The Role of Mineral Surfaces in Miller’s Experiment


 We have designed a set of experiments to test the role of mineral surfaces on the yielding of the Miller-Urey type of experiment. Two experiments were performed in borosilicate flasks, two in a Teflon flask and a third couple in a Teflon flask with pieces of borosilicate submerged in the water. The experiments were performed in CH4, N2, and NH3 atmosphere either buffered at pH 8.7 with NH4Cl or unbuffered solutions. The Gas Chromatography-Mass Spectroscopy results show important differences in the yields, in the number of products and in their structural complexity. In particular, a dipeptide, complex dicarboxylic acids, PAHs, and a complete panel of biological nucleobases form more efficiently or exclusively in the borosilicate vessel. Our results suggest the crucial role of mineral catalysis in Miller-Urey synthesis.


Introduction
The 1953's publication of the Miller-Urey experiment opened the door to the scienti c investigation of the origin of life 1 . In this brilliant experiment, Miller and Urey demonstrated that electrical sparking a mixture of methane, ammonia and hydrogen in the presence of water produces amino acids within a variety of organic compounds. The impact of these results was so high, that its mind opening relevance hardly fades over time 2 . Different gas mixtures have been explored [3][4][5][6] , and the yielding and molecular diversity were con rmed with modern analytical techniques 7 , including original sample remnants of early Miller experiments 8, 9 . Variations of the original Miller apparatus have been used but the experiments were always performed within of borosilicate asks. Interestingly, the initial pH of most of the canonical mixtures aiming to mimic the early Earth atmosphere in Miller-Urey experiments are highly alkaline. Under these alkaline conditions, silica dissolves: the higher the pH, the higher the solubility of silica (Fig. S1).
Therefore, it could be expected that upon contact of the alkaline water with the inner wall of the borosilicate ask, even this reinforced glass will slightly dissolve releasing silica, and offering silanol groups to the gas phase and to the liquid water and vapor. Motivated by the biomimetic role of silica in mineral self-organized structures, such as silica-carbonate biomorphs 10-12 as well as its catalytic role in prebiotic chemistry 13,14 , we designed a set of experiments to test the possible in uence of silica on the yielding of the classical Miller experiments. Figure 1 shows the experimental concept. Three types of experiments were carried out under two different chemical conditions, one unbuffered with a starting pH value of ca. 11, the other buffered at pH 8.7. One of the experiments was performed in a borosilicate reactor (hereafter BSR) as used in Miller-type experiments. A second was performed in a Te on® reactor (TFR), a third in a Te on reactor with centimeter pieces of borosilicate glass submerged in the water (TFBSR). After proceeding with the electrical discharges, the differences in color of the collected samples were visually evident (Fig. S2). In what follows, we describe the results of these experiments.

