2001 Annual Science Report
Carnegie Institution of Washington Reporting | JUL 2000 – JUN 2001
Hydrothermal Organic Synthesis
Hydrothermal Organic Synthesis (dm)
Task 1. Abiotic Hydrothermal Synthesis
Our approach is to explore the catalytic capabilities of transition-metal sulfides for the promotion of organic reactions that have biochemical utility. Over the past year we have demonstrated that we can synthesize pyruvate under extreme conditions. Specifically, reactions involving aqueous formic acid, FeS, and alkyl thiol promote double carbonyl insertion reactions leading to the formation of alpha-keto acids. Within the product suite we identified carbonlyated organometallic phases. We conclude that the formation of alpha-keto acids proceeds homocatalytically through reaction between the alkyl thiols and the organometallic phases. Within central metabolism, pyruvate lies at the junction of several key reaction pathways, including sugar synthesis, amino acid synthesis, as well as the oxidative and reductive citrate cycles. The synthesis of pyruvate is, therefore, a critically important prebiotic reaction for the emergence of biochemistry.
The facile synthesis of carbonylated FeS clusters is also important for the emergence of primitive biochemistry. First, the formation of such organometallic phases might provide the critical first step for the generation of ferrodoxin based electron networks. Second, the predominant organic metallic phase formed in our reaction is Fe2S2(CO)6R2. The structure of this complex is very similar to the active center (the “H-cluster”) within the enzyme hydrogenase, a ubiquitous catalytic element in the generation of reducing power in anaerobic microorganisms. Note also that hydrogenase obtains the electrons necessary for reduction of a broad range of oxidants through the oxidative decarboxylation of pyruvate.
A second component of our research involves deriving the most likely first pathway for carbon fixation. Recent theories have proposed that life arose from primitive hydrothermal environments employing chemistry analogous to the reductive citrate cycle (RCC) as the primary pathway for carbon fixation. This chemistry is presumed to have developed as a natural consequence of the intrinsic geochemistry of the young, prebiotic Earth. However, there has been no experimental evidence that there exists a natural pathway into such a cycle. We have now demonstrated, experimentally, a viable route for carbon fixation with clear relevance to the origins of life. The path involves the conversion of propene and CO2 up to a tricarboxylic acid, citric acid, via the Koch (hydrocarboxylation) reaction promoted heterocatalytically using NiS in the presence of a source of CO. These results point to a simple hydrothermal redox pathway for citric acid synthesis that may have provided a geochemical ignition point for the reductive citrate cycle.
Task 2. Hydrothermal Chemistry
We are developing techniques to monitor directly hydrothermal chemistry and its effect on biological activity, including the development of experimental procedures to monitor biological activity at extreme physical and chemical conditions. Preliminary experimental results have indicated the viability of biochemical activity at very high pressures, an order of magnitude higher than previously estimated limits on life. These results further expand the concept of what constitutes a “habitable zone,” shed light on whether pressure has any evolutionary component, and provide an experimental procedure for “in-situ” study of life in extreme environments.
Experimental techniques were also developed to monitor directly organic reactions and the effects of fluid phase behavior on their reaction kinetics. Experiments involve direct observation of fluid behavior at extreme conditions of temperature and pressures, an order of magnitude higher pressure than in previous studies. Such studies provide an important contribution to our understanding of the phase behavior of mixed fluids, the availability of fluid species for any biochemical activity, and the formation of any metastable and weakly bonded structures (such as gas hydrates). Studies were also conducted on the stability and structures of gas hydrates, resulting in the determination of two new phases of gas-hydrates at pressures exceeding 10 kbar.
