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1.
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.  相似文献   

2.
Evolution of asymbiotic nitrogen fixation   总被引:3,自引:0,他引:3  
Recent observations on the nature of the enzyme complex, nitrogenase, prepared from a variety of nitrogen-fixing micro-organisms, on its substrate specificity, energy requirements, source of reducing power and sensitivity to O2 now permit speculation on the evolution of biological nitrogen fixation in asymbiotic micro-organisms.Ability to fix N2 is restricted to procaryotic organisms and is particularly widespread among those having characteristics (e.g. hydrogenase, ferredoxin) regarded as primitive. If the primitive environment was devoid of O2, the earliest N2-fixing prokaryote would have been a strict anaerobe, not unlike Clostridium pasteurianum. Yet N2-fixation seems unnecessary in a primitive ammonia-containing environment, and ammonia represses this function in contemporary species. This apparent paradox, the development of the ability to fix N2 in circumstances in which it was apparently unnecessary suggests that a substance other than N2 might have been primary substrate of the primeval enzyme.Substances such as acetylene, cyanide, cyanogen, nitriles or isonitriles are all substrates for nitrogenase and are all probable components of the primitive terrestrial environment. Biologically useful functions which a nitrogenase-like reductase system might have served involving substrates other than N2 include: (a) a detoxification reaction to nullify the effects of cyanide or cyanogen; (b) a means of generating ATP anaerobically; (c) a hydrogen “escape valve”.Functions (b) and (c) are improbable because they would be physiologically uneconomic; function (a) is plausible.With the emergence of an oxidizing atmosphere, facultative and aerobic N2-fixing micro-organisms could only retain the nitrogenase system if the O2-sensitive component was protected from inactivation. In the Azotobacteraceae this is achieved by “conformational protection” together with a high respiration rate; in blue-green algae, a structural compartmentation occurs in the more highly evolved species.  相似文献   

3.
UV light has been the most important energy source on the primitive Earth. However, very few experiments have been performed to test directly the possible role of this energy source on the chemical evolution of the primitive atmosphere, mainly on account of experimental difficulties. Experiments are generally performed with other excitations, mainly electric discharge, and it is frequently assumed that UV irradiation would give similar results.As theoretical considerations make this assumption questionable, direct experimental controls have been undertaken: Model primitive atmospheres have been submitted to 147 nm UV light and the gaseous phase has been analysed. Preliminary qualitative results concerning CH4–NH3 atmospheres are reported.Irradiation of pure CH4 gives rise to the synthesis of a large number of hydrocarbons, mainly saturated hydrocarbons but including also unsaturated ones as, C2H2, C2H4, C3H6, C3H4. These insaturated hydrocarbons are synthetized at a very low rate when ammonia is present in the medium.Irradiations of CH4–NH3 mixtures give rise, in addition to hydrocarbons, to important amounts of HCN (about 0.1%) and to lesser amounts of CH3CN and C2H5CN. No unsaturated nitriles such as acrylonitrile and cyanoacetylene have been detected. Search for amines is in progress.These results evidence that UV irradiation may contribute largely to synthesis of HCN in CH4–NH3 atmospheres and, consequently to the synthesis of many biochemical compounds that can be derivated from HCN. However, synthesis of other compounds, such as pyrimidines, which can derivate from other nitriles, such as cyanoacetylene, cannot be initiated only by UV light, contrary to electric discharges. In addition, if electric discharges are very efficient for synthesis of nitriles in CH4–N2 atmospheres, there is not yet evidence that UV light is able to do so.Presented at the 2nd ISSOL Meeting and the 5th ICOL in Kyoto, 5–10 April, 1977.  相似文献   

