We have investigated the adsorption of adenine, adenosine, ribose, and adenosine-5′-phosphate (5′-AMP) by allophane at pH 4, 6 and 8. Adenine, adenosine and ribose gave similar isotherms, i.e. adsorption increased regularly with solution concentration and decreased in the order: pH 8 > pH 6 > pH 4. Allophane had a greater affinity for 5′-AMP than for adenine, adenosine or ribose. Further, the extent of adsorption for 5′-AMP increased in the order: pH 8 ≪ pH 6 ≈ pH 4. The adsorption of 5′-AMP at pH 4 and pH 6 was about 60 times greater than at pH 8. The strong adsorption of 5′-AMP accords with the well known high phosphate-retention capacity of allophane and allophane-rich soils. The experimental data may be rationalized in terms of the pH-dependent charge characteristics of the organic solutes and allophane. The large propensity of allophane to retain 5′-AMP is ascribed to ligand exchange between the phosphate of 5′-AMP and the hydroxyl of (HO)Al(OH2) groups, exposed at perforations on the wall of allophane spherules, giving rise to a surface (chelation) complex. The high affinity of nucleotides for allophane has implications for the possible role of allophane in the abiotic formation of RNA-type polynucleotides although nucleotide ‘immobilization’ by surface complexation might hinder RNA oligomerization.

The modification of minerals with metals can promote changes in their surface and, consequently, in their physicochemical properties. Minerals could have played an important role in the origin of life as they can protect molecules against degradation by radiation and hydrolysis, pre-concentrate molecules from dilute solutions and catalyse the formation of polymers. Thus, the current work studied the modification of montmorillonite with Cu2+ and Fe3+ ions. These modified montmorillonites were used to study the interaction with adenine dissolved in distilled water and artificial seawater 4.0 Gy (Gy = billion years ago). The most important result of this work is that the adsorption of adenine onto modified montmorillonites is a complex interaction among adenine, salts in seawater and Cu2+/Fe3+-montmorillonite (Cu2+/Fe3+-Mont) . The adsorption of Cu2+ and Fe3+ onto montmorillonite decreased its surface area and pore volume. The Sips isotherm model showed the best fit of the data and n values indicate that the adenine adsorption process was homogeneous. The highest adenine adsorption was obtained in artificial seawater 4.0 Gy onto Fe3+-Mont at 60°C and the lowest in distilled water or artificial seawater 4.0 Gy onto montmorillonite [montmorillonite washed with distilled water (Mont-STD)] at 60°C. Adenine adsorption onto Mont-STD/montmorillonite modified with 500 ml of 0.1 mol l−1 of CuCl2 and Fe3+-Mont was an exothermic process and an endothermic process, respectively. For all adsorptions ΔG was negative. The adsorption of adenine onto Fe3+-Mont was ruled out by entropy and the other samples by enthalpy and entropy, being a major contribution for Gibbs free energy from enthalpy. The Fourier transform-infrared data indicate that the interaction of adenine with minerals may occur through the NH2 functional group.

The origin of life from inanimate matter is still an open question, and our knowledge is still very limited. In this sense, prebiotic chemistry seeks to study and understand how chemical reactions may have contributed to the origin of life. Minerals are of great relevance to prebiotic chemistry, as they may have preconcentrated precursors of biomolecules or biomolecules from diluted solutions, provided protection for biomolecules against UV radiation and hydrolysis, catalysing their reactions and played the role of a primitive genetic code. Montmorillonite, a prebiotic mineral, was shown to be able to adsorb adenine and later also histidine. In addition, histidine adsorption did not displace adenine from the montmorillonite. Kinetic experiments showed that using a whole period of time (7 days) it was not possible to adjust the data to any mathematical kinetic model. Thus, the data were separated into four different adsorption ranges: range 1 (0–60 min), range 2 (60–4320 min), range 3 (4320–7200 min) and range 4 (7200–10 080 min). Range 1 showed adsorption that was too fast, meaning no variations in the adsorption data, and the data of range 3 did not fit in any model used in this work. Thus, range 2 (60–4320 min) and range 4 (7200–10 080 min) were analysed. The adsorption kinetics of histidine adsorption indicated two reaction steps, a quick step (60–4320 min), following the pseudo-first-order model, followed by a slower step (7200–10 080 min) of the pseudo-second order. With these results, isotherms were constructed with times of 1 h and 7 days. The results of the quick step (1 h) showed a reaction that was not thermodynamically favoured. For this time range, Gibbs energy values obtained ranged between 5 and 10 kJ mol−1 at temperatures of 20, 35 and 50°C, and the adsorption occurred due to the balance shift of increase in histidine concentrations. The isotherms of the slow step (7 days) presented negative values, showing a more favourable reaction with Gibbs energy values ranging between −5 and −11 kJ mol−1. The mathematical modelling of the data indicates that seawater ions are crucial in the adsorption process. Thus, the study provided essential information for prebiotic chemistry, showing that time and the reaction medium should always be taken into account.

We have determined the adsorption of RNA components (adenine, cytosine, uracil, ribose and phosphate) by Mg2+-exchanged montmorillonite. The isotherms for adenine, cytosine and uracil were of the C-type, the amount adsorbed increasing linearly with the equilibrium solute concentration. All three nucleic acid bases were apparently adsorbed by coordination to Mg2+ ions through a water bridge. Very little ribose was adsorbed by Mg-exchanged montmorillonite. The isotherm for phosphate was of the L-type. The plateau value of about 0.012 mmol g–1 indicated that phosphate adsorbed on the edge surface of montmorillonite. At comparable solute concentrations, adsorption decreased in the order adenine > cytosine > uracil. This observation reflects differences in basicity, size, and aqueous solubility among the three compounds.

Synchrotron radiation-based spectroscopic techniques are discussed. Their relevance to obtain information regarding the prebiotic evolution problem is pointed out. We present photoelectron–photoion coincidence (PEPICO) spectra of adenine and glycine obtained using 12 and 21 eV photons. The fragmentation pattern belonging to these molecules was found to present striking differences, which are discussed. Adenine partial ion yield in the energy region 12–21 eV is also presented. The neutral fragments were found to have very simple assignment. The importance of hydrogen cyanide (HCN) as a building block of these molecules is confirmed. A special instrumentation allowing precise comparisons between photon-induced desorption and energetic ion bombardment desorption is described. As an example, we show, for the first time, the frozen CO2 ice mass spectra bombarded by photons and energetic ions, under the same experimental conditions. The comparison shows that prebiotic evolution may only be properly understood if more than one particle, as energy source, is considered.