The origins of life--a status report.
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Chemistry, Organic↗
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Life: a cosmic imperative?
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Earth, Planet↗
Current status of the prebiotic synthesis of small molecules.
The prebiotic synthesis of small molecules has been accomplished using various simulated atmospheres with CH4, N2, and NH3, H2O being the most effective, but H2, CO, N2, H2O and H2, CO2, N2, H2O also give good yields of organic compounds provided H2/CO > 1 and H2/CO2 > 2. The spark discharge is a very effective source of energy in such experiments, because it is a good source of HCN. Ultraviolet light would also have been important on the primitive earth. Almost all prebiotic amino acids are made by the hydrolysis of an amino nitrile formed from an aldehyde, NH3 and HCN (Strecker synthesis). There are reasonable prebiotic syntheses worked out for the twenty amino acids that occur in proteins, with the exception of lysine, arginine and histidine. The purines are derived from the polymerization of HCN, and the precursor of the pyrimidines is cyanoacetylene. The sugars (including ribose), would have been formed from the base catalyzed polymerization of formaldehyde. There is no good prebiotic synthesis of straight chain fatty acids. Of the vitamin coenzymes, only nicotinic acid has been synthesized under prebiotic conditions. Many of the molecules that are produced in these simulated primitive earth experiments are found in a group of meteorites that contain organic compounds, called the carbonaceous chondrites. Since such prebiotic syntheses took place on the parent body of the carbonaceous chondrites, generally thought to be an asteroid, it is plausible, but not proved, that such syntheses took place on the primitive earth, and that the first living organisms were formed out of these compounds.
Amino Acids↗
Racemization and the origin of optically active organic compounds in living organisms.
The organic compounds synthesized in prebiotic experiments are racemic mixtures. A number of proposals have been offered to explain how asymmetric organic compounds formed on the Earth before life arose, with the influence of chiral weak nuclear interactions being the most frequent proposal. This and other proposed asymmetric syntheses give only sight enantiomeric excess and any slight excess will be degraded by racemization. This applies particularly to amino acids where half-lives of 10(5)-10(6) years are to be expected at temperatures characteristic of the Earth's surface. Since the generation of chiral molecules could not have been a significant process under geological conditions, the origins of this asymmetry must have occurred at the time of the origin of life or shortly thereafter. It is possible that the compounds in the first living organisms were prochiral rather than chiral; this is unlikely for amino acids, but it is possible for the monomers of RNA-like molecules.
Amino Acids↗
Cosmic questions. Proceedings of a conference. April 14-16, 1999. Washington, DC, USA.
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Biological Evolution↗
Life and space.
The studies dealing with the detection of life on planets and in space will have two periods. The first period will be associated with the application of automatical devices, which will detect living creatures, whereas the second, more late, period will be concerned with the investigations provided by man himself. In these studies one should proceed from the idea of life on the Earth, and, consequently, look for living creatures containing no water, no carbon dioxide, etc. only after obtaining negative results in searching for creatures similar to those living on the Earth. The whole course of "chemical evolution" on the Earth proves the necessity of the detection on other planets of heterotrophic microorganisms at first, and only afterwards to look for specialized forms, i.e. chemo- and photoautotrophs. It is usually underestimated that as a result of adaptation to certain ecological conditions existing on planets living creatures may appear, which will distinctly differ in their biological properties from terrestrial forms. At present there is no faultless investigation which is able to prove to existence of cosmobionts in meteorites. The appropriate experiments convince us that soil bacteria. may penetrate into the central parts of meteorites, which have been lying in the ground. Samplings at high altitudes with the aid of aerostats or the rockets are difficult to provide because of the possibility of pollution of samples by terrestrial microbes. It is therefore necessary to elaborate special devices and new methods for sampling. The most perfect instrument for the detection of microorganisms will be a device, which will record the multiplication rate of microbes in the liquid media simultaneously by means of different methods: manometry, nephelometry, potentiometry and the determination of radioactive carbon dioxide which is extracted during the breakdown of organic substances.
Atmosphere↗
Photosensitizing effect of cations on amino acids and peptides.
In connection with a study of the chemical evolution of abiogenically synthesized organic compounds on primitive Earth and the physical conditions of other planets, this paper reports the experimental results obtained by the photolysis of solutions of aliphatic amino acids (glycine, alanine, valine, leucine, n. leucine) and peptides in the atmosphere of the air, N2, Ar and CO2 in the presence of the most simple photocatalyzers-cations of sulphates. The evidence shows that the photochemical conversion of NH2 acids depends on the content of the atmosphere. The decay of NH2-group is most active in air. N2 and Ar exert no significant influence on deamination, whereas in the atmosphere of CO2 the formation of ammonia in valine, for example, was only 29 per cent of its total amount during photolysis in the air. Cu2+ and Fe2+ catalyzed while Al3+ inhibited the ammonia excretion. The formation of acetaldehyde during alanine photolysis was actually independent from the atmosphere of N2 and was inhibited in Ar and CO2. Oxydative processes inducing the formation of glyoxalic acid and formaldehyde were sharply inhibited in Ar, N2 and CO2. Under the influence of ultraviolet light of the decay of NH2-acids is also accompanied by the formation of new NH2-acids. The photosensitizing effect of cations induces a rupture of -CO-NH-bonds in peptides and, provided heavy radiation doses, prevents the formation of new NH2-acids. The longer the dipeptide chain, the more significant the quantum yield of its decomposition. The photolysis of dipeptides, leading to their decay, does not necessarily induce a hydrolytic rupture of -CO-NH-bonds resulting in the formation of three amino acids. The results obtained permit approaching problems concerning the effect of the gas content of the atmosphere and various cations (photocatalyzers) on photolytic conversion of abiogenically synthesized and biogenically significant substances, amino acids for example, at the action of ultraviolet light.
