The Abiotic Chemistry of Thiolated Acetate Derivatives and the Origin of Life.

Kuhan Chandru,Masashi Aono,H James Cleaves,Christopher Butch,Alexis Gilbert

doi:10.1038/srep29883

Abstract

Thioesters and thioacetic acid (TAA) have been invoked as key reagents for the origin of life as activated forms of acetate analogous to acetyl-CoA. These species could have served as high-energy group-transfer reagents and allowed carbon insertions to form higher molecular weight compounds such as pyruvate. The apparent antiquity of the Wood-Ljungdahl CO2 fixation pathway and its presence in organisms which inhabit hydrothermal (HT) environments has also led to suggestions that there may be a connection between the abiotic chemistry of compounds similar to TAA and the origins of metabolism. These compounds’ apparent chemical simplicity has made their prebiotic availability assumed, however, although the kinetic behavior and thermochemical properties of TAA and analogous esters have been preliminarily explored in other contexts, the geochemical relevance of these compounds merits further evaluation. Therefore, the chemical behavior of the simplest thiolated acetic acid derivatives, TAA and methylthioacetate (MTA) were explored here. Using laboratory measurements, literature data, and thermochemical models, we examine the plausibility of the accumulation of these compounds in various geological settings. Due to the high free energy change of their hydrolysis and corresponding low equilibrium constants, it is unlikely that these species could have accumulated abiotically to any significant extant.

Highlights

Several models for the origin of life invoke reactions occurring in the vicinity of volcanic or HT vent systems[1,2,3]
Several origin of life scenarios hypothesize that thioacetic acid (TAA), MTA or other thioacetate derivatives could have acted as acetyl-CoA analogues, donating acetate groups for organosynthesis in a primitive reverse tricarboxylic acid(rTCA)-like cycle[2,6,18,19] or as activating agents for carboxyl groups for transacylation reactions[7,10,20]
TAA or MTA hydrolysis leads to the formation of acetic acid (CH3COOH or AcOH) and H2S or CH3SH (Equation 1): CH3COSR + H2O CH3COOH + HSR (R = H or CH3)

Summary

Introduction

Several models for the origin of life invoke reactions occurring in the vicinity of volcanic or HT vent systems[1,2,3]. High-energy compounds such as CO, COS, thioacids (R-COSH) and thioesters (R-COS-R′) are central to many of these schemes as feedstocks for primitive self-complexifying chemical cycles and the polymerization of biomolecules[4,5,6,7,8,9,10,11,12]. These authors concluded that near pH 7 and 25 °C, given sufficiently high initial concentrations of reactants, exchange between thioesters can compete with hydrolysis This analysis, ignores questions of the sources of these compounds, and important aspects of the abiotic geochemistry of TAA, MTA and their homologs warrant detailed consideration. As TAA and thioesters are “high-energy” compounds, in that their hydrolysis is very thermodynamically favorable; their formation is unfavorable, though the problems that may preclude their formation may be kinetic rather than thermodynamic, as we show here

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