Adenine Adsorbed onto Montmorillonite Exposed to Ionizing Radiation: Essays on Prebiotic Chemistry

Abstract

Most adsorption and radiolysis experiments related to prebiotic chemistry studies are performed in distilled water or sodium chloride solutions. However, distilled water and sodium chloride solutions do not represent the composition of the primitive seas of Earth. In this work, an artificial seawater with ion abundances Mg²⁺ > Ca²⁺ >> Na⁺ ≈ K⁺ and SO₄^2- >> Cl^- was used, one that is different from the average composition of seawater today. This artificial seawater is named seawater 4.0 Ga, since it better represents the composition of the major constituents of seawater of primitive Earth. The radiolysis of adenine adsorbed onto montmorillonite was studied. The most important result is that adenine is adsorbed onto montmorillonite, when it is dissolved in artificial seawater 4.0 Ga, and the clay protects adenine against gamma radiation decomposition. However, desorption of adenine from montmorillonite was possible only with 0.10 mol L^-1 of KOH. This result indicates that adenine was strongly bonded to montmorillonite. Fourier transform infrared spectroscopy showed that NH₂ group and electrostatic interactions, between negatively charged montmorillonite and positively charged adenine, are responsible for adsorption of adenine onto montmorillonite. In addition, X-ray diffractograms showed that adenine enters in the interlayer space of montmorillonite.