Reply to Nayak, P.K. Comment on "Samulewski et al. Magnetite Synthesis in the Presence of Cyanide or Thiocyanate under Prebiotic Chemistry Conditions. Life 2020, 10, 34".

We have considered the criticisms raised by Pranaba K. Nayak [...].

Study of Ferrocyanide Adsorption onto Different Minerals as Prebiotic Chemistry Assays.

Considered one of the most promising building blocks of life on primitive Earth, cyanide and its complexes are likely to have played an important role in the emergence of life on the planet. Investigation into cyanide on Earth has primarily considered high concentrations, but the cyanide concentration in the oceans of prebiotic Earth was exceptionally low. Thus, Bernal's hypothesis has allowed investigators to work around this problem. We observed, however, that cyanide does not adsorb onto several minerals; therefore, ferrocyanide could be used as a cyanide source when adsorbed onto mineral surfaces to promote the synthesis of molecules of biological significance. When adsorbed onto bentonite, a mineral that has Fe3+ atoms in its interlayers, the formation of Prussian blue analog complexes occurs through endothermic reaction and with increased entropy. The adsorption of ferrocyanide onto kaolinite indicates an exothermic and outer-sphere interaction, which results in degeneracy breakdown for C ≡ N stretch energy into two new bands of FTIR-ATR spectrum. Magnetite, which has iron atoms in its structure, and ferrocyanide interactions have been observed by outer-sphere coordination as well as the formation of Prussian blue analogs, as confirmed by the appearance of a new doublet in the Mössbauer spectra and a broadband close to 750 nm at UV-visible spectroscopy. Magnetite and kaolinite experiments presented relevant results only when performed in seawater, which suggests the importance of seawater composition for prebiotic experiments. These obtained results prove that ferrocyanide interacts with minerals differently according to structure and composition and show that this complex, like the Prussian blue analogs, may have played a crucial role as a source of cyanide on primitive Earth.

Cysteine, thiourea and thiocyanate interactions with clays: FT-IR, Mössbauer and EPR spectroscopy and X-ray diffractometry studies.

The present study examined the adsorption of cysteine, thiourea and thiocyanate on bentonite and montmorillonite at two different pHs (3.00, 8.00). The conditions used here are closer to those of prebiotic earth. As shown by FT-IR, Mössbauer and EPR spectroscopy and X-ray diffractometry, the most important finding of this work is that cysteine and thiourea penetrate into the interlayer of the clays and reduce Fe(3+) to Fe(2+), and as consequence, cystine and c,c'-dithiodiformamidinium ion are formed. This mechanism resembles that which occurs with aconitase. This is a very important result for prebiotic chemistry; we should think about clays not just sink of molecules, but as primitive vessels of production of biomolecules. At pH 8.00, an increasing expansion was observed in the following order for both minerals: thiourea > thiocyanate > cysteine. At pH 3.00, the same order was not observed and thiourea had an opposite behavior, being the compound producing the lowest expansion. Mössbauer spectroscopy showed that at pH 8.00, the proportion of Fe(2+) ions in bentonite increased, doubling for thiourea, or more than doubling for cysteine, in both clays. However, at pH 3.00, cysteine and thiourea did not change significantly the relative amount of Fe(2+) and Fe(3+) ions, when compared to clays without adsorption. For thiocyanate, the amount of Fe(2+) produced was independent of the pH or clay used, probably because the interlayers of clays are very acidic and HSCN formed does not reduce Fe(3+) to Fe(2+). For the interaction of thiocyanate with the clays, it was not possible to identify any potential compound formed. For the samples of bentonite and montmorillonite at pH 8.00 with cysteine, EPR spectroscopy showed that intensity of the lines due to Fe(3+) decreased because the reaction of Fe(3+)/cysteine. Intensity of EPR lines did not change when the samples of bentonite at pH 3.00 with and without cysteine were compared. These results are in accordance with those obtained using Mössbauer and FT-IR spectroscopy.

Interaction between glyphosate and montmorillonite in the presence of artificial seawater.

Glyphosate (N- (phosphonomethyl) glycine) is one of the most widely used herbicides in the world. In the literature, there are several studies describing the interaction between glyphosate and clay minerals. However, there is a lack of data of this interaction in marine environments. In this research, we examined the adsorption of glyphosate onto montmorillonite in the presence of artificial seawater. Mössbauer data showed that the interaction of the phosphonate group of glyphosate with Fe2+ of montmorillonite prevents its oxidation to Fe3+. X-ray diffractograms showed that glyphosate adsorption takes place only onto the montmorillonite surface and not in its interlayers. Infrared spectroscopy data demonstrate that the interaction between glyphosate and montmorillonite could be through the amino group. FT-IR spectra of aqueous solutions of salts of seawater showed that Ca2+ interacts with glyphosate of the phosphonate group, thus causing an increase in its adsorption onto montmorillonite. However, glyphosate dissolved in 0.50 mol L-1 NaCl and 0.034 mol L-1MgCl2 solutions showed the lowest adsorption onto montmorillonite. In addition, the adsorption of glyphosate onto montmorillonite decreased when the NaCl concentration increased. The results fitted the Sips isotherm model, probably because the Ca2+ interacts with glyphosate, making the adsorption process more homogeneous. Thus, n values for Freundlich and Sips isotherm models decreased with an increase in ionic strength. Glyphosate and ions of artificial seawater increased the pHpzc of montmorillonite.

Magnetite Synthesis in the Presence of Cyanide or Thiocyanate under Prebiotic Chemistry Conditions.

Magnetite is an iron oxide mineral component of primitive Earth. It is naturally synthesized in different ways, such as magma cooling as well as olivine decomposition under hydrothermal conditions. It is probable magnetite played a significant role in biogenesis. The seawater used in the current work contained high Mg2+, Ca2+ and SO42- concentrations, unlike the seawater of today that has high Na+ and Cl- concentrations. It is likely that this seawater better resembled the ion composition of the seas of the Earth from 4 billion years ago. Cyanide and thiocyanate were common molecules in prebiotic Earth, and especially in primitive oceans, where they could act on the magnetite mechanism synthesis via Fe2+ interaction. In this research, magnetite samples that were synthesized under prebiotic conditions in the presence of cyanide or thiocyanate, (both with and without artificial seawater), showed that, besides magnetite, goethite and ferrihydrite can be produced through different Fe2+-ion interactions. Cyanide apparently acts as a protective agent for magnetite production; however, thiocyanate and seawater 4.0 Gy ions produced goethite and ferrihydrite at different ratios. These results validate that Fe3+ oxides/hydroxides were possibly present in primitive Earth, even under anoxic conditions or in the absence of UV radiation. In addition, the results show that the composition of water in early oceans should not be neglected in prebiotic chemistry experiments, since this composition directly influences mineral formation.