4.
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)  相似文献   

5.
The molecular basis for the chemical evolution of low molecular compounds was studied using electric discharges on a higly oxidized atmosphere comprised of CO2, N2 and H2O. In the gas phase, O2 and CO were formed by the decomposition of CO2 and their yields were enhanced by the addition of N2 to the gas mixture. It was demonstrated that H2O suppressed the reduction of CO2 while H2O also had a role in producing organic compounds such as formic acid and formaldehyde. Infrared analysis of the water soluble products and the inner surface of the reaction vessel indicated the production of compounds more complex than formic acid and formaldehyde. These compounds contained the chemical bonds which were identified to be OH, CO, CN and/or CC.  相似文献   

6.
Summary Sulphur may have played an important role, mainly as an energy converter, during the initial steps of Chemical Evolution.In atmospheric processes, sulphur, in the form of H2S might have been a primary energy acceptor and a source of hot hydrogen atoms. The presence of H2S in the primeval earth atmosphere with a molar ratio of about 10–2 could have allowed the formation of several volatile S-containing compounds without inhibiting the synthesis of the reactive products which are formed in the absence of H2S. An evaluation of the quantity of H2S which could have been included in the primeval atmosphere suggests that such a molar ratio may have been reached.In the primitive soup, the thiols and sulphides formed in the gaseous phase may have evolved, giving rise to various prebiotic syntheses. Studies on the addition reaction of alkanethiols on malonic nitriles in aqueous solutions show two different condensation processes: the formation of thioethers and the formation of iminothioesters. Taking into account the values of the specific rate constants for the two reactions, it is shown that these reactions may have taken place in the primitive earth conditions. These two compounds may have played an important role in the prebiochemical evolution. In particular, iminothioesters can be considered as the immediate precursors of thioesters.  相似文献   

7.
E. Sanhueza 《Plant and Soil》1982,67(1-3):61-71
In this work an analysis of the sources, atmospheric concentration, chemical reactions and sinks of the principal atmospheric nitrogen compounds is made. Atmospheric emissions of N2O and NH3 are almost entirely due to biological activity on the continents and in the oceans. The combustion of fossil fuels and biomass is the principal source of NOx. The only relevant chemical transformations in the troposphere are the oxidation of NOx to NO3 ? and the formation of ammonium salts. Only 10% of the NH3 emitted is oxidized. Washout of NH4 + and NO3 ? by rainfall is the principal mechanism for removing nitrogen compounds from the atmosphere. Part of the N2O enters the stratosphere and part must be removed in the biosphere by processes not yet established. NOx produced in the atmosphere by the burning of fossil fuels and biomass and by lightning represents between 30 and 40% of the total nitrogen fixed. A complete nitrogen balance for the troposphere is presented. Since the photochemical oxidation of NOx is rapid and atmospheric transport is relatively slow with respect to the cycling of water in the troposphere, nitrogen compounds return to the earth's surface close to where they were emitted. Fixed-nitrogen inputs to the continents and oceans due to biological and industrial fixation are slightly greater than those due to rain water. However, since rain falls everywhere, input from this source is only important on soils not subject to intensive agriculture.  相似文献   

8.
Hydrazine, hydrazone and hydrazide derivatives are nitrogen–nitrogen bond containing compounds. Such molecules are relatively scarce in nature and have been isolated from plants, marine organisms and microorganisms. These compounds exhibit remarkable structural diversity and relevant biological activities. The enzymes involved in the formation of the N–N bond are still unknown, but many lines of evidence support the involvement of N-nitrosation and N-hydroxylation activating steps. Beside the challenging N–N bond, N-acylases catalyzing the C–N bond formation contribute to the chemical diversity of N–N-containing natural products (N2NP). This review examines the state of knowledge regarding the biosynthesis of N2NP, for which only two biosynthetic gene clusters have been investigated. Biological properties and chemical synthesis of hydrazines, hydrazones and hydrazides are also reported.  相似文献   

