Showing papers in "Origins of Life and Evolution of Biospheres in 1997"
TL;DR: In this paper, a meteorite from Mars which dates to this early time appears to contain evidence of this early environment and possibly life, possibly during the end of the late heavy bombardment.
Abstract: Mars appears to have no life on its surface today. However, the presence of fluvial features provides evidence that liquid water was once present on the martian surface. By analogy with Earth, life may have originated on Mars early in its history, possibly during the end of the late heavy bombardment. Analysis of the one meteorite from Mars which dates to this early time appears to contain evidence of this early environment and possibly life. As the climate cooled and liquid water became unavailable, life would have eventually died out. The cold deserts of Antarctica provide a glimpse of what martian ecosystems might have been like as conditions worsened. The search for fossil evidence of past life on Mars may provide the first direct indication of life beyond Earth.
186 citations
TL;DR: In this article, the formation and evolution of complex circumstellar and interstellar molecules is discussed and a number of promising chemical routes are discussed which may lead to the formation of polycyclic aromatic hydrocarbon molecules, fullerenes, and unsaturated hydrocarbon chains in the outflows from stars.
Abstract: We review the formation and evolution of complex circumstellar and interstellar molecules. A number of promising chemical routes are discussed which may lead to the formation of polycyclic aromatic hydrocarbon molecules, fullerenes, and unsaturated hydrocarbon chains in the outflows from stars. Some of the problems with these chemical schemes are pointed out as well. We also review the role of grains in the formation of complex molecules in interstellar molecular clouds. This starts with the formation of simple molecules in an ice grain mantle. UV photolysis and/or thermal polymerization can convert some of these simple molecules into more complex polymeric structures. Some of these species may be released to the gas phase, particularly in the warm regions around newly formed stars. Methanol and formaldehyde seem to play an important role in this drive towards molecular complexity and their chemistry is traced in some detail.
85 citations
TL;DR: Millimeter arrays can be used to identify hot young molecular cores which contain large, highly saturated interstellar molecules, including biomolecules, which are prime locations for searching for interstellar glycine.
Abstract: Millimeter arrays can be used to identify hot young molecular cores which contain large, highly saturated interstellar molecules, including biomolecules These cores are prime locations for searching for interstellar glycine The current status of the interstellar glycine search is discussed
85 citations
TL;DR: In this paper, a space-based interferometer was used to search for the 9.6μm band of O3 for the detection of O2-rich atmospheres.
Abstract: Habitable planets are likely to exist around stars nottoo different from the Sun if current theories about terrestrialclimate evolution are correct. Some of these planets may have evolved life, and some of the inhabited planets may have evolved O2-rich atmospheres. Such atmospheres could be detectedspectroscopically on planets around nearby stars using a space-based interferometer to search for the 9.6μm band of O3.Planets with O2-rich atmospheres that lie within the habitable zone around their parent star are, in all probability, inhabited.
76 citations
TL;DR: I review the relative importance of internal and external sources of prebiotic molecules on Earth at the time of life's origin μ3.7Gyr ago and concludes that if the early atmosphere was non-reducing and CO2-dominated, external delivery might have been the dominant source.
Abstract: I review the relative importance of internal and external sources of prebiotic molecules on Earth at the time of life’s origin ~3.7 Gyr ago. The efficiency of synthesis in the Earth’s atmosphere was critically dependent on its oxidation state. If the early atmosphere was non-reducing and CO2dominated, external delivery might have been the dominant source. Interplanetary dust grains and micrometeorites currently deliver carbonaceous matter to the Earth’s surface at a rate of ~ 3 × 105 kg/yr (equivalent to a biomass in ~2Gyr), but this may have been as high as 5 × 107 kg/yr (a biomass in only ~10 Myr) during the epoch of late bombardment. Much of the incoming material is in the form of chemically inactive kerogens and amorphous carbon; but if the Earth once had a dense (~10-bar) atmosphere, small comets rich in a variety of prebiotic molecules may have been sufficiently air-braked to land non-destructively. Lingering uncertainties regarding the impact history of the Earth and the density and composition of its early atmosphere limit our ability to draw firm conclusions.
73 citations
TL;DR: 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.
Abstract: 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 α-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.
