2-bis(p-chlorophenyl)ethane (DDD) found in the urine of female Swiss mice are reported. The metabolites of DDT are DDD, 1-chloro-2,2-bis(p-chlorophenyl)ethene (DDMU), 1,1-dichloro-2,2-bis(p-chlorophenyl)ethene (DDE), 2,2-bis(p-chlorophenyl)acetic acid (DDA), 2-hydroxy-2,2-bis(p-chlorophenyl)acetic acid (αOH-DDA) and 2,2-bis(p-chlorophenyl)ethanol (DDOH), while DDD afforded DDMU, DDE, DDA, αOH-DDA and DDOH. The relative excreted levels of DDA and DDOH and the absence of 2,2-bis(p-chlorophenyl)acetaldehyde (DDCHO) are not consistent with the generally accepted path way for DDA formation, which involves sequential metabolism of DDT and DDD via DDOH to afford DDA. The quantitative results are interpreted to mean that DDA is formed by hydroxylation at the chlorinated sp³-side chain carbon of DDD to give 2,2-bis(p-chlorophenyl)acetyl chloride (DDA-Cl), which in turn is hydrolyzed to DDA. The excretion of αOH-DDA from both DDT- and DDD-treated mice has never been previously observed. It is suggested that this metabolite arises from the initial epoxidation of DDMU, a metabolite of DDT and DDD, to yield 1,2-epoxy-1-chloro-2,2-bis(p-chlorophenyl)ethane (DDMU-epoxide). This chloroepoxide is then hydrolyzed and oxidized to produce the αOH-DDA.     

Heteropolypeptides on Titan?

Since hydrogen cyanide is a component of Titan's hazy atmosphere, HCN polymers might also be present by way of a low energy pathway leading initially to the synthesis of polyaminomalonitrile. Subsequent reactions of HCN with the activated nitrile groups of this HCN homopolymer would then yield heteropolyamidines, readily converted to heteropolypeptides following contact with frozen water on the surface of Titan.Similar HCN polymers in the reducing atmospheres of Jupiter and Saturn could be major contributors to the yellow-brown-orange appearance of these giant planets.Any detection of such HCN chemistry by the Voyager missions or the pending Galileo probe would constitute evidence for the hypothesis that heteropolypeptides on the primitive Earth were synthesized directly from hydrogen cyanide and water without the intervening formation of -amino acids.Paper presented at the 6th College Park Colloquium, October 1981.     

Metabolism of a DDT metabolite via a chloroepoxide

The 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT) metabolic intermediate 1-chloro-2,2-bis(p-chlorophenyl)ethene (DDMU) is partially metabolized in vivo by mice to 2-hydroxy-2,2-bis(p-chlorophenyl)acetic acid (αOH-DDA) and other metabolites which are excreted in urine. The subsequent DDT metabolic intermediates 1-chloro-2,2-bis(p-chlorophenyl)ethane (DDMS) and 1,1-bis(p-chlorophenyl)ethene (DDNU) are metabolized to αOH-DDA to a much lesser extent. These results imply that DDMU may be metabolized via an α-chloroepoxide. The authentic DDMU-epoxide, which after oral administration is excreted as αOH-DDA, is mutagenic in the Ames assay, and thermally rearranges rapidly to the corresponding α-chloroaldehyde, 2,2-bis(p-chlorophenyl)-2-chloroacetaldehyde (αCl-DDCHO). As expected αCl-DDCHO yielded the same urinary metabolites as DDMU-epoxide. This suggested metabolic pathway for DDMU via a chloroepoxide intermediate may account for the tumorigenicity of DDT in mice.     

The outer solar system: Perspectives for exobiology

The outer solar system contains many environments of interest for studies of the origin of life. Recent observations support the idea that Jupiter and Saturn have retained the mixture of elements originally present in the solar nebula. Subsequent low temperature chemistry has produced the expected array of simple molecules giving characteristic absorption bands in the spectra of these planets. Microwave and infrared observations show that the lower atmospheres are at temperatures above 300 K. Sources of energy for non-equilibrium chemistry seem available at least on Jupiter and the presence of an array of colored materials in the Jovian cloud belts has often been cited as evidence for the existence of complex abiogenic organic molecules. Further study of both planets in an exobiological context seems well worthwhile; potentially productive methods of investigation (including planned space missions) can be described and evaluated from this point of view. Uranus and Neptune are clearly deficient in light gases, but otherwise little is known with certainty about these distant planets. Again unusually high temperatures have been reported, but not above 273 K. Pluto and many of the outer planet satellites appear to represent a class of small bodies very unlike our neighbors in the inner solar system. Titan, Saturn's largest satellite, is especially interesting for our purposes because of its atmosphere. Methane and hydrogen are both present, and Titan's unusually reddish color again suggests the presence of organic compounds. The hydrogen-methane ratio is likely to be more similar to that of a primitive reducing terrestrial atmosphere than the ratios for Jupiter and Saturn, suggesting that in some respects this satellite may provide an even better model for early organic synthesis on the Earth. The problem of Titan's heat balance and atmospheric composition are currently under active investigation.