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At least 73 records · Page 4Linked to original sources
PubMed1979

Reactions of molybdenum-sulphur compounds with cyanide: chemical evolution and deactivation of molybdoenzymes.

Reactions of molybdenum-sulphur compounds with cyanide are reported which may be relevant to (1) the chemical evolution of molybdoenzymes and (2) deactivation of molybdoenzymes by cyanide. (1) With aqueous cyanide MoS2 gave thio-bridged complex anions [(Mo(CN)6)2(mu-S)]6- and [(Mo(CN)4(mu-S))2]6-. Under prebiotic conditions such complexes could have been formed similarly from molybdenite and may have been precursors of molybdoenzymes. (2) Only those compounds which contained terminal sulphur bound to molybdenum (i.e., Mo = S groups), viz. oxothiomolybdates and the complex [(Mo(mu-S)(S)(Et2NCS2))2], reacted with cyanide; thiocyanate was formed and the molybdenum underwent two-electron reduction. That the cyanolysable sulphur of xanthine oxidase reacts in the same way with cyanide suggests the presence of a Mo = S group which could be a structural feature of the enzyme or could have been formed by initial cyanolysis of a bound persulphide or cysteine residue.

Biological Evolution↗
PubMed1980

The evolution of the protein synthesis system, II. From chemical evolution to biological evolution.

The sequence of events previously proposed for modern protein synthesis is reviewed. It begins with an abiological synthesis of a template, and evolves through two model autocatalytic systems to a primitive cell that has a rudimentary biological protein synthesis system. A possible scheme for the origin of tRNA's is described so as to fill the gap between the model and the modern system. Fragments of genes that existed in and around the primitive system are proposed to be precursors of tRNA's. Since these fragments must have been undesirable components for the system, the origin and evolution of tRNA's may be regarded as an excellent answer by the primitive system to adverse circumstances.

Animals↗
PubMed1984

The radiolysis of aqueous ammonium cyanide: polymers of interest to chemical evolution studies.

Some of the polymers isolated from the mixture of products obtained upon irradiation of dilute oxygen-free solutions of NH4CN with gamma rays from a 60Co source were examined. Their molecular weights were estimated to be in the range 4000-20000 daltons; elements of their structure were inferred from chemical and spectroscopic studies; and their interactions with nucleic acid components were examined. Significance of these results for chemical evolution studies is summarized.

Biological Evolution↗
PubMed1976

Chemical evolution: effect of high energy radiation.

Quantitative estimation - based on extrapolated data of radioactivity and reasonable assumptions about the radiolytic effect of beta-particles on amino acids- shows that an assymmetry greater than the statistical fluctuation in the number of L-, and D-amino acid molecules could have been produced by beta-decay during chemical evolution.

Amino Acids↗
PubMed1989

Chemical evolution of primitive solar system bodies.

In this paper we summarize some of the most salient observations made recently on the organic molecules and other compounds of the biogenic elements present in the interstellar medium and in the primitive bodies of the solar system. They include the discovery of the first phosphorus molecular species in dense interstellar clouds, the presence of complex organic ions in the dust and gas phase of Halley's coma, the finding of unusual, probably presolar, deuterium-hydrogen ratios in the amino acids of carbonaceous chondrites, and new developments on the chemical evolution of Titan, the primitive Earth, and early Mars. Some of the outstanding problems concerning the synthesis of organic molecules on different cosmic bodies are also discussed from an exobiological perspective.

Amino Acids↗
PubMed1985

Trace elements in chemical evolution. II: Synthesis of amino acids under simulated primitive earth conditions in the presence of trace elements.

Electric discharge experiments have been performed in a plausible primitive earth atmosphere consisting of methane, nitrogen, and water over an aqueous phase of an ammonia-ammonium buffer solution. In some experiments, ions of metal elements, calcium, magnesium, zinc, iron and molybdenum were introduced. Gas phase products and amino acids in the liquid phase were analyzed by gas chromatography. With trace metal ions, less organic compounds in the gas phase and larger amounts of amino acids were obtained than without them. The results have shown the possible importance of trace elements in chemical evolution and the origin of life on the earth.

Amino Acids↗
PubMedDate not supplied

Aminonitriles: possible role in chemical evolution.

The formation of HCN, ammonium cyanide, alkylnitriles, aminoacetonitrile and its C- and N-methyl homologs was demonstrated earlier in a simulated Jovian atmosphere. The polymeric material resulting in these experiments was shown to give glycine, alanine, sarcosine, aspacrtic acid and some imino dibasic acids on acid hydrolysis suggesting thereby the participation of the monomeric nitriles into the formation of the polymeric product(s). Further examination of products resulting from semi-corona and arc discharge through a mixture of methane and ammonia has provided evidence for the formation of alkylaminopropionitriles as a complex mixture and also some pyridyl and pyrimidyl type heterocyclic compounds. A GC-MS examination of the heterocylics showed resemblance with those found in some carbonaceous chondrites. The significance of these findings in relation to chemical evolution will be discussed.

