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  • 1
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    The Relative Reactivity of Deoxyribose and Ribose: Did DNA Come Before RNA? (1995)
    In:  CASI
    Details
    Publication Date: 2019-08-17
    Description: If it is assumed that there was a precursor to the ribose-phosphate backbone of RNA in the preRNA world (such as peptide nucleic acid), then the entry of various sugars into the genetic material may be related to the stability and non-enzymatic reactivity of the aldose. The rate of decomposition of 2-deoxyribose has been determined to be 1/3 that of ribose. In addition we have measured the amount of free aldehyde by H-1 and C-13 NMR and find that it has approximately 0.15% free aldehyde compared to 0.05% for ribose at 25 C. This suggests that deoxyribose would be significantly more reactive with early bases in the absence of enzymes. This is confirmed by urazole and deoxyribose reacting to form the deoxynucleoside 45 times faster as 25 C than urazole reacts with ribose to form the Ribonucleoside. Urazole is a potential precursor of uracil and is a plausible prebiotic compound which reacts with aldoses to form nucleosides. Thus the non-enzymatic reactivity of deoxyribose would favor its early use over ribose until enzymes could change the relative reactivities. Most of the reasons that RNA is presumed to have come before DNA are extrapolations back from contemporary metabolism (e.g. the abundance of ribose based coenzymes, the biosynthesis of histidine, deoxyribonucleotides are synthesized from ribonucleotides, etc.). It is very difficult to reconstruct biochemical pathways much before the last common ancestor, and it is even more difficult to do more than guess at the biochemistry of very early self-replicating systems. Thus we believe that these reasons are not compelling and that the non-enzymatic chemistry may be more important than enzymatic pathways for constructing the earliest of biochemical pathways. While the RNA world has been discussed at great length, there has not been an exploration of the transition out of the RNA world. We have constructed many possible schemes of genetic takeover events from preRNA to modern DNA, RNA, protein system which could generate the RNA metabolic fossils we see today.
    Keywords: Chemistry and Materials (General)
    Type: NASA/CR-95-208003 , NAS 1.26:208003
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  • 2
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    Investigation of the Prebiotic Synthesis of Amino Acids and RNA Bases from CO2 Using FeS/H2S As a Reducing Agent (1995)
    In:  CASI
    Details
    Publication Date: 2019-08-17
    Description: An autotrophic theory of the origin of metabolism and life has been proposed in which carbon dioxide is reduced by ferrous sulfide and hydrogen sulfide by means of a reversed citric acid cycle, leading to the production of amino acids. Similar processes have been proposed for purine synthesis. Ferrous sulfide is a strong reducing agent in the presence of hydrogen sulfide and can produce hydrogen as well as reduce alkenes, alkynes, and thiols to saturated hydrocarbons and reduce ketones to thiols. However, the reduction of carbon dioxide has not been demonstrated. We show here that no amino acids, purities, or pyrimidines are produced from carbon dioxide with the ferrous sulfide and hydrogen sulfide system. Furthermore, this system does not produce amino acids from carboxylic acids by reductive amination and carboxylation. Thus, the proposed autotrophic theory, using carbon dioxide, ferrous sulfide, and hydrogen sulfide, lacks the robustness needed to be a geological process and is, therefore, unlikely to have played a role In the origin of metabolism or the origin of life.
    Keywords: Chemistry and Materials (General)
    Type: NASA/CR-95-208011 , NAS 1.26:208011 , Proc. National Academy of Science; 92; 11,904-11,906
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  • 3
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    Endogenous Synthesis of Prebiotic Organic Molecules (1996)
    In:  CASI
    Details
    Publication Date: 2019-08-17
    Description: The necessary condition for the synthesis of organic compounds on the primitive earth is the presence of reducing conditions. This means an atmosphere of CH4, CO, or CO2 + H2. The atmospheric nitrogen can be N2 with a trace of NH3, but NH4(+) is needed in the ocean at least for amino acid synthesis. Many attempts have been made to use CO2 + H2O atmospheres for prebiotic synthesis, but these give at best extremely low yields of organic compounds, except in the presence of H2. Even strong reducing agents such as FeS + H2S or the mineral assemblages of the submarine vents fail to give significant yields of organic compounds with CO2. There appears to be a high kinetic barrier to the non-biological reduction of CO2 at low temperatures using geological reducing agents. The most abundant source of energy for prebiotic synthesis is ultraviolet light followed by electric discharges, with electric discharges being more efficient, although it is not clear which was the important energy source. Photochemical process would also make significant contributions. In an atmosphere Of CO2, N2, and H2O with no H2, the production rates of HCN and H2CO would be very low, 0.001 or less than that of a relatively reducing atmosphere. The concentration of organic compounds under these non-reducing conditions would be so low that there is doubt whether the concentration mechanism would be adequate for further steps toward the origin of life. A number of workers have calculated the influx of comets and meteorites on the primitive earth as a source of organic compounds. We conclude that while some organic material was added to the earth from comets and meteorites the amount available from these sources at a given time was at best only a few percent of that from earth bases syntheses under reducing conditions.