55 citations
TL;DR: Models for the origin of Titan's atmosphere, the processing of the atmosphere and surface and its exobiological role are reviewed in this paper, where direct comparisons between the conditions on present day Titan and those proposed for prebiotic Earth are also presented.
Abstract: Models for the origin of Titan’s atmosphere, the processing of the atmosphere and surface and its exobiological role are reviewed. Titan has gained widespread acceptance in the origin of life field as a model for the types of evolutionary processes that could have occurred on prebiotic Earth. Both Titan and Earth possess significant atmospheres (≥1 atm) composed mainly of molecular nitrogen with smaller amounts of more reactive species. Both of these atmospheres are processed primarily by solar ultraviolet light with high energy particles interactions contributing to a lesser extent. The products of these reactions condense or are dissolved in other atmospheric species (aerosols/clouds) and fall to the surface. There these products may have been further processed on Titan and the primitive Earth by impacting comets and meteorites. While the low temperatures on Titan (~72–180 K) preclude the presence of permanent liquid water on the surface, it has been suggested that tectonic activity or impacts by meteors and comets could produce liquid water pools on the surface for thousands of years. Hydrolysis and oligomerization reactions in these pools might form chemicals of prebiological significance. Other direct comparisons between the conditions on present day Titan and those proposed for prebiotic Earth are also presented.
54 citations
TL;DR: A reproduction of the sequence of slides which were shown in the author's lecture ‘Pyranosyl-RNA’ at the 8.
Abstract: To Leslie E. Orgel, arbiter elegantiarum of contemporary origin-of-life science and far-sighted pioneer of non-enzymic molecular replication, on the occasion of his 70th birthday, with cordial congratulations and best wishes. In the sequel of some general remarks on a chemical etiology of nucleic-acid structure, the paper presents a reproduction of the sequence of slides which were shown in the author's lecture ‘Pyranosyl-RNA’ at the 8. ISSOL Conference in Orleans. Each slide figure is accompanied by a short explanatory comment.
54 citations
TL;DR: By utilizing reactants from dilute solution at near-neutral pH, and eliminating the need for participating organic nitrogen compounds, the reaction comprises several elements considered necessary for geochemical realism in models for molecular evolution.
Abstract: Bilateral surface-active minerals with excess positive charge concentrate glycolate and trimetaphosphate ion from l0(-3) m aqueous solution to half-saturation of the internal surface sites, and induce phosphorylation of glycolate ion in the mineral with trimetaphosphate, sorbed from l0(-2) m solution By utilizing reactants from dilute solution at near-neutral pH, and eliminating the need for participating organic nitrogen compounds, the reaction comprises several elements considered necessary for geochemical realism in models for molecular evolution
43 citations
TL;DR: It is concluded that comets may have provided a primary source for phosphorus compounds of prebiotic interest and a number of proposals aimed to gain observational supporting evidence to the above conclusion are made.
Abstract: In this work we consider the role of phosphorus in chemical evolution from an interdisciplinary approach. First we briefly review the presence of this element in different cosmic sites, such as massive stellar cores, circumstellar and interstellar clouds, meteorites, lunar and Martian samples, interplanetary dust particles, cometary dust and planetary atmospheres. Thus we illustrate the fact that phosphorus seems to be, at the same time, scarce and ubiquitous in the solar system. Afterwards, by comparing the phosphorus content of our planet's main reservoirs with the amount of cometary and meteoritic matter captured by the primitive Earth, we conclude that comets may have provided a primary source for phosphorus compounds of prebiotic interest. Finally, we make a number of proposals aimed to gain observational supporting evidence to the above conclusion and other suggestions made in the article.
37 citations
TL;DR: It is shown that Earth-like planets orbiting F and K stars may well receive less harmful UV radiation at their surfaces than does the Earth itself, and neither of these concerns is necessarily fatal to the evolution of advanced life.
Abstract: Now that extrasolar planets have been found, it is timely to ask whether some of them might be suitable for life. Climatic constraints on planetary habitability indicate that a reasonably wide habitable zone exists around main sequence stars with spectral types in the early-F to mid-K range. However, it has not been demonstrated that planets orbiting such stars would be habitable when biologically-damaging energetic radiation is also considered. The large amounts of UV radiation emitted by early-type stars have been suggested to pose a problem for evolving life in their vicinity. But one might also argue that the real problem lies with late-type stars, which emit proportionally less radiation at the short wavelengths (lambda < 200 nm) required to split O2 and initiate ozone formation. We show here that neither of these concerns is necessarily fatal to the evolution of advanced life: Earth-like planets orbiting F and K stars may well receive less harmful UV radiation at their surfaces than does the Earth itself.
