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NASA NTRS2001

Thiols in Hydrothermal Solution: Standard Partial Molal Properties and Their Role in the Organic Geochemistry of Hydrothermal Environments

Modern seafloor hydrothermal systems are locations where great varieties of geochemistry occur due to the enormous disequilibrium between vent fluids and seawater. The disequilibrium geochemistry has been hypothesized to include reactions to synthesize organic compounds. Despite the incomplete understanding of the carbon budget in hydrothermal systems, the organic geochemistry of these sites has received little attention. Experimental simulations of these environments, however, indicate that organic compounds may have difficulty forming in a purely aqueous environment. On the other hand, thiols, thioesters and disulfides have been implicated as reaction intermediates between CO or CO2 in experiments of carbon reduction in hydrothermal environments, as well as in a variety of biological processes and other abiotic reactions. The reduction of CO2 to thesis, for example, is observed using the FeS-H2S/FeS2 couple to provide the reducing power. We have used recent advances in theoretical geochemistry to estimate the standard partial moral thermodynamic properties and parameters for the revised Helgeson-Kirkham-Flowers equation of state for aqueous straight-chain alkyl thesis. With these data and parameters we have evaluated the role that organic sulfur compounds may play as reaction intermediates during organic compound synthesis. We conclude that organic sulfur compounds may hold the key to the organic chemistry leading to the origin of life in hydrothermal settings. These results may also explain the presence of sulfur in a number of biomolecules present in ancient thermophilic microorganisms.

Schulte, Mitchell D.↗
NASA NTRS1987

The evolution of glutathione metabolism in phototrophic microorganisms

Of the many roles ascribed to glutathione (GSH) the one most clearly established is its role in the protection of higher eucaryotes against oxygen toxicity through destruction of thiol-reactive oxygen byproducts. If this is the primary function of GSH then GSH metabolism should have evolved during or after the evolution of oxygenic photosynthesis. That many bacteria do not produce GSH is consistent with this view. In the present study we have examined the low-molecular-weight thiol composition of a variety of phototrophic microorganisms to ascertain how evolution of GSH production is related to evolution of oxygenic photosynthesis. Cells were extracted in the presence of monobromobimane (mBBr) to convert thiols to fluorescent derivatives, which were analyzed by high-pressure liquid chromatography. Significant levels of GSH were not found in the green bacteria (Chlorobium thiosulfatophilum and Chloroflexus aurantiacus). Substantial levels of GSH were present in the purple bacteria (Chromatium vinosum, Rhodospirillum rubrum, Rhodobacter sphaeroides, and Rhodocyclus gelatinosa), the cyanobacteria [Anacystis nidulans, Microcoleus chthonoplastes S.G., Nostoc muscorum, Oscillatoria amphigranulata, Oscillatoria limnetica, Oscillatoria sp. (Stinky Spring, Utah), Oscillatoria terebriformis, Plectonema boryanum, and Synechococcus lividus], and eucaryotic algae (Chlorella pyrenoidsa, Chlorella vulgaris, Euglena gracilis, Scenedesmus obliquus, and Chlamydomonas reinhardtii). Other thiols measured included cysteine, gamma-glutamylcysteine, thiosulfate, coenzyme A, and sulfide; several unidentified thiols were also detected. Many of the organisms examined also exhibited a marked ability to reduce mBBr to syn-(methyl,methyl)bimane, an ability that was quenched by treatment with 2-pyridyl disulfide or 5,5'-bisdithio-(2-nitrobenzoic acid) prior to reaction with mBBr. These observations indicate the presence of a reducing system capable of electron transfer to mBBr and reduction of reactive disulfides. The distribution of GSH in phototrophic eubacteria indicates that GSH synthesis evolved at or around the time that oxygenic photosynthesis evolved.

NASA Discipline Exobiology↗
NASA NTRS1984

Prebiotic formation of 'energy-rich' thioesters from glyceraldehyde and N-acetylcysteine

The 'energy-rich' thioester, N-acetyl-S-lactoylcysteine, is formed from low concentrations of glyceraldehyde and N-acetylcysteine under anaerobic conditions at ambient temperature in aqueous solutions of sodium phosphate (pH 7.0). Reactions with 2mM glyceraldehyde, 2mM N-acetylcysteine, and 500 mM sodium phosphate (pH 7.0) convert about 0.3 percent/day of the glyceraldehyde to lactoyl thioester. The formation of lactoyl thioester in similar reactions with 500 mM imidazole hydrochloride (pH 7.0) is supported by the thiol-dependence of lactate formation, which is 3-fold greater in the presence of thiol (0.11 percent/day) than in the absence of thiol (0.04 percent/day). The formation of lactoly thioester is thought to proceed by the phosphate (or imidazole)-catalyzed dehydration of glyceraldehyde, which adds to the thiol to form a hemithioacetal that rearranges to the thioester. A limited amount of a second thioester, N-acetyl-S-glyceroyl-cysteine, is also formed at the beginning of these reactions. The significance of these reactions to the origin of life is discussed.

