Abiotic organic synthesis relevant to Earth’s Crust
and possible the origins of life
Research Support: NASA Exobiology and NASA Astrobiology
Background: The hypothetical path from the initial abiotic world to Earth’s first biology has been described (in purely general terms) as passing through several key stages (e.g. de Duve, 1991). In a condensed format, such a path starts with proto-metabolism (with an emphasis on abiotic nucleotide synthesis), leads into RNA replication (the nascent stages of “RNA world”), and eventually achieves encapsulation (the first membrane). The development of RNA dependent peptide synthesis, followed by the final development of translation, yields the “pre”-biotic system poised to initiate biological metabolism and herald in Earth’s first life (de Duve, 1991). Although, over the past three decades there has been substantial progress made in research covering many of these path segments, for example in the area of RNA catalysis, there remains considerable uncertainty regarding how organic chemistry intrinsic to a primitive terrestrial planet segued into the initiation of the RNA world.
What we do: Recently we have set out to establish, experimentally, whether transition metal catalyzed reactions in the presence of water could provide potentially useful protometabolic chemistry. Our early efforts focused on primitive carbon fixation pathways. We have found that the all but one of the transition metal sulfides studied thus far promote reactions that mimic the key intermediate steps of the critical enzyme complex acetyl-CoA synthase operating at the core of the anabolic metabolism of acetogens and methanogens (Cody et al. 2004).
Figure (above): The yield of decanoic acid from nonane thiol and formic acid under high pressure (200 Mpa) moderate temperature (250 °C) in the presence of a range of transition metal sulfides. Note that maximum yield in the presence of NiS (millerite) is on the order of a 30 % yield based on the starting amount of nonane thiol. See Cody et al. (2004) GCA.
We have further shown that metal sulfide catalysis can promote a series of hydrocarboxylation reactions that highlight a potential reaction pathway leading up from propene to hydroaconitic acid; i.e. an abiotic carbon fixation pathway that ends within two electrons and one water molecule away from citric acid (Cody et al. GCA, 2001).
Figure (above): The principal hydrothermal decomposition pathways (a and b) of citric acid. In the presence of a transition metal sulfide (NiS) catalyst and a reduced CO2 bearing fluid, we have identified a plausible pathway (the b’ pathway above) for the formation of citric acid that follows a route that differs completely from extant metabolic pathways (akin to reversing the a pathway) (see Cody et al. 2001 GCA).
The potential of such such a pathway is compelling as it is well known that under hydrothermal reactions citric acid can be a good source for pyruvic acid and possibly oxalacetic acid (although this compound has never been observed due to its extreme thermal instability). Pyruvic acid is a very useful prebiotic product, for example, in the presence of NH4+ a simple reductive amination reaction provides a ready source of alanine (e.g. Brandes et al. 1999).
Figure (above): The prebiotic pathway based on our current hydrothermal abiotic reactions. Whereas this complete cycle has not been completed as a “one-pot” reaction, each step has been demonstrated experimentally. The pathway moving off citric acid is under study.
Currently, we are exploring potential hydrothermal prebiotic pathways towards pyrimidine and purine synthesis, starting from various branch points in our previously defined reaction network.
How we do it:
Please go to High Pressure Organic Chemistry page.
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