    Keywords: Chemistry and Materials (General)
    Type: NASA/CR-1996-208009 , NAS 1.26:208009 , ; 201-202
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  • 4
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    Rates of Decomposition of Ribose and other Sugars: Implications for Chemical Evolution (1995)
    In:  CASI
    Details
    Publication Date: 2019-08-17
    Description: The existence of the RNA world, in which RNA acted as a catalyst as well as an informational macromolecule, assumes a large prebiotic source of ribose or the existence of pre-RNA molecules with backbones different from ribose-phosphate. The generally accepted prebiotic synthesis of ribose, the formose reaction, yields numerous sugars without any selectivity. Even if there were a selective synthesis of ribose, there is still the problem of stability. Sugars are known to be unstable in strong acid or base, but there are few data for neutral solutions. Therefore, we have measured the rate of decomposition of ribose between pH 4 and pH 8 from 40 C to 120 C. The ribose half-lives are very short (73 min at pH 7.0 and 100 C and 44 years at pH 7.0 and 0 C). The other aldopentoses and aldohexoses have half-lives within an order of magnitude of these values, as do 2-deoxyribose, ribose 5-phosphate, and ribose 2,4bisphosphate. These results suggest that the backbone of the first genetic material could not have contained ribose or other sugars because of their instability.
    Keywords: Chemistry and Materials (General)
    Type: NASA/CR-95-207600 , NAS 1.26:207600 , Proc. of National Academy of Science; 92; 8158-8160
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  • 5
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    National Educators' Workshop: Update 1997. Standard Experiments in Engineering Materials, Science, and Technology (1998)
    In:  CASI
    Details
    Publication Date: 2019-08-13
    Description: This document contains a collection of experiments presented and demonstrated at the National Educators' Workshop: Update 97, held at Boeing Commercial Airplane Group, Seattle, Washington, on November 2-5, 1997. The experiments related to the nature and properties of engineering materials and provided information to assist in teaching about materials in the education community.
    Keywords: Chemistry and Materials (General)
    Type: NASA/CP-1998-208726 , L-17782 , NAS 1.55:208726 , Standard Experiments in Engineering Materials, Science, and Technology; Nov 02, 1997 - Nov 05, 1997; Seattle, WA; United States
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  • 6
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    Hydrolysis of Dihydrouridine and Related Compounds (1996)
    In:  CASI
    Details
    Publication Date: 2019-08-15
    Description: Dihydrouridine is absent from the tRNA of almost all hyperthermophiles and most Archaea but is ubiquitous in the tRNA of Eubacteria and Eukaryotes. In order to investigate whether this could be due to instability, the rate of ring opening of dihydrouridine was measured between 25 and 120 C. The dihydrouridine ring is stable at 25 C, but the half-life at 100 C and pH 7 is 9.1 h, which is comparable to the doubling time of hyperthermophiles. This suggests an explanation for the absence of dihydrouridine from the tRNA of hyperthermophiles. The rates of ring opening of dihydrouracil, dihydrothymine, and 1-N-methyldihydrouracil were measured at 100 C and pH 6-9, as were the equilibrium constants for ring closure of the ureido acids to the dihydrouracils. The pH rate profiles for ring opening and ring closing were calculated from the data. Possible roles for dihydrouracils in the pre-RNA world are discussed.
    Keywords: Chemistry and Materials (General)
    Type: NASA/CR-96-207607 , NAS 1.26:207607 , Biochemistry (ISSN 0006-2960); 35; 1; 315-320
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  • 7
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    Thermal Fatigue Testing of ZrO2-Y2O3 Thermal Barrier Coating Systems using a High Power CO2 Laser (1997)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: In the present study, the mechanisms of fatigue crack initiation and propagation, and of coating failure, under thermal loads that simulate diesel engine conditions, are investigated. The surface cracks initiate early and grow continuously under thermal Low Cycle Fatigue (LCF) and High Cycle Fatigue (HCF) stresses. It is found that, in the absence of interfacial oxidation, the failure associated with LCF is closely related to coating sintering and creep at high temperatures. Significant LCF and HCF interactions have been observed in the thermal fatigue tests. The fatigue crack growth rate in the ceramic coating strongly depends on the characteristic HCF cycle number, N*(sub NCF), which is defined as the number of HCF cycles per LCF cycle. The crack growth rate is increased from 0.36 microns/LCF cycle for a pure LCF test to 2.8 microns/LCF cycle for a combined LCF and HCF test at N*(sub NCF) about 20,000. A surface wedging model has been proposed to account for the HCF crack growth in the coating systems. This mechanism predicts that HCF damage effect increases with increasing surface temperature swing, the thermal expansion coefficient and the elastic modulus of the ceramic coating, as well as with the HCF interacting depth. A good agreement has been found between the analysis and experimental evidence.