Weber, A. L.↗
NASA NTRS2001

The Biogeochemistry of Sulfur in Hydrothermal Systems

The incorporation of sulfur into many biomolecules likely dates back to the development of the earliest metabolic strategies. Sulfur is common in enzymes and co-enzymes and is an indispensable structural component in many peptides and proteins. Early metabolism may have been heavily influenced by the abundance of sulfide minerals in hydrothermal systems. The incorporation of sulfur into many biomolecules likely dates back to the development of the earliest metabolic strategies. Sulfur is common in enzymes and co-enzymes and is an indispensable structural component in many peptides and proteins. Early metabolism may have been heavily influenced by the abundance of sulfide minerals in hydrothermal systems. Understanding how sulfur became prevalent in biochemical processes and many biomolecules requires knowledge of the reaction properties of sulfur-bearing compounds. We have previously estimated thermodynamic data for thiols, the simplest organic sulfur compounds, at elevated temperatures and pressures. If life began in hydrothermal environments, it is especially important to understand reactions at elevated temperatures among sulfur-bearing compounds and other organic molecules essential for the origin and persistence of life. Here we examine reactions that may have formed amino acids with thiols as reaction intermediates in hypothetical early Earth hydrothermal environments. (There are two amino acids, cysteine and methionine, that contain sulfur.) Our calculations suggest that significant amounts of some amino acids were produced in early Earth hydrothermal fluids, given reasonable concentrations H2, NH3, H2S and CO. For example, preliminary results indicate that glycine activities as high as 1 mmol can be reached in these systems at 100 C. Alanine formation from propanethiol is also a favorable reaction. On the other hand, the calculated equilibrium log activities of cysteine and serine from propanethiol are -21 and -19, respectively, at 100 C. These results indicate that while amino acid formation with thiols as intermediates is favored in some cases, other mechanisms may have been necessary to produce significant amounts of other amino acids. Coupled with our previous results for thiols, these studies imply that sulfur may have been easily incorporated into the organic geochemistry of early Earth hydrothermal systems, leading to its widespread use in biomolecules. Formation of more complex biomolecules in hydrothermal systems may have required sulfur-bearing organic compounds as reaction intermediates.

Schulte, Mitchell↗
NASA NTRS1981

The effects of organosulfur compounds upon the storage stability of Jet A fuel

This study examined the effect of sulfur-containing compounds on the storage stability of Jet A turbine fuel. It was found that alkyl sulfides and disulfides increased the fuel's stability while all thiols and thiophene derivatives tested decreased fuel stability (increased deposit formation) at temperatures and sulfur concentrations selected. Linear Arrhenius plots of sulfur-spiked fuel samples demonstrated that deposit formation decreased with increased slope for all alkyl sulfides, alkyl disulfides, thiols, and thiophene derivatives. A plot of insoluble deposit vs. concentration of added alkyl sulfide produces a negative slope. It appears that the inhibiting mechanism for alkyl sulfides is a result of the compound's reactivity with intermediate soluble precursors to deposit in the fuel. A method of approximating the relative basicity of weak organosulfur bases was developed via measurement of their resonance chemical shifts in proton NMR. Linear plots of log gm. deposit vs. change in chemical shift (shift differences between sulfur bases neat and complexed with I2) were found for alkyl sulfides and alkyl thiols. This suggests the possibility that increased deposit formation is due to base catalysis with these compound classes.