TL;DR: The formation and early state of the solar system are reviewed in order to better understand the nature of the early Earth, and to constrain the conditions present during the origin and early evolution of life on this planet.
Abstract: Life arose on an early Earth which was the product of the conditions present, and processes operating, during formation of the solar system. The formation and early state of the solar system are reviewed in order to better understand the nature of the early Earth, and to constrain the conditions present during the origin and early evolution of life on this planet.
TL;DR: The previously reported condensation reaction of glycylglycine with trimetaphosphate was reinvestigated and shown to be catalyzed by magnesium ion, indicating the presence of a considerable catalytic effect of magnesium ion.
Abstract: The previously reported condensation reaction of glycylglycine with trimetaphosphate (Yamanaka et al, 1988) was reinvestigated and shown to be catalyzed by magnesium ion Aqueous solutions containing glycylglycine (05 M), trimetaphosphate (05 M) and magnesium chloride (05 M) were incubated at 38 degrees C at pH 4, 5, 6, 7 and 8 After incubation for ten days at pH 5, the maximum yields of tetraglycine and hexaglycine as condensation products were found to be about 12 and 14%, respectively This result indicated the presence of a considerable catalytic effect of magnesium ion compared with the maximum yield of about 2% for tetraglycine and approximately 0% for hexaglycine in the absence of magnesium ion
TL;DR: The self-condensation of nucleoside 5′-phosphate 2-methylimidazolide in the presence of Mg2+ in concentrated solutions, such as might have been found in an evaporating lagoon on prebiotic Earth, is reported.
Abstract: Phosphoimidazolide activated ribomononucleotides (*pN) are useful substrates for the non-enzymatic synthesis of polynucleotides. However, dilute neutral aqueous solutions of *pN typically yield small amounts of dimers and traces of polymers; most of *pN hydrolyzes to yield nucleoside 5'-monophosphate. Here we report the self-condensation of nucleoside 5'-phosphate 2- methylimidazolide (2-MeImpN with N = cytidine, uridine or guanosine) in the presence of Mg2(+) in concentrated solutions, such as might have been found in an evaporating lagoon on prebiotic Earth. The product distribution indicates that oligomerization is favored at the expense of hydrolysis. At 1.0 M, 2-MelmpU and 2-MelmpC produce about 65% of oligomers including 4% of the 3',5'-Iinked dimer. Examination of the product distribution of the three isomeric dimers in a self-condensation allows identification of reaction pathways that lead to dimer formation. Condensations in a concentrated mixture of all three nucleotides (U,C,G mixtures) is made possible by the enhanced solubility of 2-MeImpG in such mixtures. Although percent yield of intemucleotide linked dimers is enhanced as a function of initial monomer concentration, pyrophosphate dimer yields remain practically unchanged at about 20% for 2-MelmpU, 16% for 2-MeImpC and 25% of the total pyrophosphate in the U,C,G mixtures. The efficiency by which oligomers are produced in these concentrated solutions makes the evaporating lagoon scenario a potentially interesting medium for the prebiotic synthesis of dimers and short RNAs.
TL;DR: The evolutionary response of an in vitro replicating system to the selection pressure for fast growth is described and what happens to the amplified molecules within this replication system is shown.