9.
Important prebiotic organic compounds might have been transported to Earth in dust or produced in vapor clouds resulting from atmospheric explosions or impacts of comets. These compounds coalesced in the upper atmosphere with particles ejected from craters formed by impacts of large objects. Coalescence during exposure to UV radiation concentrated organic monomers and enhanced formation of oligomers. Continuing coalescence added material to the growing particles and shielded prebiotic compounds from prolonged UV radiation. These particles settled into the lower atmosphere where they were scavenged by rain. Aqueous chemistry and evaporation of raindrops containing nomomers in high temperature regions near the Earth's surface also promoted continued formation of oligomers. Finally, these oligomers were deposited in the oceans where continued prebiotic evolution led to the most primitive cell. Results of our studies suggest that prebiotic chemical evolution may be an inevitable consequence of impacting comets during the late accretion of planets anywhere in the universe if oceans remained on those planetary surfaces.  相似文献   

10.
Probably the first nitrogen fixers were anaerobic, non-photosynthetic, bacteria, i.e. fermenters. During the evolution of N2 fixation they still needed nitrogen on the oxidation level of ammonia. Because of the complexities in structure and function of nitrogenase this evolution must have required a long time. The photosynthetic and later the respiring bacteria inherited the capacity for N2 fixation from the fermenters, but the process did not change a great deal when it was taken over.Because of the long need for NH3, which is unstable in a redoxneutral atmosphere, a long-persisting reducing atmosphere was needed. The transition to a redoxneutral atmosphere, dominated by CO2, H2O and N2, cannot have been rapid, and the NH3 in it was recycled. Probably the atmosphere contained for a long time, as was suggested by Urey but is often denied now, a great deal of methane as a reductant. The recycling occurred with participation of intermediates like cyanide, through energy input as UV radiation or as electric discharges. A stationary state was set up.The hypothesis is recalled that coloured, photosynthetic, NH3 bacteria, analogous to coloured sulphur bacteria, may have existed, or may still exist, in reducing conditions. A few remarks are made about the origin of nitrification in the later, oxidizing atmosphere.  相似文献   

11.
In the past, it was generally assumed that the early atmosphere of the Earth contained appreciable quantities of methane (CH4) and ammonia (NH3). This was the type of atmosphere believed to be the most suitable environment for chemical evolution, the nonbiological formation of complex organic molecules, the precursors of living systems. Photochemical considerations suggest that a CH4–NH3 dominated early atmosphere was probably very short-lived, if it ever existed at all. Instead, an early atmosphere of carbon dioxide (CO2) and nitrogen (N2) is favored by photochemical as well as geological and geochemical considerations. Photochemical calculations also indicate that the total oxygen column density of the prebiological paleoatmosphere did not exceed 10–7 of the present atmospheric level.Paper presented at the 6th College Park Colloquium, October 1981  相似文献   

12.
Original extracts from an unpublished 1958 experiment conducted by the late Stanley L. Miller were recently found and analyzed using modern state-of-the-art analytical methods. The extracts were produced by the action of an electric discharge on a mixture of methane (CH4), hydrogen sulfide (H2S), ammonia (NH3), and carbon dioxide (CO2). Racemic methionine was formed in significant yields, together with other sulfur-bearing organic compounds. The formation of methionine and other compounds from a model prebiotic atmosphere that contained H2S suggests that this type of synthesis is robust under reducing conditions, which may have existed either in the global primitive atmosphere or in localized volcanic environments on the early Earth. The presence of a wide array of sulfur-containing organic compounds produced by the decomposition of methionine and cysteine indicates that in addition to abiotic synthetic processes, degradation of organic compounds on the primordial Earth could have been important in diversifying the inventory of molecules of biochemical significance not readily formed from other abiotic reactions, or derived from extraterrestrial delivery.  相似文献   

13.
We report the first experimental study of nitrogen fixation by corona discharge on the anoxic primitive Earth. The energy yields of nitric oxide (NO) and nitrous oxide (N2O) were experimentally determined over a wide range of CO2-N2 mixtures simulating the evolution of the Earth's atmosphere during the Hadean and Archean eras (from 4.5 ba to 2.5 ba). NO, the principal form of fixed nitrogen in lightning and coronal discharge in early Earth, is produced ten times less efficiently in the latter type of electrical discharge with an estimated maximum annual production rate of the order of 1010 g yr−1. For N2O the maximum production rate was estimated to be ∼109 g yr−1. These low rates of syntheses indicate that corona discharges as point discharges on the clouds and ground did not play a significant role in the overall pool of reactive nitrogen needed for the emergence and sustainability of life.  相似文献   

