**3.3 Prebiotic S chemistry**

Thioesters (R-(C=O)-SR') are organics with high energy (C=O)-S bonds and was invoked to act as an alternative and prebiotic version of phosphoesters (such as ATP) to drive endergonic reactions coupled with the hydrolysis of (C=O)-S bond. The reasons are as follows as described in a recent paper [83]: (1) both thioester and phosphoester bonds have similar standard free energies of hydrolysis; (2) the thioester synthesis precedes the synthesis of phosphoesters in metabolism, such as in glycolysis and the Wood-Ljungdahl pathway; (3) based on computational studies employing network extension algorithms to a phosphate-free core metabolism, thioesters can promote similar reactions like phosphoesters through energetic coupling, for example, reductive TCA cycle and biosynthesis of amino acid [84]. In addition, using thioester as a prebiotic energy coupling agent can partially solve the problem of the scarcity of phosphate in the ferruginous Archean ocean [85] despite the presence of other reactive P sources (e.g., phosphite) [86]. Researchers have reported the synthesis of thioester by abiotic processes. Driven by active CO, at 100°C, the Wächtershäuser group reported C-C bond formation with the generation of CH3-CO-SCH3 thioesters proceeds on FeS and NiS from a mixture of CO and CH3SH [27]. Sandan et al. reported the generation of thioester by reacting of thioacetate and thiols in presence of Fe3+ at 70°C in water [83]. In addition, by further adding ferredoxin–hememaquettes, [4Fe-4S] cluster was formed based on the characteristic UV–Vis absorption band at 384 and 447 nm. This process also generates iron sulfide minerals. Recently, Kitadai et al. demonstrated the synthesis of S-methyl thioacetate (MTA) synthesis from CO and methanethiol on NiS at room temperature at neutral pH [54]. NiS was partially reduced to Ni under simulated geo-electrochemical conditions. This partially reduced Ni0 /NiS mixture catalyst is important to accumulate surface-bound CO by electro-reduction of CO2. CO accumulation process on Ni0 /NiS surface was crucial for subsequent thioester formation in early ocean hydrothermal systems. The pH and temperature conditions are mild and geologically plausible.