Heneman, F. C.↗
NASA NTRS1985

Alanine synthesis from glyceraldehyde and ammonium ion in aqueous solution

The formation of alanine (ala) form C(14)-glyceraldehyde and ammonium phosphate in the presence or absence of a thiol is reported. At ambient temperature, ala synthesis was six times more rapid in the presence of 3-mercaptopropionic acid than in its absence (0.6 and 0.1 percent, respectively, after 60 days). Similarly, the presence of another thiol, N-acetylcysteinate, increased the production of ala, as well as of lactate. The reaction pathway of thiol-catalyzed synthesis of ala, with the lactic acid formed in a bypath, is suggested. In this, dehydration of glyceraldehyde is followed by the formation of hemithioacetal. In the presence of ammonia, an imine is formed, which eventually yields ala. This pathway is consistent with the observation that the rate ratio of ala/lactate remains constant throughout the process. The fact that the reaction takes place under anaerobic conditions in the presence of H2O and with the low concentrations of simple substrates and catalysts makes it an attractive model prebiotic reaction in the process of molecular evolution.

Weber, A. L.↗
NASA NTRS1988

The evolution of glutathione metabolism in phototrophic microorganisms

The low molecular weight thiol composition of a variety of phototropic microorganisms is examined in order to ascertain how evolution of glutathione (GSH) production is related to the evolution of oxygenic photosynthesis. Cells were extracted in the presence of monobromobimane (mBBr) to convert thiols (RSH) to fluorescent derivatives (RSmB) which were analyzed by high performance liquid chromatography (HPLC). Significant levels of GSH were not found in green sulfur bacteria. Substantial levels were present in purple bacteria, cyanobacteria, and eukaryotic algae. Other thiols measured included cysteine, gamma-glutamylcysteine, thiosulfate, coenzyme A, and sulfide. Many of the organisms also exhibited a marked ability to reduce mBBr to syn-(methyl,methyl)bimane, an ability which was quenched by treatment with 2-pyridyl disulfide or 5,5 prime-bisdithio - (2-nitrobenzoic acid) prior to reaction with mBBr. These observations indicate the presence of a reducing system capable of electron transfer to mBBr and reduction of reactive disulfides. The distribution of GSH in phototropic eubacteria indicates that GSH synthesis evolved at or around the time that oxygenic photosynthesis evolved.

Fahey, Robert C.↗
NASA NTRS2000

Radioprotective Thiolamines WR-1065 and WR-33278 Selectively Denature Nonhistone Nuclear Proteins

Differential scanning calorimetry was used to study the interactions of nuclei isolated from Chinese hamster V79 cells with the radioprotector WR-1065, other thiol compounds, and polyamines. Differential scanning calorimetry monitors denaturation of macromolecules and resolves the major nuclear components (e.g. constrained and relaxed DNA, nucleosome core, and nuclear matrix) of intact nuclei on the basis of thermal stability. WR-1065 treatment (0.5-10 mM) of isolated nuclei led to the irreversible denaturation of nuclear proteins, a fraction of which are nuclear matrix proteins. Denaturation of 50% of the total nonhistone nuclear protein content of isolated nuclei occurred after exposure to 4.7 mM WR-1065 for 20 min at 23 C. In addition, a 22% increase in the insoluble protein content of nuclei isolated from V79 cells that had been treated with 4 mM WR-1065 for 30 min at 37 C was observed, indicating that WR-1065-induced protein denaturation occurs not only in isolated nuclei but also in the nuclei of intact cells. From the extent of the increase in insoluble protein in the nucleus, protein denaturation by WR-1065 is expected to contribute to drug toxicity at concentrations greater than approximately 4 mM. WR-33278, the disulfide form of WR1065, was approximately twice as effective as the free thiol at denaturing nuclear proteins. The proposed mechanism for nucleoprotein denaturation is through direct interactions with protein cysteine groups with the formation of destabilizing protein-WR-1065 disulfides. In comparison to its effect on nuclear proteins in isolated nuclei, WR-1065 had only a very small effect on non-nuclear proteins of whole cells, isolated nuclear matrix, or the thiol-rich Ca (2+) ATPase of sarcoplasmic reticulum, indicating that WR-1065 can effectively denature protein only inside an intact nucleus, probably due to the increased concentration of the positively charged drug in the vicinity of DNA.

Booth, Valerie K.↗
NASA NTRS2000

Thermodynamics of Aqueous Organic Sulfur Compounds: A Key to the Organic Geochemistry of Hydrothermal Systems?