Abstract: The conditions necessary for evolution are amplification, mutagenesis and selection. Here we describe the evolutionary response of an in vitro replicating system to the selection pressure for fast growth and show what happens to the amplified molecules within this replication system. Our emphasis is on methodology, on the monitoring and the automation of experiments in molecular evolution. In order to perform in vitro studies on the evolution of RNA molecules, a modified self-sustained sequence replication (3SR) method was used. In the first step of the 3SR reaction, the RNA template is reversely transcribed by HIV-1 reverse transcriptase, followed by a second strand synthesis and the transcription of the resulting dsDNA by T7 RNA polymerase. The selection pressure (fast growth) was achieved by applying the principle of serial transfer pioneered in the laboratories of Sol Spiegelman and Leslie Orgel. At the end of the exponential growth phase of the 3SR reaction, an aliquot of the reaction mixture is transfered into a new sample containing only buffer, nucleotides and enzymes while RNA template molecules are provided by the transfer. The conditions in the exponential growth phase allow the RNA molecules to be amplified in a constant environment; all enzymes (HIV-1 reverse transcriptase and T7 RNA polymerase) and nucleotides are present in large excess. Therefore, transfering reproducibly within the exponential growth phase is equivalent to selecting for fast growth; those molecules which can replicate faster will displace others after several transfers. The experiments were performed using a serial transfer apparatus (STA) which allows the nucleic acid concentration to be monitored on-line by measuring the laser-induced fluorescence caused by intercalation of thiazole orange monomers into the RNA/DNA amplification products. The serial transfer experiments were carried out with an RNA template (220b RNA) that represents a 220-base segment of the HIV-1 genome and comprises the in vivo primer binding site (PBS) for the HIV-1 reverse transcriptase. It could be shown that after only two serial transfers two RNA species (EP1 and EP2) emerged that were much shorter. EP1 (48b) and EP2 (54b) were formed by deletion mutations within the original 220b RNA template in the very beginning of the serial transfer experiment; due to their higher replication rate (calculated from the growth curves derived on-line) these two deletion mutants displaced the original 220b RNA template in the course of the following thirty transfers. We assume that these two RNA species evolved independently of each other. Their formation was probably induced by a strand-transfer reaction of HIV-1 reverse transcriptase. Sequence analyses of these two evolution products seem to confirm such a presented pathway. 30 years after Spiegelman's experiment, the study described here is another answer to the question he posed: ‘How do molecules evolve if the only demand is the biblical injunction: multiply?’. The answer, derived from a modified 3SR amplification system (mimicking a part of the HIV-1 replication cycle in vitro), is the same as thirty years ago: The RNA molecules adapt to the new conditions by throwing away any ballast not needed for fast replication. Clearly, this is only one aspect of molecular evolution; however, it shows that we should be careful in designating unidentified genetic material as ‘junk DNA’.
TL;DR: From the phylogenetic distribution it is shown that distribution of ALA synthase is restricted to the α subclass of purple bacteria in prokaryotes, and further distributed to mitochondria of eukaryotes.
Abstract: 5-Aminolevulinic acid (ALA), a universal precursor of tetrapyrrole compounds can be synthesized by two pathways: the C5 (glutamate) pathway and ALA synthase. From the phylogenetic distribution it is shown that distribution of ALA synthase is restricted to the α subclass of purple bacteria in prokaryotes, and further distributed to mitochondria of eukaryotes. The monophyletic origin of bacterial and eukaryotic ALA synthase is shown by sequence analysis of the enzyme. Evolution of ALA synthase in the α subclass of purple bacteria is discussed in relation to the energy-generating and biosynthetic devices in subclasses of this bacteria.
TL;DR: It is argued that the phosphorus chemistry of Earth's earliest hydrosphere, whether primarily exogenic or endogenic in origin, was most likely dominated by compounds less oxidized than phosphoric acid and its esters.
Abstract: The available evidence indicates that the origin of life on Earth certainly occurred earlier than 3.5 billion years ago and perhaps substantially earlier. The time available for the chemical evolution which must have preceded this event is more difficult to estimate. Both endogenic and exogenic contributions to chemical evolution have been considered; i.e., from chemical reactions in a primitive atmosphere, or by introduction in the interiors of comets and/or meteorites. It is argued, however, that the phosphorus chemistry of Earth's earliest hydrosphere, whether primarily exogenic or endogenic in origin, was most likely dominated by compounds less oxidized than phosphoric acid and its esters. A scenario is presented for the early production of a suite of reactive phosphonic acid derivatives, the properties of which may have foreshadowed the later appearance of biophosphates.
TL;DR: The existence of protoplanetary disks has been inferred on the basis of emission from T T Tauri stars that does not arise from a stellar photosphere as mentioned in this paper, and the existence of extra-solar planets is supported by increasing circumstantial evidence.
