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We do not understand the steps leading from the abiotic early earth to the RNA world. Consequently, we cannot estimate the time required for the origins of life. Attempts to circumvent this essential difficulty are based on misunderstandings of the nature of the problem.
Helix formation between various combinations of 3'-5' linked oligoribouridylates and oligoriboadenylates from dimer to dodecamer has been studied to gain information on the chain-length requirement for the template-directed condensation of oligoribonucleotides. We have measured the helix formation under high oligoribonucleotide concentration in the presence of magnesium ion at 0-50 degrees C by UV or CD, as many model processes of oligoribonucleotides replication have been carried out under such conditions. Adenylic acid, (pA), diadenylic acid, (pA)2, or triadenylic acid, (pA)3, forms a helix with poly(U) or oligo(U) with a chain length of more than eight. On the other hand, neither uridylic acid, (pU), nor diuridylic acid. (pU)2, can form a helix with oligo(A) or poly(A). Triuridylic acid, (pU)3, or the longer oligo(U) forms a helix with oligo(A) with a chain length of over six. The results suggest that a trimer is the minimum unit as an incorporating nucleotide for conducting any set of nonenzymatic template-directed synthesis, A --> U and U --> A, as the nonenzymatic template-directed condensation of oligoribonucleotides correlates well with the results of helix formation of complementary oligoribonucleotides. We have further found the partial helix formation between 2'-5' linked decauridylate, (pU)10, and pA or 2'-5' linked (pA)2 at 0 degrees C, which indicates the possibility of the template activity of long 2'-5' linked oligonucleotides for the nonenzymatic oligonucleotide synthesis.
Our previous kinetic and thermodynamic studies upon the reactional system HCHO/HCN/NH3 in aqueous solutions are completed. In the assumed prebiotic conditions of the primitive earth ([HCHO] and [HCN] near 1 g L-1, T = 25 degrees C, pH = 8, [NH3] very low), this system leads to 99.9% of alpha-hydroxyacetonitrile and 0.1% of alpha-aminoacetonitrile (precursor of the alpha-amino acid). The classical base-catalyzed hydration of nitriles, slow and not selective, can not modify significantly this proportion. On the contrary, we found two specific and efficient reactions of alpha-aminonitriles which shift the initial equilibrium in favor of the alpha-aminonitrile pathway. The first reaction catalyzed by formaldehyde generates alpha-aminoamides, precursors of alpha-aminoacids. The second reaction catalyzed by carbon dioxide affords hydantoins, precursors of N-carbamoyl-alpha-aminoacids. In the primitive hydrosphere, where the concentration in carbon dioxide was estimated to be higher than that of formaldehyde, the formation of hydantoins was consequently more efficient. The rates of hydrolysis of the alpha-aminoacetamide and of the hydantoin at pH 8 being very similar, the synthesis of the N-carbamoyl-alpha-amino acid seems then to be the fatal issue of the HCHO/HCN/NH3 system that nature used to perform its evolution. These N-protected alpha-amino acids offer new perspectives in prebiotic chemistry, in particular for the emergence of peptides on the prebiotic earth.
On the basis of experimental studies of the initial stages of glycine oligomerization in aqueous suspension of zeolite and kaolinite catalysts, a model is suggested for the prebiotic synthesis of oligopeptides from alpha-amino acids. The formation of linear dipeptides by hydrolysis of one amide bond in the cyclic piperazinedione intermediate (formed from glycine spontaneously) is found to be the critical stage of the reaction. This stage is base catalyzed and its rate increases when pH of the medium goes up. The linear glycyl-glycine yield rises under effect of hydroxyl anions generated from different sources including insoluble silicates and soluble sodium bicarbonate. During prebiotic evolution silicates capable of cation-exchange can serve as local sources of the hydroxyl anions which dramatically accelerate formation of linear dipeptides from cyclic ones. Oligopeptides of higher molecular weight are then easily formed from the linear dipeptides at neutral pH, even in the absence of catalysts or sources of energy (e.g. such as light). The described catalytic synthesis could occur in the proximity of submarine hydrothermal vents.