Results
We used a single ask Miller apparatus where electrodes, water, and the components of the atmosphere were joined in one single reaction ask made either of borosilicate or of Te on. The borosilicate ask (Duran ®) had a volume of 3L, the Te on ask of 1,5L (Fig. S3). A Tesla coil provided the 30 kV to ignite the electric arc between the tungsten electrodes. The asks were lled with water to a volume of 200 mL, so the sparking took place in the gas phase ( Figure S4 and S5). All the experiments were performed at room temperature, with a water vapor pressure of ca. 24 mbars, in order to remove thermal effects for a more effective comparison (see further details in SI# 1). We selected one of the most effective Miller  Table S1 (buffered condition) and Table S2 (unbuffered condition), the mass to charge (m/z) ratio values and relative peak abundances of products are in SI #2 (Table S3), while GC chromatograms and original m/z fragmentation spectra are in SI #3 and SI #4, respectively. As shown in Fig. 2 and Tables S1-S2, a large panel of elemental prebiotic chemical precursors (ECP) 1-4, amino acids and alkyl amines 5-24, carboxylic acids 25-35, RNA and DNA nucleobases 36-40, and aromatic and heteroaromatic derivatives 41-48 were synthesized in different yield and selectivity depending on the speci c experimental conditions. The total yield of compounds 1-48 grouped per chemical class similarity is reported in Table 1.
Overall, these results con rm the visual assessment that the brown broth obtained in the borosilicate experiments contained much more organic compounds than those of the  Tables S2-S3), and several amino acids, a dipeptide, carboxylic acids and aromatic miscellanea (for a total of 17 compounds) were produced only in the presence of borosilicate (Tables S2-S3) (Fig. 3A).
Borosilicate increased the yield of ECP 1-4 relative to Te on alone ( Table 1,  were detected in the crude (Fig. 2). The total yield of amino acids was higher in the borosilicate asks than in Te on alone (Table 1, entry 2). In addition, amino acids 6-7, 12-13, and 17, and the dipeptide 22, formed exclusively in the presence of borosilicate (Fig. 2, Tables S1-S2). The synthesis of 22, as well as that of formylated amino acids 20-21 (Fig. 2), is of prebiotic relevance and was probably favored by the formation of carbodiimide from 1, a borosilicate-catalyzed process 23 .
Once formed, carbodiimide can activate amino acids towards the formation of the peptide bond with contemporaneous release of urea 24 . Carboxylic acids 25-35 (from C-1 to C-9) were also identi ed in the reaction mixture (Fig. 2), the highest total yield being obtained in the presence of borosilicate (Table S3, entry 3). Carboxylic acids 25, 30-31, 32 and 35 were absent in the experiment performed in Te on alone (Tables S1-S2). The bene cial role of borosilicate was further con rmed in the synthesis of nucleobases. In this latter case, borosilicate systems afforded the complete set of nucleobases 36-40, while only 36, 39 and 40 were detected in the Te on ask (Tables S1-S2). Again, the total yield of nucleobases was highest in the presence of borosilicate (Table 1, entry 4). A slightly different behavior was observed in the formation of aromatic miscellanea 41-48, including polycyclic aromatic derivatives 45-48 (PAHs) (Fig. 2, Tables S1-S2). PAHs are important contributors to the overall pool of organic carbon in the universe, and potential candidates in the "aromatic world" hypothesis 25 . Aromatic derivatives prevailed in the borosilicate ask under buffered conditions, but this trend was reversed in the absence of the buffer, in which case the highest total yield was obtained in Te on alone (Table 1,  The key role of the borosilicate reactor in the diversity and yielding of the molecules forming in the discharge experiment is likely due to the existence of silanol groups on the surface of the glass 26 . The presence of Si-O-H groups enhanced by the alkaline conditions facilitates the absorption of the organic molecules synthesized in the gas and in the liquid water in contact with the glass 27 . This explains the formation, few hours after sparking, of a thin brown lm covering the inner surface of the borosilicate ask. This lm, which was noticed by Miller in his early experiments (1,3), does not form in the Te on reactors. The lm appears as a translucent orange matrix under the optical microscope ( Fig. 3B-C). The infrared spectra of the fresh formed lm show the characteristic absorption bands for HCN oligomers 28 .
GC-MS con rms that the lm is mainly made of HCN oligomers, in accordance with previously reported data. It also shows that it works as a matrix embedding and concentrating organic molecules, including urea 3, glycine 5, lactic acid 28, adenine 36, cytosine 39, guanidine 49, succinic acid 50, 2,4-diamino-6hydroxypyrimidine 51, hypoxanthine 52, and four polycyclic aromatic hydrocarbons, namely anthracene 53, chrysene 54, pyrene 55, and dibenz(a,h) anthracene 56 (Fig. 2, Table S5. Among them, 49-56 were not previously detected in the liquid fraction of the experiment. As a general trend, the total yield of these latter compounds was found to increase after acid hydrolysis 28 , highlighting the possibility that the treatment favored their extraction from the solid matrix (See supplementary information Table S5 condition A vs condition B). The EDX analysis of the lm reveals the existence of a signi cant amount of silica ( Fig. 3F-H and Figure S8). The formation of organosilicon compounds is most likely responsible for the incomplete mass-balance relative to the crude (Table 1). In addition, the highest total yield for the reaction products observed under unbuffered condition is in accordance with a possible role of borosilicate as catalyst for prebiotic processes (Table 1, entry 7).

Discussion
We conclude that the understanding and relevance of the Miller-Urey discharge experiment for the origin of life require extending the classical purely gaseous phase scenario to one including mineral surfaces. This conclusion is especially important in the framework of the new ideas about the Hadean Earth in which the concomitance of a reduced atmosphere, electrical storms, silicate-rich rocky surfaces, and liquid water is expected 29,30 . Our results demonstrate that mineral surfaces, particularly silica and silicates, drastically enhance known prebiotic synthetic routes in diversity and yielding. In addition, they also trigger the formation of porous insoluble organic matrices that serve as niches for preservation and concentration of forming prebiotic molecules. These abiotic organic lms can form during the early stages of Earth-like planets and moons as in the case of Mars and several moons of the solar system [31][32][33] .    Supplementary Files This is a list of supplementary les associated with this preprint. Click to download.