Abstract: Within the last few decades, the existence of protoplanetary disks has been inferred on the basis of emission from T Tauri stars that does not arise from a stellar photosphere. More recently, high-resolution interferometric techniques have resolved the dust continuum emission, and millimeter arrays have imaged circumstellar molecular gas. These measurements corroborate the disk interpretation; many T Tauri stars are surrounded by centrifugally supported circumstellar disks with radial sizes of order 100 AU. Further proof issues from Hubble Space Telescope images of disks that are illuminated externally. The morphology of circumstellar dust is revealed in striking detail and affirms the prevalence and dimensions of disks imaged at longer wavelengths. The fate of circumstellar material around young stars must be understood in order to discern the degree to which these disks are proto-planetary. Observational studies of circumstellar disks which are in the beginning of a dispersal phase are challenging and place great demands on astronomical techniques. Nevertheless, the connection between disks and the formation of extra-solar planets is supported by increasing circumstantial evidence. Optically thin dust continuum emission persists in T Tauri stars and is detected around some young main sequence stars. Since the dust is subject to rapid dispersal by radiation pressure and Poynting-Robertson drag, some mechanism of replenishment is required. Disks around nearby young main sequence stars show evidence for inner voids and disk asymmetries that should also disappear on short timescales. The presence of large orbiting bodies which collide and interact with the resulting debris can explain both the persistence of optically thin dust and the maintenance of otherwise-ephemeral dynamical features. Together with recent detections of extra-solar planets, these observations lend some support to the hypothesis that circumstellar disks commonly give birth to planetary systems.
TL;DR: In particular, a series of absorption bands have been observed near 3.4 μm (approximately 2940 cm−1) towards bright infrared objects which are seen through large column densities of interstellar dust as discussed by the authors.
Abstract: Star formation and the subsequent evolution of planetary systems occurs in dense molecular clouds, which are comprised, in part, of interstellar dust grains gathered from the diffuse interstellar medium (DISM). Radio observations of the interstellar medium reveal the presence of organic molecules in the gas phase and infrared observational studies provide details concerning the solid-state features in dust grains. In particular, a series of absorption bands have been observed near 3.4 μm(~2940cm−1) towards bright infrared objects which are seen through large column densities of interstellar dust. Comparisons of organic residues, produced under a variety of laboratory conditions, to the diffuse interstellar medium observations have shown that aliphatic hydrocarbon grains are responsible for the spectral absorption features observed near 3.4 μm (~2940 cm−1). These hydrocarbons appear to carry the -CH2- and -CH3 functional groups in the abundance ratio CH2/CH3~2.5, and the amount of carbon tied up in this component is greater than 4% of the cosmic carbon available. On a galactic scale, the strength of the 3.4 μm band does not scale linearly with visual extinction, but instead increases more rapidly for objects near the Galactic Center. A similar trend is noted in the strength of the Si-O absorption band near 9.7 μm. The similar behavior of the C-H and Si-O stretching bands suggests that these two components may be coupled, perhaps in the form of grains with silicate cores and refractory organic mantles. The ubiquity of the hydrocarbon features seen in the near infrared near 3.4 μm throughout our Galaxy and in other galaxies demonstrates the widespread availability of such material for incorporation into the many newly forming planetary systems. The similarity of the 3.4 μm features in any organic material with aliphatic hydrocarbons underscores the need for complete astronomical observational coverage in the 2–30 μm region, of lines of sight which sample dust in both dense and diffuse interstellar clouds, in order to uniquely specify the composition of interstellar organics. This paper reviews the information available from ground-based observations, although currently the Infrared Satellite Observatory is adding to our body of knowledge on this subject by providing more extensive wavelength coverage. The Murchison carbonaceous meteorite has also been used as an analog to the interstellar observations and has revealed a striking similarity between the light hydrocarbons in the meteorite and the ISM; therefore this review includes comparisons with the meteoritic analog as well as with relevant laboratory residues. Fundamental to the evolution of the biogenic molecules, to the process of planetary system formation, and perhaps to the origin of life, is the connection between the organic material found in the interstellar medium and that incorporated in the most primitive solar system bodies.
TL;DR: The nuclear processes in stars, the early Universe, and elsewhere that have produced these common elements, also encounter the production of lithium, gold, uranium, and other elements of sociological, if not biological, importance.