Task 3. Amphiphiles
The synthesis of amphiphilic molecules capable of aqueous self-assembly into membrane-like structures, including bilayers and micelles, is an essential step in the emergence of a protocell. Mechanisms for the prebiotic synthesis and assembly of amphiphiles are thus of considerable interest. We observe amphiphile synthesis from pyruvate in a CO2-water fluid subjected to temperatures from 250â??C to 350â??C and pressures from 0.05 to 0.2 GPa (500 to 2000 atmospheres) for 2 hours. Principal run products include acetic acid and CO2 (from decarbonation of pyruvate) and methyl succinate (from dimerization of pyruvate and subsequent decarboxylation). We also observe up to 50% conversion of pyruvate to a water-insoluble, yellow-brown, strongly aromatic oily residue. Analysis by GCMS reveals this material to be a complex suite, including cyclic aromatic compounds, presumably formed by polymerization and subsequent cycloaddition reactions. We characterized this material by extracting and analyzing the chloroform-soluble fraction. We examined this material by 2D TLC chromatography, which revealed a pattern of seven distinct fluorescent regions â?? a pattern that is qualitatively similar to that observed for amphiphilic components of the Murchison meteorite. Each of these seven TLC areas was extracted, dried, washed in a phosphate buffer (pH = 8.5), and examined by fluorescence microscopy. One of these regions consisted of a significant fraction of surface-active molecules, which form apparent monomolecular films at air-water interfaces. When placed in the phosphate-buffered aqueous solution, these molecules also organize into fluorescent, membranous vesicle-like structures. We conclude that hydrothermal processes, perhaps similar to those that occurred on the Murchison parent body, may lead to the production of amphiphilic, membrane-forming molecules.
Task 4. Amino Acid Synthesis Under Hydrothermal Conditions
We are building on our earlier work on the synthesis of amino acids from H2CO and HCN on various common mineral surfaces. We have recently considered the question of an endogenous source of amino acids, and we have carried out calculations showing that a primitive CO2/N2 atmosphere in equilibrium with water and an FMQ (fayalite/magnetite/quartz) redox buffer could not have yielded sensible quantities of H2CO and HCN for amino acid formation. The calculations show that the expected aqueous species are ammonium and formate, at concentrations well into the mM range for CO2 = N2 = 1 bar.
We thus conclude that an endogenous basis for life must be based upon those ions. Since C-C bond formation is a necessary feature of early life-related chemistry, formate as the principal reactant in Fischer-Tropsch-synthesis (FTS) is an attractive consideration. The FTS variant described by Koelbel and Trapper satisfies another critical element of the early chemistry â?? C-N bond formation â?? and it can be shown further that at temperatures below 100â??C an FMQ-controlled aqueous system provides hydrogen fugacities sufficient to produce quantities of glycine > 1 M via
2 CO2 + 0.5 N2(g) + 4.5 H2(g) H2NCH2CO2H + 2 H 2O
This approach is not without its challenges, however. The third essential ingredient of the Earth’s early organic chemistry must be a process providing branching and polyfunctionality (i.e., routes leading to both carboxyl and amino substitution). Here a Fischer-Tropsch approach must be carefully appraised since conventional FTS yields solely alkanes or monofunctionalized alkanes.
Our current activities consider the likelihood that the elevated temperatures in conventional FTS trigger secondary, high-activation-energy reactions that eliminate initial polyfunctionality. These reactions would vanish kinetically at lower temperatures, and our current efforts include studies seeking such products in FTS at lower temperatures.
Task 5. Nitrogen Fixation (dm)
The reduction of nitrite and nitrate to ammonia is an important step in the prebiotic synthesis of ammonia. We have investigated the hydrothermal formation of ammonia at 300â??C and 50 MPa in the presence of a variety of transition metal oxide and sulfide minerals. We find that nitrate reduction occurs readily in the presence of most of these minerals. This result suggests that hydrothermal systems produced significant quantities of ammonia in the prebiotic oceans.
PROJECT MEMBERS:James Bischoff
RELATED OBJECTIVES:Objective 1.0
Determine whether the atmosphere of the early Earth, hydrothermal systems or exogenous matter were significant sources of organic matter.
Develop and test plausible pathways by which ancient counterparts of membrane systems, proteins and nucleic acids were synthesized from simpler precursors and assembled into protocells.
Identify the environmental limits for life by examining biological adaptations to extremes in environmental conditions.
Search for evidence of ancient climates, extinct life and potential habitats for extant life on Mars.
Determine the presence of life's chemical precursors and potential habitats for life in the outer solar system.
Determine (theoretically and empirically) the ultimate outcome of the planet-forming process around other stars, especially the habitable ones.
Define an array of astronomically detectable spectroscopic features that indicate habitable conditions and/or the presence of life on an extrasolar planet.