PubMed Health⌕ Search

SEARCH · PubMed Health

Results for “Evolution, Chemical”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 883 records · Page 49Linked to original sources
PubMed1995

Were micrometeorites a source of prebiotic molecules on the early Earth?

"Interplanetary Dust Particles" with sizes approximately 10 micrometers collected in the stratosphere (IDPs), as well as much larger "giant" micrometeorites retrieved from Antarctic ice melt water (AMMs), are mostly composed of unequilibrated assemblages of minerals, thus being related to primitive unequilibrated meteorites. Two independent evaluations of the mass flux of micrometeorites measuring approximately 50 micrometers to approximately 200 micrometers, recovered from either the Greenland or the Antarctic ice sheets have been reported (approximately 20,000 tons/a). A comparison with recent evaluation of the flux of meteorites reaching the Earth's surface (up to masses of 10,000 tons), indicates that micrometeorites represent about 99.5% of the extraterrestrial material falling on the Earth's surface each year. As they show carbon concentrations exceeding that of the most C-rich meteorite (Orgueil), they are the major contributors of extraterrestrial C-rich matter accreting to the Earth today. Moreover they are complex microstructured aggregates of grains. They contain not only a variety of C-rich matter, such as a new "dirty" magnetite phase enriched in P, S, and minor elements, but also a diversity of potential catalysts (hydrous silicates, oxides, sulfides and metal grains of Fe/Ni composition, etc.). They could have individually functioned on the early Earth, as "micro-chondritic-reactors" for the processing of prebiotic organic molecules in liquid water. Future progress requires the challenging development of meaningful laboratory simulation experiments, and a better understanding of the partial reprocessing of micrometeorites in the atmosphere.

Antarctic Regions↗
PubMed1995

Radiation and thermal stabilities of adenine nucleotides.

We have investigated in detail radiation and thermal stabilities and transformations of adenosine mono- and triphosphates in liquid and frozen solid aqueous solutions within a wide range of absorbed radiation dose (up to 75 kGy) and temperature (up to 160 degrees C). Dephosphorylation is the main pathway of high temperature hydrolysis of adenine nucleotides. Basic thermodynamic and kinetic parameters of this process have been determined. Radiolysis of investigated compounds at room temperature results in scission of N-glycosidic bond with a radiation yield about of 1 mol/100 eV. Solution freezing significantly enhances radiation stability of nucleotides as well as other biomolecules. This circumstance is essential in the discussion of panspermia concepts.

Adenine Nucleotides↗
PubMed1995

Exobiology research on Space Station Freedom.

The Gas-Grain Simulation Facility (GGSF) is a multidisciplinary experiment laboratory being developed by NASA at Ames Research Center for delivery to Space Station Freedom in 1998. This facility will employ the low-gravity environment of the Space Station to enable aerosol experiments of much longer duration than is possible in any ground-based laboratory. Studies of fractal aggregates that are impossible to sustain on Earth will also be enabled. Three research areas within exobiology that will benefit from the GGSF are described here. An analysis of the needs of this research and of other suggested experiments has produced a list of science requirements which the facility design must accommodate. A GGSF design concept developed in the first stage of flight hardware development to meet these requirements is also described.

Cosmic Dust↗
PubMed1995

Life on Mars? II. Physical restrictions.

The primary physical factors important to life's evolution on a planet include its temperature, pressure and radiation regimes. Temperature and pressure regulate the presence and duration of liquid water on the surface of Mars. The prolonged presence of liquid water is essential for the evolution and sustained presence of life on a planet. It has been postulated that Mars has always been a cold dry planet; it has also been postulated that early mars possessed a dense atmosphere of CO2 (> or = 1 bar) and sufficient water to cut large channels across its surface. The degree to which either of these postulates is true correlates with the suitability of Mars for life's evolution. Although radiation can destroy living systems, the high fluxes of UV radiation on the martian surface do not necessarily stop the origin and early evolution of life. The probability for life to have arisen and evolved to a significant degree on Mars, based on the postulated ranges of early martian physical factors, is almost solely related to the probability of liquid water existing on the planet for at least hundreds of millions to billions of years.

Atmospheric Pressure↗
PubMed1995

Phosphorus as a potential guide in the search for extinct life on Mars.