Abstract: The chemical elements most widely distributed in terrestrial living creatures are the ones (apart from inert helium and neon) that are commonest in the Universe — hydrogen, oxygen, carbon, and nitrogen. A chemically different Universe would clearly have different biology, if any. We explore here the nuclear processes in stars, the early Universe, and elsewhere that have produced these common elements, and, while we are at it, also encounter the production of lithium, gold, uranium, and other elements of sociological, if not biological, importance. The relevant processes are, for the most part, well understood. Much less well understood is the overall history of chemical evolution of the Galaxy, from pure hydrogen and helium to the mix of elements we see today. One implication is that we cannot do a very good job of estimating how many stars and which ones might be orbited by habitable planets.
TL;DR: The evolution of icy grain mantles is governed by the environment in which they exist as discussed by the authors, and the effect of processing on grain mantle composition and the connection between organics in grain mantle and prebiotic chemistry is introduced.
Abstract: The evolution of icy grain mantles is governed by the environment in which they exist. This review presents an overview of the study of the molecules that make up the mantles and discusses their relevance to the origin of life. Models predict two phases of mantle growth during cloud collapse: simple polar and nonpolar molecules dominate the mantle layers at early and late times, respectively (Section 1). The effect of processing on grain mantle composition and the connection between organics in grain mantles and prebiotic chemistry is introduced. Section 2 describes how infrared spectroscopy of dense cloud sources, combined with theoretical models and laboratory data, gives us information on the composition and abundance of the ices in varying regions. The observed features and how they are used as diagnostics of mantle evolution are discussed in Section 3. This section also discusses the importance of these molecules to prebiotic chemistry. Section 4 compares grain mantle composition in different low-mass star forming regions, which best represent the solar birthplace. The final section (Section 5) summarizes the information presented, emphasizing the link between the study of interstellar dust and the origin of life.
TL;DR: A biogeochemical model for the evolution of template-and-sequence-directed (TSD) syntheses of biological templates (proto-RNAs) and catalysts (peptides) is described and is the basis for a computer program.
Abstract: A biogeochemical model for the evolution of template-and-sequence-directed (TSD) syntheses of biological templates (proto-RNAs) and catalysts (peptides) is described. A fluctuating environment characterized by hydrating (cool) and dehydrating (warm) phases with cycles of consecutive organic reactions, as well as a constant supply of the polymeric building blocks is assumed. The scenario starts with the catalyzed formation of a primordial population of small random peptides, based on the relatively-ineffective mineral catalysts. The resulting peptides initiate a catalytic takeover process, during which the catalytic functions are gradually taken over by peptides. The evolution of TSD peptides is based on a combination of Lahav's (1991) co-evolution and Moller and Janssen's (1990) specific recognition sites hypotheses. During the emergence of TSD systems the fraction of TSD peptides and proto-RNA constituents rises from almost insignificance to dominance in a TSD Reactions Takeover. The TSD system is characterized by autocatalysis, positive feedback loops and a primordial genetic code. The model is the basis for a computer program (Part II of present series).
TL;DR: The sequence-dependent conformational diversity was extended to poly(Asp-Leu) and poly(Leu- asp-Asp -Leu), which adopts an α -helix in the presence of divalent cations.
Abstract: Sequential oligo- and polypeptides based on glutamic acid and leucine residues have been synthesized. In pure water, they exhibit a random coil conformation. Addition of very small amounts of divalent metallic cations induces the formation of ordered structure in the peptides which remain in solution. Higher salt concentrations precipitate the peptides. Polypeptides with alternating glutamic acid and leucine residues undergo a coil to beta-sheet transition in the presence of Ca2+, Ba2+, Mn2+, Co2+, Zn2+ and Hg2+. Addition of Cu2+ or Fe3+ induces the formation of an alpha-helix. Solid amorphous CdS generates water soluble beta-sheets, as well. Sequential poly(Leu-Glu-Glu-Leu) adopts an alpha-helix in the presence of divalent cations. The sequence-dependent conformational diversity was extended to poly(Asp-Leu) and poly(Leu-Asp-Asp-Leu).
TL;DR: The solar system is thought to have begun in a flattened disk of gas and dust referred to traditionally as the solar nebula as discussed by the authors, which is a natural product of the collapse of dense parts of giant molecular clouds, the vast star-forming regions that pepper the Milky Way and other galaxies.