Air↗
Energy supply in early development of living cell.
It is proposed that before light and oxidation of organic compounds became the predominant energy suppliers of the living cell, electron charge energy derived from heat energy by mineral particles, was the first energy source. The basis of this hypothesis is the finding that Al2O3 in electrolytic condensors can produce an electron driving force with a potential high enough for electrolysis of water and subsequent reduction of CO2 into organic molecules. This electron driving force is likely to originate from one-way movement of electrons in tunnel structures of the Al2O3 layer, driven by temperature kinetic energy.
Adaptation, Biological↗
Viruses: origin, evolution and biodiversity. Proceedings of a meeting. Tourtour, France, July 24-30, 2000.
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Evolution, Chemical↗
Origin of life. II. From prebiotic replicators to protocells.
Primitive microvesicles (coacervates, microspheres, marigranules, etc.), free-born in aqueous media, are only protometabolic proteinoids surrounded by an amphiphilic protomembrane. In contrast, surface-born microvesicles could be initiated in the pores of watered rocks providing primary boundaries coated by amphiphilic compounds and acting as sinks for primitive peptides and their coding nucleobases N-P anchored on polyphosphates. Only presumed replication of these prenucleic infopolymers would qualify the basipetally budded microvesicles as protocells.
Cell Membrane↗
Kinetic study of sinephrine oxidation by mushroom tyrosinase.
Mushroom tyrosinase catalyzes the oxidation of sinephrine showing a marked lag period during appearance of adrenochrome and simultaneously adrenaline accumulation in the reaction medium can be detected. The adrenaline accumulation follows a sigmoidal curve until a constant level of adrenaline is reached when the system is in the steady-state. These experimental results agree with a model of enzymatic catalysis that includes the chemical evolution of adrenoquinone and permit us to explain these phenomenon as well as the influence that enzyme and sinephrine concentration present on the lag period and the level of adrenaline accumulated in the steady-state.
Adrenochrome↗
Minimizing complexity in cellular automata models of self-replication.
Understanding self-replication from an information processing perspective is important because, among other things, it can shed light on molecular mechanisms of biological reproduction and on prebiotic chemical evolution. Intuition, biological knowledge, and early computational models of self-replication all suggested that self-replication is an inherently complex process. In this paper we describe recent computational studies that challenge this viewpoint. We summarize our recent work with cellular automata models of simple yet non-trivial self-replicating structures called unsheathed loops. For example, one unsheathed loop consists of only six components and requires only 20 rules to specify the local intercomponent interactions needed to bring about replication. The implication of this work is that, when viewed as an emergent property of numerous local, concurrent interactions between components, self-replicating systems can be substantially simpler than is generally recognized.
Computer Simulation↗
Origin of the biologically important elements.
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.
Carbon↗
Evolution of color vision loss induced by occupational exposure to chemicals.
The evolution of occupationally induced color vision loss was studied in workers exposed to various chemicals. Exposure was evaluated by biological monitoring or personal air samplers, and color vision using the Lanthony D-15 desaturated panel (D-15 d). The effect of short-term interruption of exposure was studied in 39 Styrene (St) exposed workers: at a first examination a dose-related color vision loss was disclosed; a re-test performed after one month's interruption of exposure did not show any improvement of the effect. The evolution during longer periods was studied in another group of 30 St workers. Exposure and color vision were evaluated, then a follow-up was done 12 months later: the exposure was unmodified or slightly decreased in 20 subjects, and D-15 d outcomes remained unchanged, while St levels had increased and color vision loss progressed in the other 10. Similar results were obtained in 33 PCE exposed dry-cleaners: no change in color perception was observed in 14 workers whose exposure decreased, while in the other 19 a rise in PCE levels was followed by a significant color vision worsening. In 21 Hg exposed workers whose mean urinary excretion of Hg was threefold the BEI proposed by ACGIH, a dose-related impairment in color perception was observed. 12 months after a marked reduction of exposure, an almost complete recovery of the impairment was observed. Our data show that an increase in exposure can induce a worsening in color vision loss. A short interruption in exposure did not reduce the effect. A more prolonged reduction of dose reversed color vision loss in Hg exposed workers, while in solvent-exposed individuals the progression deserves further evaluation. D-15 d proved a useful test for studies on the evolution of color perception in workers exposed to eye-toxic chemicals.