A series of shock experiments were conducted to assess the feasibility of the delivery of organic compounds to the Earth via cometary impacts. Aqueous solutions containing near-saturation levels of amino acids (lysine, norvaline, aminobutyric acid, proline, and phenylalanine) were sealed inside stainless steel capsules and shocked by ballistic impact with a steel projectile plate accelerated along a 12-m-long gun barrel to velocities of 0.5-1.9 km sec-1. Pressure-temperature-time histories of the shocked fluids were calculated using 1D hydrodynamical simulations. Maximum conditions experienced by the solutions lasted 0.85-2.7 microseconds and ranged from 5.1-21 GPa and 412-870 K. Recovered sample capsules were milled open and liquid was extracted. Samples were analyzed using high performance liquid chromatography (HPLC) and mass spectrometry (MS). In all experiments, a large fraction of the amino acids survived. We observed differences in kinetic behavior and the degree of survivability among the amino acids. Aminobutyric acid appeared to be the least reactive, and phenylalanine appeared to be the most reactive of the amino acids. The impact process resulted in the formation of peptide bonds; new compounds included amino acid dimers and cyclic diketopiperazines. In our experiments, and in certain naturally occurring impacts, pressure has a greater influence than temperature in determining reaction pathways. Our results support the hypothesis that significant concentrations of organic material could survive a natural impact process.
A reliable model for the composition and evolution of interstellar ices in regions of active star formation is fundamental to our quest to determine the organic inventory of planetesimals in the early Solar System. This has become a realistic goal since the launch of the Infrared Space Observatory, which provides a facility for infrared spectroscopy unhindered by telluric absorption over the entire spectral range of vibrational modes in solids of exobiological interest. Interstellar molecules detected in the solid phase to date include H2O, NH3, CO, CO2, CH3OH, CH4, H2CO, OCS and HCOOH, together with a C identical to N-bonded absorber generically termed 'XCN'. In this article, we focus on cosmic synthesis of CN-bearing species, as this important class of prebiotic molecules may not have formed endogenously in significant quantities on early Earth if conditions were not highly reducing. Experiments in which interstellar ice analogs are subject to UV photolysis or energetic ion bombardment yield CN-rich residues with a spectral signature that matches a corresponding feature observed in young protostars enshrouded in dust and gas. CN-bearing species are also present in cometary ices, with a combined abundance comparable to the lower end of the range observed in protostars. Energetic processing of interstellar ices is thus a viable and potentially significant source of CN compounds in protoplanetary disks.
Median hardness and sodium levels in groundwater were calculated for 244 Texas counties from measurements at 7728 water wells. The data were mapped and analyzed with a geographic information system (GIS). County median hardness levels varied widely, from 4-2304 mg L-1. More than 60% of the counties had hardness medians above 180 mg L-1. County medium sodium concentrations ranged from 6-1170 mg L-1, with more than 90% of those values exceeding 20 mg L-1. There was a significant positive correlation between hardness and sodium concentrations in six of Texas' nine major aquifers. A significant negative correlation between hardness and sodium was observed in two aquifers. Several factors control hardness and sodium variations in Texas aquifers including rock/sediment composition, groundwater chemical evolution, and seepage from nearby formations. Probable human controls include agricultural return flow and pumping-induced saltwater intrusion.
This article is concerned with the spontaneous onset of homochiral oligopeptide sequences. We will show that the polymerization of hydrophobic NCA (N-carboxyanhydride = cyclic anhydride)-amino acid racemates (i.e. tryptophane, leucine and isoleucine) in aqueous solution yields oligopeptides that are characterized by a high degree of homochiral sequences. Furthermore we will show that quartz enhances efficiently the mole fraction of oligopeptides with homochiral sequence by selectively adsorbing the more stereoregular oligopeptides from an aqueous solution of oligo-D,L-leucine. We find in particular that the mole fraction of the adsorbed homochiral 7mers is 17 times larger than the mole fraction calculated for a theoretical, random process. Experimentally the stereoisomer distribution for each oligomer length can be determined by the use of enantio-labeling and LC-MS (Liquid Chromatography-Mass Spectrometry). Furthermore, if we start the polymerization with an enantiomeric excess (e.e.) of 20% of L-leucine (L-amino acid: D-amino acid = 6:4, molar ratio) we observe a chiral amplification in the enantiomeric homochiral oligopeptides. We think that such processes are relevant to the chemical evolution of single handedness.
