Engineering topics
Publications and source records attributed to Miller, S. L..
Reduction of C60 with rhodium(0) on alumina and hydrogen in deuterated benzene (C6D6) at ambient temperature and pressure yields a mixture of hydrogenated compounds; C60H2 has been characterized as the major product in 14% yield based on 1H NMR.
The work of Bartel and Szostak, in which RNA molecules were selected to enhance the ability to catalyze a reaction similar to a step in protein-catalyzed RNA replication, is discussed. An important aspect of this experiment was the ability to reach a high level of functional organization in ten evolutionary steps. Further steps necessary to obtain an RNA enzyme with RNA replicase activity include performing the reaction with mononucleoside 5'-triphosphates, generalizing the reaction to include a variety of sequences without loss of template-dependent specificity, and overcoming template self-structure that could prevent some regions from being copied efficiently.
A model [Wachtershauser, G. (1988) Microbiol. Rev. 52, 452-484], according to which life started in the form of a monomolecular layer of interacting anionic metabolites electrostatically bound to a positively charged surface, is examined critically. The model raises a number of thermodynamic and kinetic difficulties.
In order to study the production of organic compounds in plasmas (and shocks), various mixtures of N2, CH4, and H2, modeling the atmosphere of Titan, were exposed to discrete sparks, laser-induced plasmas (LIP) and ultraviolet light. The yields of HCN and simple hydrocarbons were measured and compared to those calculated from a simple quenched thermodynamic equilibrium model. The agreement between experiment and theory was fair for HCN and C2H2. However, the yields of C2H6 and other hydrocarbons were much higher than those predicted by the model. Our experiments suggest that photolysis by ultraviolet light from the plasma is an important process in the synthesis. This was confirmed by the photolysis of gas samples exposed to the light, but not to the plasma or shock waves. The results of these experiments demonstrate that, in addition to the well-known efficient synthesis of organic compounds in plasmas, the yields of saturated species, e.g., ethane, may be higher than predicted by theory and that LIP provide a convenient and clean way of simulating planetary lightning and impact plasmas in the laboratory.
The prebiotic synthesis of hydrogen cyanide and formaldehyde was studied by the action of electric discharges on various model primitive atmospheres containing CH4, CO, and CO2. Photochemical production rates would also have been important and were calculated for HCN and H2CO. A reasonable rate of synthesis of amino acids from these sources is about 10 n moles/(sq cm yr) or 0.10 moles/sq cm in 10(exp 7) yrs. This would give a concentration of 3 x 10(exp -4) M in an ocean of the present size (300 liters/sq cm). The amino acids cannot accumulate over a longer period because the entire ocean passes through the 350 C submarine vents in 10(exp 7) yrs, which decomposes all the organic compounds. A number of workers have calculated the influx of comets and meteorites on the primitive earth, both as a destructive process for organic compounds and for any life that was present, as well as a source of organic compounds. Some of the amino acids from the meteorite proposed to have hit the earth 65 x 10(exp 6) yrs ago were detected at the Cretaceous/Tertiary boundary sediments. The problem with proposing a large scale input of organic compounds from meteorites and comets is that they must survive passage through the atmosphere and impact. There are some processes that would allow survival such as showers of centimeter to meter sized meteorites and various aerodynamic braking processes for larger objects. Even if a significant amount of the organic material survived impact, the destructive processes in the hydrothermal vents would remove these compounds on the average in 10(exp 7) yrs or less. If it is assumed that the input rate was sufficient to overcome these destructive processes, then too much carbon and water, especially from comets, would have been added to the surface of the earth. It was concluded 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 only a few percent of that from earth based syntheses.
One of the most important sets of model prebiotic experiments consists of reactions that synthesize complementary oligonucleotides from preformed templates under nonenzymatic conditions. Most of these experiments are conducted at 4 degrees C using 0.01-0.1 M concentrations of activated nucleotide monomer and template (monomer equivalent). In an attempt to extend the conditions under which this type of reaction can occur, we have concentrated the reactants by freezing at -18 degrees C, which is close to the NaCl-H2O eutectic at -21 degrees C. The results from this set of experiments suggest that successful syntheses can occur with poly(C) concentrations as low at 5 x 10(-4) M and 2MeImpG concentrations at 10(-3) M. It was also anticipated that this mechanism might allow the previously unsuccessful poly(A)-directed synthesis of oligo(U)s to occur. However, no template effect was seen with the poly(A) and ImpU system. The failure of these conditions to allow template-directed synthesis of oligo(U)s supports the previously proposed idea that pyrimidines may not have been part of the earliest genetic material. Because of the low concentrations of monomer and template that would be expected from prebiotic syntheses, this lower temperature could be considered a more plausible geologic setting for template-directed synthesis than the standard reaction conditions.