Color Perception↗
The natural selection of the chemical elements.
Evolution is treated here in a novel way. DNA or any other code is considered to be conservative and therefore, once life began, it would prevent change. Change was imposed upon the DNA code as a stress resulting in vulnerability to "advantageous" DNA damage and mutation. In this respect it is the stress, the changing environment, that opened up a possibility of evolution once an early life form had optimised itself in primitive circumstances. Here I examine the initial slow-coming-to-terms with the environment of primitive life, and then its evolution as the environment forced the DNA into novel development by introducing chemical elements in new forms. The situation today is no different. Environmental change is hostile to present day life and will lead to further evolution.
Biological Evolution↗
Chemical and structural evolution of humic acids during organic waste composting.
The effects of the composting process on the chemical and structural properties of humic acids have been studied in seven different organic waste mixtures from different origin. Only slight changes in elemental composition have been found in the humic acids after the composting process pointing to a more aromatic structure with higher proportions of oxygen and nitrogen. Functional groups were the most sensitive to the changes caused by the composting process showing a marked increase in the total acidity and phenolic, carboxyl and carbonyl groups. Gel permeation chromatography showed a slight increase in the average molecular weight of the humic acids. Infrared spectroscopy did not show important differences in humic acid structure but a clear decrease in the intensity of the bands in the region 3000-2850 cm(-1) corresponding to the aliphatic fractions. As a general result, the composting process yields humic acids in which the elemental and functional composition are closer to that of the more humified soil humic acids.
Biodegradation, Environmental↗
HPLC analysis of complete BZ systems. Evolution of the chemical composition in cerium and ferroin catalysed batch oscillators: experiments and model calculations.
In the last few years many new reaction routes and intermediates have been discovered in the mechanism of the Belousov-Zhabotinsky (BZ) reaction with the aid of high performance liquid chromatography (HPLC). These previous HPLC studies, however, were limited to the Ce(4+)-organic substrate (malonic or bromomalonic acid) systems only. Very recently some measurements were made on a cerium catalysed full BZ system but only in its induction period. The present work follows the evolution of the main chemical components in a cerium and in a ferroin catalysed full BZ system from the start until the end of the oscillatory regime in a batch reactor. While recording the potential oscillations of a bromide selective electrode we measured from time to time the concentration of the following components: malonic and bromomalonic acids and bromate as main components; malonyl malonate, ethanetetracarboxylic and bromoethenetricarboxylic acids which are recombination products of organic free radicals; oxidized intermediates: tartronic, oxalic (OA) and mesoxalic (MOA) acids, and brominated products: dibromoacetic and tribromoacetic acids. Recombination products are generated in the intervals when the autocatalytic reaction is "switched off". In the course of the autocatalytic periods, however, the organic radicals react with the inorganic bromine dioxide radical mainly which leads to the formation of MOA and OA. Due to a very fast Ce(4+)-MOA reaction, MOA can be detected in the ferroin catalysed BZ system only. Our model calculations deal exclusively with the cerium catalysed system. The suggested new Marburg-Budapest-Missoula (MBM) model includes both negative feedback loops (bromous acid-bromide ion Oregonator type and bromine dioxide-organic free radicals Radicalator type feedback) and the recently discovered radical-radical recombination reactions. Comparison of the experimental data with the model calculations shows a good qualitative agreement but some open problems still remain. To overcome these problems oxygen atom transfer and other redox reactions are proposed.
Journal Article↗
Assessment of safety/risk of chemicals: inception and evolution of the ADI and dose-response modeling procedures.
This article reviews the procedures for the assessment of safety/risk of chemicals to human health. Because the nature and severity of toxicity and the extent of the database vary from chemical to chemical, the assessment is done on a case by case basis. Essentially 5 steps are involved in the assessment: (a) identification of hazards based on appropriate human and animal data; (b) determination of the dose-response relationship of the adverse effects of the chemical; (c) extrapolation of the dose-response data from test subjects to human populations; (d) estimation of the exposure; and (e) assessment of the safety/risk of the chemical under a specified exposure. Emphasis in this article, however, is placed on the extrapolation of the dose-response data to the human situation. The extrapolation is done by the identification of a no-observed-adverse-effect level (NOAEL) and the application of a safety factor, thereby arriving at an acceptable daily intake (ADI). The safety factor is selected on the basis of, inter alia, the severity of the adverse effect and the adequacy of the database. On the other hand, with genotoxic carcinogens, mathematical modeling is used for extrapolation. This is because the effects of genotoxic carcinogens are generally believed to have no threshold. The ADI approach, which involves the identification of a NOAEL, is therefore not applicable. A number of mathematical models have been developed to assess, from the dose-response data, either the risks that may be associated with a specified dose, or the 'virtually safe dose' at a specified risk level. The evolution, application and shortcomings of these procedures and the potential improvements in the ADI approach and in the dose-response characterization based on these mathematical models are also discussed.
Animals↗