Abstract: The solar system is thought to have begun in a flattened disk of gas and dust referred to traditionally as the solar nebula Such a construct seems to be a natural product of the collapse of dense parts of giant molecular clouds, the vast star-forming regions that pepper the Milky Way and other galaxies Gravitational, magnetic and thermal forces within the solar nebula forced a gradual evolution of mass toward the center (where the sun formed) and angular momentum (borne by a small fraction of the mass) toward the outer more distant regions of the disk This evolution was accompanied by heating and a strong temperature contrast from the hot, inner regions to the cold, more remote parts of the disk The resulting chemistry in the disk determined the initial distribution of organic matter in the planets; most of the reduced carbon species, in condensed form, were located beyond the asteroid belt (the ‘outer’ solar system) The Earth could have received much of its inventory of pre-biological material from comets and other icy fragments of the process of planetary formation in the outer solar system
TL;DR: Comparisons are made with existing information on the base content of codons and abundance of amino acid in different organisms and between the three groups of amino acids suggested by the model and the two classes of aminoacetyl-tRNA synthetases.
Abstract: We compare predictions from a proposed model for the origin of the genetic code (J. Theor. Biol (1993) 164, 291–305) with existing information on the base content of codons and abundance of amino acid in different organisms. A comparison is also made between the three groups of amino acids suggested by the model and the two classes of aminoacetyl-tRNA synthetases. The observed agreements tend to suppot the model.
TL;DR: A polarographic method for analyzing natural solution for formaldehyde and acetaldehyde is described and a review of other occurrences of formaldehyde in natural solutions is also included.
Abstract: Low concentrations (microM) of formaldehyde have been found in four hot springs, two in Iceland, one in Mexico and one in Southern California. A polarographic method for analyzing natural solution for formaldehyde and acetaldehyde is described. A review of other occurrences of formaldehyde in natural solutions is also included.
TL;DR: 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 nonenzythmic template- directed condensation of oligoribonucleotides correlates well with the results of helix formation of complementary oligorIBonucleotsides.
Abstract: 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°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 °C, which indicates the possibility of the template activity of long 2‘–5’ linked oligonucleotides for the nonenzymatic oligonucleotide synthesis.
TL;DR: Thermodynamic data indicate that there is little support for the idea that metabolic sulfur cycles were involved in prebiotic and early life processes, but it is suggested that metabolic sulfide cycles may have become important when oxygen evolved, when iron(II) ions disappeared from the seas, and when large amounts of sulfur were released from their iron sulfide sources.
Abstract: Thermodynamic data (Sillen, L. G.: 1966, Arkiv Kemi, 25, 159) indicate that there is little support for the idea that metabolic sulfur cycles were involved in prebiotic and early life processes, since under reducing conditions the ‘equilibrium’ concentration of sulfur in the primordial seas should have been very low, < 10−8 M. However, it is suggested that metabolic sulfur cycles may have become important when oxygen evolved, when iron(II) ions disappeared from the seas, and when large amounts of sulfur were released from their iron sulfide sources.
TL;DR: The iGd-tetrads are more stable with regard to their thermal denaturation and to their resistance against enzymatic phosphodiester hydrolysis than those formed by dG.
Abstract: Single stranded DNA-fragments containing short runs of isoguanine such as d(T4iG4T4) (5) or d(iG4T4) (6) form quartet structures by self-assembly of the isoguanine residues. The stoichiometry of the complexes is deduced from mixed aggregates formed between d(T4iG4T4) and d(iG4T4). The iGd-tetrads are more stable with regard to their thermal denaturation and to their resistance against enzymatic phosphodiester hydrolysis than those formed by dG.
TL;DR: In this article, a brief overview of what has been learned from the data available so far is presented, making a comparison between the compositions of ices in molecular clouds, protostellar condensations and comets.
Abstract: The Infrared Space Observatory (ISO) was launched by the European Space Agency on 17 November 1995. The availability of spectra from the Short Wavelength Spectrometer (SWS) on ISO is a landmark in the study of interstellar ices and organics; they provide a wealth of data in the 2–20 μm region of the spectrum covering the principal solid state resonances of condensed matter in interstellar clouds. We thus have the opportunity to study many species likely to be relevant to the inventory of CNO-bearing interstellar material present at the formation of our own and other planetary systems. This paper presents a brief overview of what has been learned from the data available so far. A comparison is made between the compositions of ices in molecular clouds, protostellar condensations and comets. Key areas of uncertainty are highlighted as a basis for future research.