## 4 Discussion  
Purine ribonucleosides (guanosine and adenosine) appeared more reactive under mechanochemistry than corresponding pyrimidines (cytidine and uridine). The presence of water in our mechanochemical apparatus was inevitable (i.e., either from metal salts or ribonucleoside degradation). As demonstrated in previous work [25], water had a relevant role in the degradation mechanism of guanosine and uridine, being one source of catalytic H\({}^{+}\). However, the ribonucleoside-metal samples often became \"sticky\" after 1.5 h of ball-mill mechanochemistry treatment and could not be used for an additional period. Applying the pre-heating treatment only to the samples with nickel salt revealed ribonucleoside degradation, although much less intense compared with the ribonucleoside-metal mixture. Despite the thermal stability of canonical ribonucleosides (ranging from 244 to 271 \({}^{\circ}\)C) [36, 37] blanks containing canonical purine and pyrimidine ribonucleosides were submitted to the pre-heating treatment at the same temperature as that of samples of metal ions together with ribonucleosides, \(\sim\)90 \({}^{\circ}\)C (see Section 2), and then analyzed in HPLC-UV; however, substantial degradation was not observed for samples without ion metals. The pre-heating treatment had no impact on ribonucleoside reactivity, except for iron sulfate samples. These had a starting oxidation state of metal of +2, however, contact with molecular oxygen of the atmosphere might have led to a mixture of Fe\({}^{2+}\)/Fe\({}^{3+}\), and the appearance of intermediate species, such as Fe(III)SO\({}_{4}\)OH [38], which could somehow increase ribonucleoside degradation (in these samples, temperature was crucial). Nevertheless, both oxidation states are present in meteorite minerals (e.g., olivine, (Mg,Fe)\({}_{2}\)SiO\({}_{4}\), and hematite, \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\)) [17]. In the case of the samples containing sodium carbonate, no significant degradation was detected with or without the pre-heating treatment of the sample before mechanochemistry. Previous studies demonstrated that carbonate could act as an activator for the degradation of ribonucleosides; however, without the catalytic component (either H\({}^{+}\) or a metal ion) [25], the degradation seems ineffective. Moreover, it was revealed that chloride, Cl- (all metal ions, except iron, used were chloride salts), did not affect the degradation of ribonucleosides, since samples containing NaCl, as the unique source of chloride, were tested [25]. In some samples (G.2.1, C.2.1-C.2.3, and C.3.1-C.3.2), other species necleobase and starting ribonucleoside were detected, but their chemical characterization was not accomplished. These \"unknown species\" are present in samples containing aluminum or iron ions (with and without pre-heating treatment), resembling either a different degradation product or a type of ribonucleoside/nucleobase-metal complex. Additional characterization was attempted by \({}^{1}\)H NMR spectroscopy, but no complementary conclusion was obtained. It should be noted that some of the mechanochemical treated samples could not be removed from the reactor without solvent (i.e., H\({}_{2}\)O), and hence some were preserved in solution at room temperature (G.2.1, G.2.3, and G.3.2; A.2.1, A.2.3 and A.3.2; C.2.1, C.2.3 and C.3.2; U.1.1, U.2.1 to U.2.4 and U.3.1). Ribonucleosides' stability is affected in aqueous media at different pH. Since, under our experimental conditions, the pH value can change, acid/base catalysis could happen. Solutions prepared before mechanochemical treatment had, typically, one unit of pH higher compared with the afterward sample (i.e., after mechanochemistry, the resulting solution was more acid), suggesting the formation of other species. The half-life, t\({}_{1/2}\) for guanosine, adenosine, cytidine, and uridine in a solution with pH = 1, at 37 \({}^{\circ}\)C < T < 100 \({}^{\circ}\)C ranges from -20 h to 8 days; hence, acid hydrolysis can be considered insignificant for the time frame of these experiments.