Previous attempts to produce nonenzymatic template-directed oligomerizations of activated pyrimidines on polypurine templates have been unsuccessful. The only efficient reactions are those where the template is composed primarily of pyrimidines, especially cytosine. Because molecular evolution requires that a synthesized daughter polynucleotide be capable of acting as a template for the synthesis of the original polynucleotide, the one-way replication achieved thus far is inadequate to initiate an evolving system. Several uracil analogs were used in this investigation in order to search for possible replacements for uracil. The monomers used in this investigation were the imidazolides of UMP, xanthosine 5'-monophosphate, the bis-monophosphates of the acyclic nucleosides of uracil, and 2,4-quinazolinedione. The concentrations of various salts, buffers, pH, and temperature were among the different variables investigated in attempts to find conditions that would permit template-directed oligomerizations. Although the different monomers in this study demonstrated varying abilities to form very short oligomers, we were unable to detect any enhancement of this oligomerization that could be attributed to the poly(A) template. Although special conditions might be found that would allow purine-rich templates to work, these reactions cannot be considered robust. The results of our experiments suggest that pyrimidines were not part of the original replicating system on the primitive Earth. It has already been shown that ribose is an unlikely component of the first replicating systems, and we now suggest that phosphate was absent as well. This is due to the low solubility of phosphate in the present ocean (3 x 10(-6) M), as well as the difficulty of prebiotic activation of phosphates.
The prebiotic formation of histidine (His) has been accomplished experimentally by the reaction of erythrose with formamidine followed by a Strecker synthesis. In the first step of this reaction sequence, the formation of imidazole-4-acetaldehyde took place by the condensation of erythrose and formamidine, two compounds that are known to be formed under prebiotic conditions. In a second step, the imidazole-4-acetaldehyde was converted to His, without isolation of the reaction products by adding HCN and ammonia to the reaction mixture. LC, HPLC, thermospray liquid chromatography-mass spectrometry, and tandem mass spectrometry were used to identify the product, which was obtained in a yield of 3.5% based on the ratio of His/erythrose. This is a new chemical synthesis of one of the basic amino acids which had not been synthesized prebiotically until now.
The Comet Rendezvous Asteroid Flyby mission is strongly supported by the scientific community. Two major events during the past year have forced a re-analysis of the mission options, with the goal of selecting a baseline mission for launch in 1993. Venus and earth gravity assists can be used to allow rendezvous with comet Tempel 2 in 1996; the scientific potential of the resulting mission is excellent. Although backup mission opportunities have been identified, the significantly longer flight times weaken the scientific appeal of these missions.
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.
NASA's Comet Rendezvous Asteroid Flyby (CRAF) mission to P/Tempel 2 is described with attention given to CRAF spacecraft design. Infrared/visible spectrometers, dust counters, magnetometers, and plasma-wave analyzers are an integral part of the CRAF payload. CRAF's subsystems consist of the following: (1) a structure subsystem, (2) a temperature control subsystem, (3) a propulsion module subsystem, (4) an attitude control subsystem, (5) a command and data subsystem, (6) a radio frequency subsystem, and (7) a power and pyrotechnics subsystem. It is concluded that the CRAF mission will enable the first detailed study of the cometary nucleus.
The recent Voyager flyby of Titan produced evidence for at least nine organic compounds in that atmosphere that are heavier than methane. Several models of Titan's atmosphere, as well as laboratory simulations, suggest the presence of organics considerably more complex that those observed. To ensure that the in situ measurements are definitive with respect to Titan's atmosphere, experiment concepts, and the related instrumentation, must be carefully developed specifically for such a mission. To this end, the possible composition of the environment to be analyzed must be bracketed and model samples must be provided for instrumentation development studies. Laboratory studies to define the optimum flight experiment and sampling strategy for a Titan entry probe are currently being conducted. Titan mixtures are being subjected to a variety of energy sources including high voltage electron from a DC discharge, high current electric shock, and laser detonation. Gaseous and solid products are produced which are then analyzed. Samples from these experiements are also provided to candidate flight experiments as models for instrument development studies. Preliminary results show that existing theoretical models for chemistry in Titan's atmosphere cannot adequetely explain the presence and abundance of all trace gases observed in these experiments.