Hydrothermal environments are locations of varied geochemistry due to the disequilibrium between vent fluids and seawater. The disequilibrium geochemistry has been hypothesized to include reactions to synthesize organic compounds. Observations of the organic geochemistry of hydrothermal vent sites has received little attention. Experimental simulations of these environments, however, indicate that organic compounds may have difficulty forming in a purely aqueous environment. On the other hand, thiols. thioesters and disulfides have been implicated as reaction intermediates between CO or CO2 in experiments of carbon reduction in hydrothermal environments as well as in a variety of biological processes and other abiotic reactions (Wachtershauser, 1990, OLEB 20, 173; Heinen and Lauwers, 1996, OLEB 26, 13 1, Huber and Wachtershauser, 1997, Science 276, 245; Russell et al., 1998, in Thermophiles: The keys to molecular evolution and the origin of life?). The reduction of CO2 to thiols, for example, is observed using the FeS-H2S/FeS2 couple to provide the reducing power (see Schoonen et al., 1999, OLEB 29, 5). In addition, the enzyme involved in final stage of methanogenesis, coenzyme-M, is itself a thiol. Thus, organic sulfur compounds may hold the key to the organic chemistry leading to the origin of life at high temperatures. Understanding the biochemical processes of microorganisms that can live to temperatures at least as high as 113 C (Blochl et al., 1996, Extremophiles 1, 14) requires knowledge of the properties of the chemical reactions involved. In order to assess the role of aqueous organic sulfur compounds in hydrothermal organic geochemistry, we have been attempting to determine their thermodynamic properties. We have culled the literature to obtain the properties of organic sulfur compounds. We are able to calculate a number of essential properties, such as free energies of formation, from solubility data available in the literature together with standard properties of organic sulfur gases. However, a number of the properties for aqueous organic sulfur compounds have not been experimentally determined. Furthermore, most of thermodynamic data that are available are for 25 C and 1 bar. In order to determine reaction properties to temperatures and pressures appropriate to the hydrothermal conditions in which thermophilic organisms actually live, we use equations of state developed by Helgeson and co-workers (Helgeson et al., 1981, AJS 281, 1249). A key piece of information needed to go up in temperature is the partial molal heat capacity, which is one of the properties for which experimental data are unavailable for nearly all organic sulfur compounds. We have used correlation methods to determine the partial molal heat capacities and volumes of many organic solutes. These estimates allow us to asses the role of organic sulfur compounds during the reduction of carbon in hydrothermal settings. We will present these data, along with examples of the thermodynamic properties of reactions involving aqueous organic sulfur compounds.

Schulte, Mitchell↗
NASA NTRS2005

Metabolic Reactions among Organic Sulfur Compounds

Sulfur is central to the metabolisms of many organisms that inhabit extreme environments. Numerous authors have addressed the energy available from a variety of inorganic sulfur redox pairs. Less attention has been paid, however, to the energy required or gained from metabolic reactions among organic sulfur compounds. Work in this area has focused on the oxidation of alkyl sulfide or disulfide to thiol and formaldehyde, e.g. (CH3)2S + H2O yields CH3SH + HCHO + H2, eventually resulting in the formation of CO2 and SO4(-2). It is also found that reactions among thiols and disulfides may help control redox disequilibria between the cytoplasm and the periplasm. Building on our earlier efforts for thiols, we have compiled and estimated thermodynamic properties for alkyl sulfides. We are investigating metabolic reactions among various sulfur compounds in a variety of extreme environments, ranging from sea floor hydrothermal systems to organic-rich sludge. Using thermodynamic data and the revised HKF equation of state, along with constraints imposed by the geochemical environments sulfur-metabolizing organisms inhabit, we are able to calculate the amount of energy available to these organisms.

Schulte, M.↗
NASA NTRS1986

Oligoglyceric acid synthesis by autocondensation of glyceroyl thioester

The autocondensation of the glyceroyl thioester, S-glyceroyl-ethane-thiol, yielded olioglyceric acid. The rates of autocondensation and hydrolysis of the thioester increased from pH 6.5 to pH 7.5 in 2,6-lutidine and imidazole buffers. Autocondensation and hydrolysis were much more rapid in imidazole buffers as compared to 2,6-lutidine and phosphate buffers. The efficiency of ester bond synthesis was about 20% for 40 mM S-glyceroyl-ethane-thiol in 2,6-lutidine and imidazole buffers near neutral pH. The size and yield of the olioglyceric acid products increased when the concentration of the thioester was increased. The relationship of these results to prebiotic polymer synthesis is discussed.

