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Project Progress

Task 1. Hydrothermal Organic Chemistry (Cody, Boctor, Brandes, Hazen, Morowitz, Yoder)

Recent theories have proposed that life arose from primitive hydrothermal environments employing chemical reactions 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. There has been no experimental evidence, however, demonstrating that there exists a natural pathway into such a cycle. Toward this end the results of hydrothermal experiments involving citric acid are used as a method of deducing such a pathway. Our 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.

The ability of a group of sulfides containing nickel, cobalt, iron, copper, and zinc to promote a model carbon fixation reaction has been investigated. The assay reaction is the Koch reaction (hydrocarboxylation), where decanoic acid is synthesized via carbonyl insertion of a metal-sulfide-bound nonyl group. We have shown that all of the metal sulfides with the exception of CuS promote hydrocarboxylation. Two other significant reactions include the catalytic reduction of CO to form a surface-bound methyl group, detected after nucleophilic attack by nonane thiol to form methyl nonyl sulfide, and the formation of dinonyl sulfide via a similar reaction. The results of this study support the hypothesis that transition-metal sulfides may have provided useful catalytic functionality for carbon fixation in a pre-biotic world devoid of peptide enzymes.

Task 2. Stability of Organic Compounds at High Pressure (Hemley, H. Scott)

Hemley and NAI Fellow Scott are studying the stability of simple organic compounds under high pressure hydrothermal conditions. The rationale for this study is that other than Earth and perhaps Mars, the only plausible habitats for extant life in our Solar System are at the base of thick H₂O-ice layers (and therefore at moderately high pressure) within icy satellites. Specifically, the three known prerequisites for life are likely present within Jupiter’s Europa and Ganymede: liquid water, energy, and organic species of carbon. For example, Ganymede has an approximately 800-km-thick outer layer of predominantly H₂O-ice, and organic compounds have been observed on its surface. Furthermore, it currently maintains an internally generated magnetic field that is inferred to be produced in a liquid metallic core; this implies central temperatures exceeding 1000°C. However, the base of its H₂O layer is at a pressure of approximately 1.5 GPa; this is far higher than that of the terrestrial hydrothermal vents where life may have originated. Available thermodynamic data cannot accurately address the stability of organic material in hydrothermal environments beyond a pressure of 0.5 GPa.

To investigate this issue the group is using diamond anvil cells (DACs) to compress organic compounds in the presence of H₂O, and in some cases silicates (e.g., CaSiO₃-wollastonite) as well, while simultaneously heating the samples. To date they have concentrated on nCH₂-polyethylene, and an infrared laser was used to heat the sample to temperatures of 1500°C. They observed the formation of methane and graphite from polyethylene in the presence of water (i.e., stability of organic carbon); however, the addition of wollastonite leads to the production of CaCO₃-calcite (i.e., oxidation of organic carbon). These reactions are being currently investigated at lower temperatures by externally heating the DAC. Ultimately the team would like to assess the maximum pressure and temperature conditions for which organic compounds are stable within icy satellites.

Task 3. Amino Acid Synthesis Under Hydrothermal Conditions (Bischoff, Ross, Lemke)

A nagging obstacle to the view that peptides and proteins were created in the early oceans through successive amide-forming condensations of individual amino acids is the 1-2 kcal/mol uphill nature of each step. Thus after just a few additions the cumulative exponential decline in peptide concentration effectively ends the process. A second critical issue is the decarboxylation of amino acids, a highly exoergic process that can compete kinetically with peptide growth and over relatively short time periods irreversibly remove amino acids from the reaction system. On these bases life’s initiation was in the very least a taxing event, and with the view that perhaps the basic precepts of the model were incorrect the following study was conducted: (i) development of a detailed kinetic model for amino acid oligomerization via the currently accepted mechanistic scheme, (ii) experimental work in a gold-bag hydrothermal reactor to develop data for comparison to the model, and (iii) recasting of the mechanistic base for homogeneous oligomerization.
Three key results at 260^ºC/200 bar are summarized here, with gly₂ and gly₃ as the di- and tripeptide of glycine. (1) The rate of overall glycine loss via decarboxylation was far below that anticipated by the model. After 73 hrs the observed glycine level was about 3% of the starting concentration. For the standard model that period corresponds to about 35 half-lives (or a remaining 10^-9 % of the starting glycine). (2) Gly₃ was produced at rates at least two orders of magnitude more rapidly than the standard model predicts. The model, moreover, forecasts a brief growth period followed by a sharp decline in accord with the expected decarboxylation rates. The observed profile showed large growth over a large extended period, with a slow decline then developing, presumably leading to the formation of larger peptides (which at present we are not able to detect). (3) Perhaps the core feature of our current findings is a gly₃/gly₂ ratio that ultimately reached a value of around unity, or about two orders of magnitude greater than predicted by the standard model and the presumed thermochemistry.

These results point to a kinetics and thermochemistry framework governing prebiotic peptide formation considerably more favorable than considered at present. Specifically, both the accepted rate of decarboxylation and the present thermochemical basis for an exponential decline in peptides with growth appear to be incorrect and must be replaced. It is proposed that the known “unzipping” of peptides through loss of cyclic amino acid dimers (DKPs), which is manyfold faster than hydrolysis, be considered in reverse for peptide building. Estimates based on bond additivity show that the reverse reaction (i.e., A_n + DKP = A_n+2) should be downhill by about 1.7 kcal/mol, a value in good agreement with experimental findings. This new interpretation suggests that, while not entirely unproblematic, spontaneous polypeptide formation could be decidedly favored, if not inevitable.