Prebiotic experiments are usually reported in terms of carbon yields, i.e., the yield of product based on the total carbon in the system. These experiments usually involve a large input of energy and are designed to maximize the yields of product. However, large inputs of energy result in multiple activation of the reactants and products. A more realistic prebiotic experiment is to remove the products of the activation step so they are not exposed a second time to the energy source. This is equivalent to transporting the products synthesized in the primitive atmosphere to the ocean, and thereby protecting them from destruction by atmospheric energy sources. Experiments of this type, using lower inputs of energy, give energy yields (moles of products/joule) which can be used to estimate the relative importance of the different energy sources on the primitive earth. Simulated prebiotic atmospheres containing either CH4, CO or CO2 with N2, H2O and variable amounts of H2 were subjected to a high frequency Tesla coil. Samples of the aqueous phase were taken at various time intervals from 1 hr to 7 days, and the energy yields were obtained by extrapolation to zero time. The samples were analyzed for HCN with the cyanide electrode and for H2CO by chromotropic acid. The spark energy was estimated by calorimetry. The temperature rise in an insulated discharge flask was compared with the temperature rise from a resistance heater in the same flask. These results will be compared with calculated production rates of HCN and H2CO from lightning and a number of photochemical processes on the primitive Earth.
Two methods are described for graphically displaying and numerically analyzing surveys of the celestial sphere as conducted by astronomical satellites. The methods are applicable to the design phase of a mission, for planning during operations, and for post-mission display of results. Both methods have been tested in use, including application to the Infrared Astronomical Satellite (IRAS).
The generally accepted theory for the origin of life on the Earth requires that a large variety of organic compounds be present to form the first living organisms and to provide the energy sources for primitive life either directly or through various fermentation reactions. This can provide a strong constraint on discussions of the formation of the Earth and on the composition of the primitive atmosphere. In order for substantial amounts of organic compounds to have been present on the prebiological Earth, certain conditions must have existed. There is a large body of literature on the prebiotic synthesis of organic compounds in various postulated atmospheres. In this mixture of abiotically synthesized organic compounds, the amino acids are of special interest since they are utilized by modern organisms to synthesize structural materials and a large array of catalytic peptides.
Clathrate hydrates are crystalline compounds in which an expanded ice lattice forms cages that contain gas molecules. There are two principal hydrate structures. Structure I, with a 12 A cubic unit cell, contains 46 water molecules and 8 cages of two types, giving an ideal formula (for CH4) of CH4.5.75H2O. The actual formula contains somewhat more water as the cages are not completely filled. Other examples that form Structure I hydrates are C2H6, C2H4, C2H2, CO2, SO2, OCS, Xe, H2S. Structure II, with a 17 A cubic unit cell, contains 136 water molecules, and 8 large and 16 small cages. The ideal formula for CHCl3 is CHCL3.17H2O. Other examples of Structure II hydrates include C3H8, C2H5Cl, acetone, and tetrahydrofuran. Small molecules such as Ar, Kr and probably N2 and O2 also form a Structure II hydrate. The small molecules occupy both the large and small cages, giving an ideal formula of Ar.5.67H2O. The conditions of pressure and temperature for hydrate formation are discussed.
Yields based on carbon are usually reported in prebiotic experiments, while energy yields (moles/cal) are more useful in estimating the yields of products that would have been obtained from the primitive atmosphere of the earth. Energy yields for the synthesis of HCN and H2CO from a spark discharge were determined for various mixtures of CH4, CO, CO2, H2, H2O, N2 an NH3. The maximum yields of HCN and H2CO from CH4, CO, and CO2 as carbon sources are about 4 x 10 to the -8th moles/cal.
The spark discharge synthesis of amino acids using an atmosphere of CH4+N2+H2O+NH3 has been investigated with variable pNH3. The amino acids produced using higher hydrocarbons (ethane, ethylene, acetylene, propane, butane, and isobutane) instead of CH4 were also investigated. There was considerable range in the absolute yields of amino acids, but the yields relative to glycine (or alpha-amino-n-butyric acid) were more uniform. The relative yields of the C3 to C6 aliphatic alpha-amino acids are nearly the same (with a few exceptions) with all the hydrocarbons. The glycine yields are more variable. The precursors to the C3-C6 aliphatic amino acids seem to be produced in the same process, which is separate from the synthesis of glycine precursors. It may be possible to use these relative yields as a signature for a spark discharge synthesis provided corrections can be made for subsequent decomposition events (e.g. in the Murchison meteorite).