Biopolymers like DNA and proteins are strongly selective towards the chirality of their monomer units. The use of homochiral monomers is regarded as essential for the construction and function of biopolymers; the emergence of the molecular asymmetry is therefore considered as a fundamental step in Chemical Evolution. This work focuses on physicochemical mechanisms for the origin of biomolecular asymmetry. Very recently two groups, one from Allamandola at NASA Ames and the other from our Inter-European team, demonstrated simultaneously the spontaneous photoformation of a variety of chiral amino acid structures under simulated interstellar conditions. Since both groups used unpolarized light for the photoreaction the obtained amino acids turned out racemic as expected. The obtained experimental data support the assumption that tiny ice grains can furthermore play host to important asymmetric reactions when irradiated by interstellar circularly polarized ultraviolet light. It is possible that such ice grains could have become incorporated into the early cloud that formed our Solar System and ended up on Earth, assisting life to start. Several lines of evidence suggest that some of the building blocks of life were delivered to the primitive Earth via (micro-) meteoroids and/or comets. These results suggest that asymmetric interstellar photochemistry may have played a significant part in supplying Earth with some of the enantioenriched organic materials needed to trigger life. The search for the origin of biomolecular homochirality leads to a strong interest in the fields of asymmetric photochemistry with special emphasis on absolute asymmetric synthesis. We outline here the theoretical background on asymmetric interstellar ice photochemistry, summarize recent concepts and advances in the field, and discuss briefly its implications. The obtained data are crucial for the design of the enantioselective COSAC GC-MS experiment onboard the ROSETTA spacecraft to a comet to be launched in the very near future.
Bernal, and subsequently other authors, have suggested that the prebiotic synthesis of the precursors of biopolymers could have occurred on a solid surface such as that provided by clay or some other mineral. The separation of products from the other components of the reaction mixture in such a system is reminiscent of modern solid-phase synthesis of polypeptides and polynucleotides. One such scheme envisages that growing polymers were localized by adsorption to a mineral surface where an activating agent or activated monomers were supplied continuously or cyclically. We have been trying to test this scheme using reactions which we believe may be related to those that occurred during prebiotic evolution. We have already reported that oligonucleotides adsorbed onto hydroxylapatite provide suitable templates for the oligomerization of (guanosine 5'-phosphor)-2-methylimidazolide (2-MeImpG). However, this is not a suitable test reaction, as 2-MeImpG oligomerization proceeds almost to completion in a single step. Here we report that a sequence of reactions in which initially formed oligo(G)s are reactivated by conversion to phosphorimidazolides in the presence of poly(C) and then allowed to ligate is ideal, in that repeated cycles can be carried out on the surface of hydroxylapatite, whereas in the liquid phase the cycle could be achieved only with considerable difficulty.
Photosynthesis produces reduced organic carbon and an oxidized partner in equivalent molar amounts. These compounds can react with one another, again in equivalent molar amounts, so that no net change occurs in the overall level of oxidation of the biosphere (here taken to mean the biota together with the part of the Earth with which living things interact). But, the reduced and oxidized partners have different susceptibilities to transport by environmental processes and so, typically, they become separated. Conservation of matter implies that for every mole of excess oxidant in an oxidizing region of the biosphere there must be a mole of excess reductant in a reducing region. Today, the oxidized partner in photosynthesis is usually free oxygen, which floats upward to accumulate in excess in the atmosphere, whereas organic matter settles downward to collect in sediments and stagnant pools. On the anoxic Archean Earth, the oxidized partner was probably iron. As oxidized iron is markedly less soluble and mobile than organic carbon, differential transport in the Archaean biosphere would have had an effect just the opposite of that in the modern biosphere. The oxidized partner would have settled downward more rapidly than the reduced partner, resulting in the accumulation of excess oxidant in sediments and stagnant pools. An equivalent excess of the more volatile reduced compounds would have been left behind in ocean and atmosphere in the form of dissolved organic carbon and gaseous hydrocarbons. On average, therefore, the Archean biosphere may have been oxidizing at the bottom and reducing on top.
The Galactic Centre is the most active and heavily processed region of the Milky Way, so it can be used as a stringent test for the abundance of deuterium (a sensitive indicator of conditions in the first 1,000 seconds in the life of the Universe). As deuterium is destroyed in stellar interiors, chemical evolution models predict that its Galactic Centre abundance relative to hydrogen is D/H = 5 x 10(-12), unless there is a continuous source of deuterium from relatively primordial (low-metallicity) gas. Here we report the detection of deuterium (in the molecule DCN) in a molecular cloud only 10 parsecs from the Galactic Centre. Our data, when combined with a model of molecular abundances, indicate that D/H = (1.7 +/- 0.3) x 10(-6), five orders of magnitude larger than the predictions of evolutionary models with no continuous source of deuterium. The most probable explanation is recent infall of relatively unprocessed metal-poor gas into the Galactic Centre (at the rate inferred by Wakker). Our measured D/H is nine times less than the local interstellar value, and the lowest D/H observed in the Galaxy. We conclude that the observed Galactic Centre deuterium is cosmological, with an abundance reduced by stellar processing and mixing, and that there is no significant Galactic source of deuterium.
