A new view on gold speciation in sulfur-bearing hydrothermal fluids from in situ X-ray absorption spectroscopy and quantum-chemical modeling

International audienc

Activation of C1 compounds by iron and mackinawite under prebiotic conditions

The role of iron sulfide minerals for the terrestrial origin of life has been discussed over several decades, but until now not fully resolved. Here, mackinawite (FeSm) was found to be selectively formed from the reaction of a wettened mixture of iron and sulfur powders. The structure and composition of the FeSm was analyzed by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (TEM) and Mössbauer spectroscopy. When the FeSm from the elements was let in contact with different C1 substrates (CO2, CO, KCN, KSCN, KOCN, CS2, CH3SH, [Fe(CN)6]3-/4- and [Fe(CN)5(NO)]2-) a range of reaction products (CS2, COS, C1 to C5 hydrocarbons, C1 to C4 thiols and oxygenated species) were formed at 25 ° and 80 °C in the presence of various acids (HCl, H2SO4 and H3PO4). Based on the product array, a novel reduction mechanism could be formulated where the sulfide end-groups of the FeSm particles interact with the C atom of the different substrates. To support this assumption, the reduction of KCN was investigated by DFT calculations on an all ferrous Fe4S44 cluster. The nucleophilic attack of a deprotonated end-group towards the C atom was thereby found to be exothermic by – 75.5 kcal mol-1. The subsequent reduction of the C residue into CH4 and CH3SH proceeds in an overall exothermic manner if electrons and protons are available. The selective reaction between iron and sulfur has the potential to change our current view on prebiotic processes as the established chemistry of iron sulfides can now additionally be discussed for environments, other than undersea hydrothermal systems. The transfer of the FeSm chemistry towards on-land scenarios enables reactions that would not be possible in the omnipresence of H2O. Furthermore, substances like CH3CHO and the cyclic organosulfur compounds are interesting precursors for biologically relevant molecules like DNA, RNA, or organometallic complexes like [FeFe]-hydrogenases