Organic sulfur compounds resulting from the interaction of iron sulfide, hydrogen sulfide and carbon dioxide in an anaerobic aqueous environment

Abstract

Abstract. The reaction of iron sulfide (FeS) with H2S in water, in presence of CO2 under anaerobic conditions was found to yield H2 and a variety of organic sulfur compounds, mainly thiols and small amounts of CS2 and dimethyldisulfide. The same compounds were produced when H2S was replaced by HC1, in the H2S-generating system FeS/HCl/COí. The identification of the products was confirmed by GC-MS analyses and the incorporation of H2 in the organic sulfur compounds was demonstrated by experiments in which all hydrogen compounds were replaced by deuterium compounds. Generation of H2 and the synthesis of thiols were both dependent upon the relative abundance of FeS and HC1 or H2S, i.e. the FeS/HCl-or FeS/I-hS-proportions. Whether thiols or CS2 were formed as the main products depended also on the FeS/HCl-ratio: All conditions which create a H2 deficiency were found to initiate a proportional increase in the amount of CS2 . The quantities of H2 and thiols generated depended on temperature: the production of H2 was significantly accelerated from 50 °C onward and thiol synthesis above 75 °C. The yield of thiols increased with the amount of FeS and HC1 (H2S), given a certain FeS/HCl-ratio and a surplus of CO2, A deficiency of CO2 results in lower thiol systhesis. The end product, pyrite (FeSa), was found to appear as a silvery granular layer floating on the aqueous surface. The identity of the thiols was confirmed by mass spectrometry, and the reduction of CO2 demonstrated by the determination of deuterium incorporation with DC1 and D2O. The described reactions can principally proceed under the conditions comparable to those obtaining around submarine hydrothermal vents, or the global situation about 4 billion years ago, before the dawn of life, and could replace the need for a reducing atmosphere on the primitive earth.

The rocky roots of the acetyl-CoA pathway

“…Only selected metal sulphide centres of CODH and ACS are shown; the protein structure is represented by shading. The structures of methylsulphide, which has been synthesized from H 2 S and CO 2 via FeS catalysis [24], and methyl thioacetate (acetyl methylsulphide), which has been synthesized from CO and methylsulphide via NiS and FeS catalysis [25], are shown at the top for comparison. A reaction mechanism for the latter mineral-catalysed synthesis has been proposed [25] [23], the proposed active site of which is called the 'H-cluster'.…”

“…A reaction mechanism for the latter mineral-catalysed synthesis has been proposed [25] [23], the proposed active site of which is called the 'H-cluster'. In other words, the structure and atomic coordination of catalytically essential (Fe,Ni)S centres of (Fe,Ni)S proteins are not inventions of the biological world, rather they are mimics of minerals that are indisputably older and which themselves have catalytic activity in the absence of protein [24,25].…”

“…The significance of metal sulphides in the hydrothermal reactor model is their catalytic activity in CO 2 -and COreducing reactions, both as metal sulphur clusters in proteins [16,28,29] (Figure 2) and as the free minerals themselves [24,25]. The significance of the 3D compartments comprising the reactor (Figures 1 and 3), which form a barrier to diffusion into the ocean, is their retention of just-synthesized organic molecules.…”

“…Experimental evidence supports this idea. The laboratory synthesis of methylsulphide (CH 3 SH) from H 2 S and CO 2 using only FeS as the catalyst has been shown [24]. Both the thioester acetyl methylsulphide (CH 3-COSCH 3 ) and its hydrolysed product, acetate (CH 3 COO 2 ), can be produced from CO and CH 3 SH by using only FeS and NiS as catalysts [25].…”

“…But chemical energy from acetate production could nonetheless have been harnessed, provided that organic thiols were available in the reactor, which is extremely likely, for example, in the form of methylsulphide [24]. The thermodynamically favourable production of thioesters as highly reactive, energy-rich intermediates would fit very well with De Duve's [7] suggestions that thioesters were central to early biochemistry, but would exclude neither a role for pyrophosphates as early energy stores [34] nor a role for additional redox potential stemming from photolytically generated marine Fe(III) [21].…”