14.
Preincubation of the blue-green alga (cyanobacterium) Nostoc muscorum under hydrogen or argon (nongrowing conditions, neither CO2 nor N2 or bound nitrogen present) in the light resulted in a two- to fourfold increase of light-induced hydrogen evolution and a 30% increase of acetylene reduction. Preincubation under the same gases in the dark led to a decrease of both activities. Cultivation of algae under a hydrogen-containing atmosphere (N2, H2, CO2) increased neither hydrogen nor ethylene evolution by the cells. Formation of both ethylene and hydrogen is due to nitrogenase activity, which apparently was induced by the absence of N2 or bound nitrogen and not by the presence of hydrogen. Inhibitors of protein biosynthesis prevented the increase of nitrogenase activity. Hydrogen uptake by the cells was almost unaffected under all of these conditions. With either ammonia or chloramphenicol present, nitrogenase activity decreased under growing conditions (i.e., an atmosphere of N2 and CO2). The kinetics of decrease were the same with ammonia or chloramphenicol, which was interpreted as being due to rapid protein breakdown with a half-life of approximately 4 h. The decay of nitrogenase activity caused by chloramphenicol could be counteracted by nitrogenase-inducing conditions, i.e., by the absence of N2 or bound nitrogen. A cell-free system from preconditioned algae with an adenosine 5′-triphosphate-generating system exhibited the same increase or decrease of nitrogenase activity as the intact cell filaments, indicating that this effect resided in the nitrogenase complex only. We tentatively assume that not the whole nitrogenase complex, but merely a subunit or a special protein with regulatory function, is susceptible to fast turnover.  相似文献   

15.
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.  相似文献   

16.
In order to understand the role of sulfur in the primitive atmosphere, we have studied the action of a silent discharge on mixtures of CH4 and H2S at low pressure. The nature of the products formed in the gaseous phase, and the influence of several parameters, especially the H2S percentage, on the yield of the products are reported. The analysis of the products is carried out by gas liquid chromatography and infrared spectrometry. The formation of sulfur-containing compounds, such as thiols and sulfides, is reported. CS2 is formed in high yield (a few percent) in mixtures containing 40-50% of H2S, while the maximum concentration of thiols (i.e., CH3SH and C2H5SH) is reached with lower percentages of H2S. The formation of hydrocarbons decreases rapidly with increasing proportions of H2S. These results show the important inhibitor effect of H2S on the formation of hydrocarbons and the possibility of occurrence of many sulfur compounds in prebiological evolution.  相似文献   

17.
It is generally thought that the terrestrial atmosphere at the time of the origin of life was CO2-rich and that organic compounds such as amino acids would not have been efficiently formed abiotically under such conditions. It has been pointed out, however, that the previously reported low yields of amino acids may have been partially due to oxidation by nitrite/nitrate during acid hydrolysis. Specifically, the yield of amino acids was found to have increased significantly (by a factor of several hundred) after acid hydrolysis with ascorbic acid as an oxidation inhibitor. However, it has not been shown that CO2 was the carbon source for the formation of the amino acids detected after acid hydrolysis with ascorbic acid. We therefore reinvestigated the prebiotic synthesis of amino acids in a CO2-rich atmosphere using an isotope labeling experiment. Herein, we report that ascorbic acid does not behave as an appropriate oxidation inhibitor, because it contributes amino acid contaminants as a consequence of its reactions with the nitrogen containing species and formic acid produced during the spark discharge experiment. Thus, amino acids are not efficiently formed from a CO2-rich atmosphere under the conditions studied.  相似文献   