    Keywords: Chemistry and Materials (General)
    Type: NASA-TM-107439 , E-10704 , NAS 1.15:107439 , ARL-TR-1354 , Metallurgical Coatings and Thin Films; Apr 21, 1997 - Apr 25, 1997; San Diego, CA; United States
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  • 8
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    Are Polyphosphates or Phosphate Esters Prebiotic Reagents? (1995)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: It is widely held that there was a phosphate compound in prebiotic chemistry that played the role of adenosine triphosphate and that the first living organisms had ribose-phosphate in the backbone of their genetic material. However, there are no known efficient prebiotic synthesis of high-energy phosphates or phosphate esters. We review the occurrence of phosphates in nature, the efficiency of the volcanic synthesis of P4O10, the efficiency of polyphosphate synthesis by heating phosphate minerals under geological conditions, and the use of high-energy organic compounds such as cyanamide or hydrogen cyanide. These are shown to be inefficient processes especially when the hydrolysis of the polyphosphates is taken into account. For example, if a whole atmosphere of methane or carbon monoxide were converted to cyanide which somehow synthesized polyphosphates quantitatively, the polyphosphate concentration in the ocean would still have been insignificant. We also attempted to find more efficient high-energy polymerizing agents by spark discharge syntheses, but without success. There may still be undiscovered robust prebiotic syntheses of polyphosphates, or mechanisms for concentrating them, but we conclude that phosphate esters may not have been constituents of the first genetic material. Phosphoanhydrides are also unlikely as prebiotic energy sources.
    Keywords: Chemistry and Materials (General)
    Type: NASA/CR-95-207605 , NAS 1.26:207605 , Journal of Molecular Evolution; 41; 693-702
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  • 9
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    Prebiotic Synthesis of Diaminopyrimidine and Thiocytosine (1996)
    In:  CASI
    Details
    Publication Date: 2019-08-17
    Description: The reaction of guanidine hydrochloride with cyanoacetaldehyde gives high yields (40-85%) of 2,4-diaminopyrimidine under the concentrated conditions of a drying lagoon model of prebiotic synthesis, in contrast to the low yields previously obtained under more dilute conditions. The prebiotic source of cyanoacetaldehyde, cyanoacetylene, is produced from electric discharges under reducing conditions. The effect of pH and concentration of guanidine hydrochloride on the rate of synthesis and yield of diaminopyrimidine were investigated, as well as the hydrolysis of diaminopyrimidine to cytosine, isocytosine, and uracil. Thiourea also reacts with cyanoacetaldehyde to give 2-thiocytosine, but the pyrimidine yields are much lower than with guanidine hydrochloride or urea. Thiocytosine hydrolyzes to thiouracil and cytosine and then to uracil. This synthesis would have been a significant prebiotic source of 2-thiopyrimidines and 5-substituted derivatives of thiouracil, many of which occur in tRNA. The applicability of these results to the drying lagoon model of prebiotic synthesis was tested by dry-down experiments where dilute solutions of cyanoacetaldehyde, guanidine hydrochloride, and 0.5 M NaCl were evaporated over varying periods of time. The yields of diaminopyrimidine varied from 1 to 7%. These results show that drying lagoons and beaches may have been major sites of prebiotic syntheses.
    Keywords: Chemistry and Materials (General)
    Type: NASA/CR-96-208013 , NAS 1.26:208013 , Journal of Molecular Evolution; 43; 543-550
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  • 10
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    Potentially Prebiotic Syntheses of Condensed Phosphates (1996)
    In:  CASI
    Details
    Publication Date: 2019-08-16
    Description: In view of the importance of a prebiotic source of high energy phosphates, we have investigated a number of potentially prebiotic processes to produce condensed phosphates from orthophosphate and cyclic trimetaphosphate from tripolyphosphate. The reagents investigated include polymerizing nitriles, acid anhydrides, lactones, hexamethylene tetramine and carbon suboxide. A number of these processes give substantial yields of pyrophosphate from orthophosphate and trimetaphosphate from tripolyphosphate. Although these reactions may have been applicable in local areas, they are not sufficiently robust to have been of importance in the prebiotic open ocean.
    Keywords: Chemistry and Materials (General)
    Type: NASA/CR-96-207606 , NAS 1.26:207606 , Origins of Life and Evolution of the Biosphere; 26; 15-25
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