In contrast to the search for extant organisms, the quest for fossil remains of life on Mars need not be guided by the presence of water and organic compounds on the present surface. An appropriate tracer might be the element phosphorus which is a common constituent of living systems. Utilizing terrestrial analogues, it should preferentially exist in the form of sedimentary calcium phosphate (phosphorites), which would have readily resisted changing conditions on Mars. Moreover, higher ratios of P/Th in phosphorites in comparison to calcium phosphates from magmatic rocks give us the possibility to distinguish them from inorganically formed phosphorus deposits at or close to the Martian surface. Identification of anomalous phosphorus enrichments by remote sensing or in situ analysis could be promising approaches for selecting areas preferentially composed of rocks with remains of extinct life.

Calcium Phosphates↗
PubMed1995

A preliminary comparison of two perennially ice-covered lakes in Antarctica: analogs of past Martian lacustrine environments.

Perennially ice-covered lakes in the Antarctic have been suggested as analogs to lakes which may have existed on the surface of Mars 3.5 billion years ago. During the 1991-1992 austral summer, a joint Russian/American research effort was directed at studies of ice-covered lakes in the Bunger Hills Oasis, Antarctica (66 degrees S, 100 degrees E). The primary objective of the expedition was to investigate this ice-free area for features analogous to ancient martian environments that may have been capable of supporting life and to compare the ice-covered lakes of the Bunger Hills with those in the McMurdo Dry Valleys of southern Victoria Land (77 degrees S, 166 degrees E) as part of the continuing studies of Antarctic-Mars analogs.

Antarctic Regions↗
PubMed1995

Very low temperature formaldehyde reactions and the build-up of organic molecules in comets and interstellar ices.

We have investigated thermally promoted reactions of formaldehyde (H2CO) in very low temperature ices. No such reactions occurred in ices of pure formaldehyde. However, addition of trace amounts of ammonia (NH3) were sufficient to catalyze reactions at temperatures as low as 40 K. Similar reactions could take place in interstellar ices and in Comets and produce considerable amounts of organic molecules.

Ammonia↗
PubMed2003

Kinetic analysis of artificial peptide self-replication. Part I: the homochiral case.

Computational kinetic analysis of a lately discovered homochiral peptide self-replicator is presented. A 6-step kinetic model was designed that addresses the main reactions and hydrophobic interactions involved in this template-directed, autocatalytic system and that gave rise to excellent fitting of 4 previously published independent experimental series. The model sheds light on the mechanistic principle of the reaction system and illustrates directly a number of dynamic properties such as the observed autocatalytic efficiency. It was found that the dynamics are basically governed by two reversible hydrophobic interactions: between the template and a peptide fragment and between two template species. The later association was determined to be considerably more favored, which leads to the predominant presence of the catalytically inactive template dimer in the reaction system. Our results show that the involvement of a template trimer is not necessary to obtain the observed fittings.

Algorithms↗
PubMed2003

Kinetic analysis of artificial peptide self-replication. Part II: the heterochiral case.

A kinetic model has been designed to describe and to analyze the stereoselective behavior of a recently discovered heterochiral template-directed peptide self-replicator by Ghadiri and co-workers [Nature 409 (2001) 797-801]. It turned out that previous assumptions stating that exclusively homochiral species participate in a stereoselective and autocatalytic pathway and that heterochiral species originate only from uncatalyzed background reactions could not be validated by our model. On the contrary, excellent fitting of experimental data indicated that the whole combinatorial variety of possible cross-catalytic processes involving L- and D- peptide species play an important role and need to be taken into account. The system shows no net creation of chiral matter but only a redistribution of the initially present chiral material. Both, the separation of an optically inactive meso-type template dimer and a slight chiroselective autocatalytic effect, contribute to a predicted amplification of enantiomeric excess that, in some cases, can simultaneously result in a substantial amount of optically active matter.

Algorithms↗
PubMed1997

Origin of biochemical organization.

The problems of the origin of life and biochemical evolution are analyzed in the context of the biochemical organization concept proposed earlier by the authors. The traditional view according to which the evolution of biological systems occurred from lower levels to systems of higher levels is criticized. It is suggested that the formation of the cell and biological systems of lower levels (subcellular structures, supramolecular structures, biomacromolecules) occurred in a coordinated manner. The liposome and inorganic hypotheses of the origin of life are discussed. Regosome model based on ideas of Nussinov and Mekler which combines advantages of the liposome and inorganic hypotheses has been adopted and modernized. It is suggested that the initial metabolism of protocells based itself on C2-C3-compounds. Autodevelopment of catalytic systems of protocells is discussed.