Phosphates would probably have been essential compounds for prebiotic evolution on the primitive Earth. In this context, there have been several studies of condensation of water-soluble phosphates to polyphosphates and phosphorylation and condensation or polymerization of biomolecules with polyphosphates. But most of the phosphorus on the early Earth would have been in the form of water-insoluble apatite, and the origin of the water-soluble polyphosphates required for prebiotic evolution has therefore been a mystery. Here we show, both from experiments that simulate magmatic conditions and from analysis of volatile condensates in volcanic gas, that volcanic activity can produce water-soluble polyphosphates through partial hydrolysis of P4O10. This mechanism seems to be the only viable route identified so far for the production of these species on the primitive Earth.
The insolubility of calcium phosphate in water is a significant stumbling block in the chemistry required for the origin of life. The discovery of alkyl phosphonic acids in the Murchison meteorite suggests the possibility of delivery of these water-soluble, phosphorus-containing molecules by meteorites or comets to the early Earth. This could have provided a supply of organic phosphorus for the earliest stages of chemical evolution; although probably not components of early genetic systems, phosphonic acids may have been precursors to the first nucleic acids. Here we report the synthesis of several phosphonic acids, including the most abundant found in the Murchison meteorite, by ultraviolet irradiation of orthophosphorous acid in the presence of formaldehyde, primary alcohols, or acetone. We argue that similar reactions might explain the presence of phosphonic acids in Murchison, and could also have occurred on the prebiotic Earth.
The interstellar material from which the solar system formed has been modified by many processes: evaporation and condensation in the solar nebula, accretion into protoplanetary bodies and post-accretion processes within these bodies. Meteorites provide a record of these events and their chronology. Carbonaceous CI chondrites are among the most primitive, undifferentiated meteorites, but nevertheless show evidence of post-accretion alteration; they contain carbonates that are believed to have formed by reactions between anhydrous CI precursor materials and circulating fluids in the meteorite parent body (or bodies), yet little is known about the nature of these reactions or the timescale on which they occurred. Here we report measurements of excess 53Cr--formed by the decay of short-lived 53Mn--in five carbonate fragments from the CI chondrites Orgueil and Ivuna. Our results show that aqueous alteration on small protoplanetary bodies must have begun less than 20 Myr after the time of formation of the oldest known solar-nebula condensates (Allende refractory inclusions). This upper limit is much shorter than that of 50 Myr inferred from previous studies, and clearly establishes aqueous alteration as one of the earliest processes in the chemical evolution of the solar system.
The primordial ratio of deuterium to hydrogen nuclei (D/H), created as a result of the Big Bang, provides the most sensitive measure of the cosmological density of baryons. Measurements of the D/H ratio in the interstellar medium of our Galaxy place a strict lower limit on the primordial ratio, because processing of gas by stars reduces the abundance of deuterium relative to hydrogen. Absorption of radiation from distant quasars by intervening clouds of gas offers a means of probing D/H ratios at large redshifts, where the effects of stellar processing should be negligible. Measurements on one absorption system have indicated an extremely high primordial abundance ratio of 24 x 10(-5). Here we report a measurement of the D/H ratio in another high-redshift absorption system, and obtain a value that is an order of magnitude lower than that reported previously. The measured ratio of 2.3 x 10(-5) is consistent with that in the interstellar medium (after allowing for Galactic chemical evolution), and indicates that the absorption spectra on which the earlier estimates are based may have been subject to strong contamination. We calculate a baryon density that is 5% of the critical density required to close the Universe.
The H3+ ion is widely believed to play an important role in interstellar chemistry, by initiating the chains of reactions that lead to the production of many of the complex molecular species observed in the interstellar medium. The presence of H3+ in the interstellar medium was first suggested in 1961, and its infrared spectrum was measured in the laboratory in 1980. But attempts to detect it in interstellar space have hitherto proved unsuccessful. Here we report the detection of H3+ absorption in the spectra of two molecular clouds. Although the present results do not permit an accurate determination of the H3+ abundances, these ions appear nevertheless to be present in sufficient quantities to drive much of the chemistry in molecular clouds. It should soon be possible to obtain more accurate measurements, and thus better quantify the role of ion-neutral reactions in the chemical evolution of molecular clouds.