18.
Nitrogen forms an integral part of the main building blocks of life, including DNA, RNA, and proteins. N2 is the dominant gas in Earth's atmosphere, and nitrogen is stored in all of Earth's geological reservoirs, including the crust, the mantle, and the core. As such, nitrogen geochemistry is fundamental to the evolution of planet Earth and the life it supports. Despite the importance of nitrogen in the Earth system, large gaps remain in our knowledge of how the surface and deep nitrogen cycles have evolved over geologic time. Here, we discuss the current understanding (or lack thereof) for how the unique interaction of biological innovation, geodynamics, and mantle petrology has acted to regulate Earth's nitrogen cycle over geologic timescales. In particular, we explore how temporal variations in the external (biosphere and atmosphere) and internal (crust and mantle) nitrogen cycles could have regulated atmospheric pN2. We consider three potential scenarios for the evolution of the geobiological nitrogen cycle over Earth's history: two in which atmospheric pN2 has changed unidirectionally (increased or decreased) over geologic time and one in which pN2 could have taken a dramatic deflection following the Great Oxidation Event. It is impossible to discriminate between these scenarios with the currently available models and datasets. However, we are optimistic that this problem can be solved, following a sustained, open‐minded, and multidisciplinary effort between surface and deep Earth communities.  相似文献   

19.
S-Nitrosocompounds are formed when aqueous solutions of cysteine or glutathione are exposed to ultrasound (880 kHz) in air. The yield of the S-nitrosocompounds was as high as 10% for glutathione and 4% for cysteine of the initial thiol concentrations (from 0.1 to 10 mM) in the aqueous solutions. In addition to the formation of S-nitrosocompounds, thiol oxidation to disulfide forms was observed. After the oxidation of over 70% of the sulfhydryl groups, formation of peroxide compounds as well as cysteic acid derivatives was recorded. The formation of the peroxide compounds and peroxide radicals in the ultrasound field reduced the yield of S-nitrosocompounds. S-Nitrosocompounds were not formed when exposing low-molecular-weight thiols to ultrasound in atmospheres of N2 or CO. In neutral solutions, ultrasound-exposed cysteine or glutathione released NO due to spontaneous degradation of the S-nitrosocompounds. N2O3, produced due to the spontaneous degradation of the S-nitrosocompounds in air, nitrosylated sulfhydryl groups of glutathione manifested in the appearance of new absorption bands at 330 and 540 nm. The nitrogen compounds formed in an ultrasound field modified the sulfhydryl groups of apohemoglobin and serum albumin. The main target for ultrasound-generated oxygen free radicals were cystine residues oxidized to cysteic acid residues.  相似文献   

20.
Based upon arguments concerning properties of the environment and the energetics of nitrogen transformation reactions, new hypotheses regarding their evolution are presented. These hypotheses are supported by new calculations and observations germane to understanding the evolution of the nitrogen cycle. From calculations of shock production by meteor impact, we suggest that impact produced fixed nitrogen could have resulted in the entire reservoir of Earth's N2 being converted into fixed nitrogen at the end of accretion. We have significantly improved upon previous calculations of the abiotic fixation rate on the early earth and find a rate of fixation by lightning of 1–3 × 1016 Molecules NO/J, which is 2 to 3 times greater than previous estimates. This strengthens the suggestion, corroborated by the predominance of a single nitrogenase enzyme, that biological nitrogen fixation may have been a late evolutionary development, after the development of an aerobic atmosphere. In addition, we show for the first time that HNO, predicted to be the main product of atmospheric photochemical reactions involving NO on the primitive Earth by photochemical models, would eventually become NO2 and NO3 after reaching the Earth's surface. Based upon microbe-environment interactions on an ecological as well as a biochemical scale we suggest that denitrification arose prior to aerobic respiration and that nitrification arose after the advent of an aerobic atmosphere. We hypothesize the following evolutionary sequence for the biological transformation of nitrogen compounds: Ammonification Denitrification Nitrification Nitrogen fixation.  相似文献   

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