A way to thioacetate esters compatible with non-oxidative prebiotic conditions

Prebiotic formation of thioesters via cyclic anhydrides as a key step in the emergence of metabolism

Thioesters are high-energy derivatives of carboxylic acids that are essential in the functioning of today's living cells. Their central role argues in favor of their early introduction in the abiotic reaction network which led to the emergence of life on Earth. We propose that the first thioesters appeared during the establishment of the reverse tricarboxylic acid (rTCA) cycle, an effective metabolic cycle for the synthesis of organic molecules from CO2. Most of the acids in this cycle are 1,4-diacids. We show that the formation of a cyclic anhydride from aqueous solutions of succinic or citric acid is possible using drying conditions over silica, as it could happen in an evaporating pond. When these 1,4-diacids are dried in the presence of thiols, thioesters are obtained. Our experimental and theoretical results demonstrate that analogs of succinyl-CoA and citryl-CoA, thioesters from the rTCA cycle, can be produced. Such a process highlights the importance of 1,4-diacids, which would have been introduced in the metabolism then under construction because of their ability to form anhydrides and to be activated in the absence of triphosphates or of any other activating agent. At its beginning, the rTCA cycle should therefore be interpreted mainly as a "1,4-diacid cycle".

Rapid hydrolysis rates of thio- and phosphate esters constrain the origin of metabolism to cool, acidic to neutral environments

Universal to all life is a reliance on energy carriers such as adenosine triphosphate (ATP) which connect energy-releasing reactions to energy-consuming processes. While ATP is ubiquitously used today, simpler molecules such as thioesters and polyphosphates are hypothesized to be primordial energy carriers. Investigating environmental constraints on the non-enzymatic emergence of metabolism, we find that hydrolysis rates-not hydrolysis energies-differentiate phosphate esters and thioesters. At temperatures consistent with thermophilic microbes, thioesters are favored at acidic pH and phosphate esters at basic pH. Thioacids have a high stability across pH 5-10. The planetary availability of sulfur and phosphate is coincident with these calculations, with phosphate being abundant in alkaline and sulfur in acidic environments. Since both sulfur esters and phosphate esters are uniquely required in metabolism, our results point to a non-thermophilic origin of early metabolism at cool, acidic to neutral environments.

Catalysis before Enzymes: Thiol-Rich Peptides as Molecular Diversity Providers on the Early Earth

The multiplicity of simple molecules available on the primitive Earth probably made possible the development of extremely diverse prebiotic chemistry. The importance of thiols is widely recognized in the community studying the origin of life. De Duve’s “thioester world” has been considered a major contribution in this regard, where thioester bonds have high energies and thus can contribute to several chemical reactions. Herein, we propose specific models of thiols that exhibit unique activities toward several chemical reactions. Thanks to aminothiol and aminonitrile behaviors, we were able to obtain thiol-rich peptides with interesting catalytic activities leading to the formation of structurally diverse molecules. In a broader context, such chemistry could be introduced into systems chemistry scenarios in which it would be associated with the chemistry of nucleic acids or their precursors, as well as that of fatty acids.

Preliminary Free Energy Map of Prebiotic Compounds Formed from CO2, H2 and H2S

What kinds of CHOS compounds might be formed in a prebiotic milieu by reducing CO2 in the presence of H2 and H2S? How might the presence of sulfur influence the chemical composition of the mixture? We explore these questions by using first-principles quantum chemistry to calculate the free energies of CHOS compounds in aqueous solution, by first generating a thermodynamic map of one- and two-carbon species. We find that while thiols are thermodynamically favored, thioesters, thioacids, and thiones are less favorable than their non-sulfur counterparts. We then focus on the key role played by mercaptoacetaldehyde in sulfur analogs of the autocatalytic formose reaction, whereby the thiol group introduces asymmetry and potential thermodynamic selectivity of some compounds over others.