Weber, A. L.↗
NASA NTRS1987

Oligoglyceric acid synthesis by autocondensation of glyceroyl thioester

The autocondensation of the glyceroyl thioester, S-glyceroyl-ethane-thiol, yielded olioglyceric acid. The rates of autocondensation and hydrolysis of the thioester increased from pH 6.5 to pH 7.5 in 2,6-lutidine and imidazole buffers. Autocondensation and hydrolysis were much more rapid in imidazole buffers as compared to 2,6-lutidine and phosphate buffers. The efficiency of ester bond synthesis was about 20 percent for 40 mM S-glyceroyl-ethane-thiol in 2,6-lutidine and imidazole buffers near neutral pH. The size and yield of the olioglyceric acid products increased when the concentration of the thioester was increased. The relationship of these results to prebiotic polymer synthesis is discussed.

Weber, Arthur L.↗
NASA NTRS1997

Redox Energy and Sulfur Chemistry in Prebiotic Polymer Synthesis and Replication

In the past year we have made significant progress in three research areas: (1) Most importantly, we discovered a new pathway of prebiotic amino acid synthesis in which formaldehyde and glycolaldehyde (substrates of the formose reaction) react with ammonia yielding alanine and homoserine in the presence of thiol catalysts. This thiol-dependent synthesis of amino acids undoubtedly occurs via amino acid thioester intermediates capable of forming peptides. This 'one-pot' reaction system operates under mild aqueous conditions, and like modern amino acid biosynthesis, uses sugar intermediates which are converted to amino acids by energy-yielding redox disproportionation. (2) Finally, in preparation for the analysis of Martian meteorite samples, we upgraded our HPLC system and developed an improved method capable of detecting a I femtomole of amino acid enantiomers. (3) We completed our analysis of the energetics of metabolism that revealed that life depends on biosynthetic processes driven by chemical energy made available by the redox disproportionation of carbon groups of sugars. We established that the favorable energy of redox disproportionation is based on the universal reduction potentials of carbon groups. We concluded that it is hard to imagine any other organic molecule besides sugars (formaldehyde oligomers) having the energy and reactivity needed to drive either modem biosynthesis or the chemical processes behind its origin.

Weber, Arthur L.↗
NASA NTRS1995

Investigation of the Prebiotic Synthesis of Amino Acids and RNA Bases from CO2 Using FeS/H2S As a Reducing Agent

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.

Keefe, Anthony D.↗
NASA NTRS1995

Investigation of the Prebiotic Synthesis of Amino Acids and RNA Bases from CO2 using FeS/H2S as a Reducing Agent

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, purines, 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.

Keefe, Anthony D.↗
NASA NTRS1998

Prebiotic Polymer Synthesis and the Origin of Glycolytic Metabolism

Our research resulted in several discoveries which contributed to understanding the origin and operation of life. (1) Most importantly, we discovered a new pathway of prebiotic amino acid synthesis in which formaldehyde and glycolaldehyde (formose reaction substrates) react with ammonia to give alanine and homoserine in the presence of thiol catalysts. The thiol-dependent synthesis of amino acids undoubtedly occurs via amino acid thioester intermediates capable of forming peptides. This 'one-pot' reaction system operates under mild aqueous conditions, and like modern amino acid biosynthesis, uses sugar intermediates which are converted to amino acids by energy-yielding redox disproportionation. Preliminary evidence suggests that this type of process can be "evolved" by a serial transfer methods that lead to enrichment of autocatalytic molecules. (2) We established that prebiotic peptide polymers can be made by condensation of amino acid thioesters (homocysteine thiolactone and S-(N-beta-orotidyl- diaminopropionic acid) ethanethiol), and that prebiotic polydisulfide polymers can be generated by oxidation of dithiols with iron(III) in minerals. (3) In our analysis of metabolism we discovered the primary energy source of biosynthesis -- chemical energy made available by the redox disproportionation of substrate carbon groups. We concluded that the energy and reactivity of sugars make them the optimal substrate for the origin and operation of terrestrial (or extraterrestrial) life. (4) Since it is likely that the use of optimal sugar substrates in biosynthesis sets the average oxidation number of functional biocarbon throughout the Universe near 0.0 (the reduction level of formaldehyde), we proposed that a line(s) in the microwave spectrum of formaldehyde could be rationally selected as a frequency for interstellar communication that symbolizes life. (5) Finally, in preparation for the analysis of Martian meteorite samples, we upgraded our HPLC system to one femtomole sensitivity, and developed a new electrophoretic method of sample preparation for HPLC analysis of the meteoritic amino acids. In a sample of the KT boundary layer from Sussex Wyoming, we found about 300 picomoles per gram of meteoritic alpha-aminoisobutyric acid per gram of KT layer.

Weber, Arthur L.↗