Biochemical Phenomena↗
PubMed2003

Kinetics and activation parameter analyses of hydrolysis and interconversion of 2',5'- and 3',5'-linked dinucleoside monophosphate at extremely high temperatures.

Kinetic analysis of hydrolytic stability of 2',5'- and 3',5'-linked dinucleoside monophosphate (N(2)'pN and N(3)'pN) was successfully performed in aqueous solution at 175-240 degrees C using a new real-time monitoring method for rapid hydrothermal reactions. The half-lives of NpN were in the range 2-8 s at 240 degrees C and apparent activation energy decreases in the order U(2)'pU>A(2)'pA>G(2)'pG>U(3)pU approximately C(3)'pC>A(3)pA. The stability of phosphodiester bond was dependent on the types of base moiety and phosphodiester linkages, but no systematic correlation was found between the structure and stability. The interconversion of 2',5'-adenylyladenosine monophosphate (A(2)'pA) and 3',5'-adenylyladenosine monophosphate (A(3)'pA) was enhanced in the presence of D- or L-histidine. The rate constants of degradation of NpN were dissected into the rate constants of hydrolysis and interconversion between N(2)'pN and N(3)'pN using a computer program SIMFIT. Kinetic analysis supports the mechanism that imidazolium ion and imidazole catalyze interconversion and hydrolysis even under hydrothermal environments. The activation parameters for the hydrolysis and interconversion of NpN were systematically determined for the first time from the temperature dependence of the rate constants, where both DeltaH(app)( not equal ) and DeltaS(app)( not equal ) for 2',5'-linked NpN are larger than those for 3',5'-linked NpN. These parameters support the pseudorotation mechanism through pentacoordinate intermediate from 2',5'- and 3',5'-linked NpN, where the average value of DeltaH( not equal ) (pseudorotation) was estimated to be 30+/-18 kJ mol(-1) at 175-240 degrees C.

Dinucleoside Phosphates↗
PubMed1998

The origin of life--a review of facts and speculations.

Three popular hypotheses attempt to explain the origin of prebiotic molecules: synthesis in a reducing atmosphere, input in meteorites and synthesis on metal sulfides in deep-sea vents. It is not possible to decide which is correct. It is also unclear whether the RNA world was the first biological world or whether some simpler world preceded it.

Evolution, Chemical↗
PubMed1996

Did minerals perform prebiotic combinatorial chemistry?

It has long been suspected that mineral surfaces may have been important in prebiotic chemistry. The recent demonstrations of extended oligomerization of nucleotides, amino acids and carbohydrates on mineral surfaces add support to the potential prebiotic importance of minerals, but leave a number of questions unanswered.

Catalysis↗
PubMed2001

Thermal hysteresis proteins.

Extreme environments present a wealth of biochemical adaptations. Thermal hysteresis proteins (THPs) have been found in vertebrates, invertebrates, plants, bacteria and fungi and are able to depress the freezing point of water (in the presence of ice crystals) in a non-colligative manner by binding to the surface of nascent ice crystals. The THPs comprise a disparate group of proteins with a variety of tertiary structures and often no common sequence similarities or structural motifs. Different THPs bind to different faces of the ice crystal, and no single mechanism has been proposed to account for THP ice binding affinity and specificity. Experimentally THPs have been used in the cryopreservation of tissues and cells and to induce cold tolerance in freeze susceptible organisms. THPs represent a remarkable example of parallel and convergent evolution with different proteins being adapted for an anti-freeze role.

Adaptation, Biological↗
PubMed2001

Chiroselective self-directed octamerization of serine: implications for homochirogenesis.

Serine undergoes chiroselective self-directed oligomerization to form a singly protonated octamer under positive ion electrospray conditions, as identified by ion trap tandem mass spectrometry. The experiments also show a series of higher-order clusters (metaclusters) corresponding to [(Ser8H)n]n+, n = 1, 2, 3. There is a magic number effect favoring formation of the protonated octamer over its homologues and also a strong preference for octamer formation from homochiral serine molecules. Collision-induced dissociation suggests that the protonated octamer is composed of four hydrogen-bonded dimers, stabilized by further extensive hydrogen bonding. Density functional calculations support this model and show that the protonated homochiral octamer is energetically stabilized relative to its possible fragments (dimer plus protonated hexamer, etc). The calculations also show that heterochiral octamers are less stable than homochiral octamers (e.g., the protonated 7:1 cluster is 2.1 kcal/mol less stable than the 8:0 analogue). The implications of these results for the origin of homochirality are discussed.

Evolution, Chemical↗