Ammonia↗
PubMed1976

The possible role of solid surface area in condensation reactions during chemical evolution: reevaluation.

Published data on adsorption and condensation of amino acids, purine and pyrimidine bases, sugars, nucleosides, and nucleotides are analyzed in connection with Bernal's hypothesis that clays and other minerals may have provided the most likely surface for adsorption and condensation of these molecules in prebiotic times. Using surface concentration and reaction rate as the main criteria for the feasibility of condensation reactions, four types of prebiotic environments were analyzed: (1) an ocean-sediment system, (2) a dehydrated lagoon bed produced by evaporation, (3) the surface of a frozen sediment, and (4) a fluctuating system where hydration (rainstorms, tidal variations, flooding) and dehysration (evaporation) take place in a cyclic manner. With the possible exception of nucleotides, low adsorption of organomonomers on sediment surfaces of a prebiotic ocean (pH 8) is expected, and significant condensation is considered unlikely. In dehydrated and frozen systems, high surface concentrations are probable and condensation is more likely. In fluctuating environments, condensation rates will be enhanced and the size distribution of the oligomers formed during dehydration may be influenced by a "redistribution mechanism" in which adsorbed oligomers and monomers are desorbed and redistributed on the solid surface during the next hydration-dehydration cycle.

Adsorption↗
PubMed1985

Soluble minerals in chemical evolution. I. Adsorption of 5'-AMP on CaSO4--a model system.

The adsorption of 5'-AMP onto solid CaSO4 2H2O was studied in a saturated suspension as a function of pH and electrolyte concentration. The adsorption is pH-dependent and is directly correlated with the charge on the 5'-AMP molecule which is determined by the state of protonation of the N-1 nitrogen of the purine ring and the phosphate oxygens. It is proposed that the binding occurs between the nucleotide and the salt is electrostatic in nature. The adsorption decreases with increasing ionic strength of the solution which means that in a fluctuating environment of wetting and drying cycles, a biomolecule similar to 5'-AMP could be expected to desorb during the drying phase. The results indicate that CaSO4 2H2O can serve as a concentrating surface for biomolecules. The significance of this is discussed with regard to the possible role of soluble minerals and their surfaces in a geochemical model consistent with the evolution of the Earth and the origin of life.

Adenosine Monophosphate↗
PubMed1980

The radiolysis of aqueous ammonium cyanide: compounds of interest to chemical evolution studies.

Oxygen-free aqueous solutions of NH4CN (0.1 M, pH9) were exposed to gamma rays from a 60Co source, the mixture of nonvolatile products was fractionated, and the fractions were analyzed. The procedures were chosen to make effective investigations of radiolytic products, and to minimize the contributions of chemical changes which are known to occur in aqueous solution in the absence of ionizing radiation. It has been found that the main constituents are: urea, 25.9%; an oligomer, very likely oligoimine (18.4%); and several fractions (about 50%) which release amino acids on hydrolysis. These fractions differ considerably, as shown by amino acid assay, enzymatic digestion, IR spectra, and biuret reaction. All these tests were found to be positive for two fractions; in two further fractions the enzymatic cleavage was absent, but other tests were positive. Negative enzymatic and biuret tests, and no bands characteristic of amide or peptide, were found for a fraction whose hydrolysate consisted of 55% glycine. Although most of the isolated materials were found to be composite, the results of the analyses were sufficient for getting a reliable over-all picture of the chemical action of the ionizing radiation. The role of free radicals in reactions leading to the formations of radiolytic products was considered.

Amino Acids↗
PubMed1997

Chemical evolution on Titan: comparisons to the prebiotic earth.

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 (> or = 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 (approximately 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.

Atmosphere↗
PubMed1969

Photochemical reactions and the chemical evolution of purines and nicotinamide derivatives.

The photochemical rearrangement of a tetramer of hydrogen cyanide to 4-aminoimidazole-5-carbonitrile, a critical step in the proposed prebiotic synthesis of purines, proceeds in high yield in the absence of oxygen without photodestruction of the reaction product. The mechanism of the rearrangement involves the conversion of the excited singlet of the hydrogen cyanide tetramer to 2-imino-3-cyano-4-aminoazetine, which then rearranges to the imidazole product. The photolysis of the vinylogous enaminonitrile 1,3-dicyano4-amino-1,3-butadiene yields 6-aminonicotinonitrile. The latter reaction affords an efficient route to a nicotinamide derivative from cyanoacetylene.

Biological Evolution↗
PubMed1995

Chemical evolution of circumstellar matter around young stellar objects.

Recent observational studies of the chemical composition of circumstellar matter around both high- and low-mass young stellar objects are reviewed. The molecular abundances are found to be a strong function of evolutionary state, but not of system mass or luminosity. The data are discussed with reference to recent theoretical models.

Astronomical Phenomena↗