Studies on interactions of astrobiologically relevant molecules with mineral surfaces under space conditions

The study of the interaction between electromagnetic radiation and bio-molecules in heterogeneous environments is prebiotically relevant. Minerals may have a pivotal role in the prebiotic evolution of complex chemical systems, mediating the effects of electromagnetic radiation, influencing the photostability of bio-molecules, catalyzing important chemical reactions and/or protecting molecules against degradation. In particular, nucleobases are relevant bio-molecules to investigate both in the prebiotic context, because they are coding components of nucleic acids, and from the standpoint of the survival of biological systems in space conditions. Several studies on the photodynamics of nucleobases suggest that their structure could have been naturally selected for the ability to dissipate electronic energy through ultrafast photophysical decay. Considering the putative involvement of minerals in the prebiotic chemistry, it is necessary to study the photostability of nucleobases under space conditions in the presence of mineral matrices, to investigate both the prebiotic processes that might have had a role in the development of the first living entities on Earth and the physical and chemical processes occurring in extraterrestrial environments. Based on these issues, the adsorption properties of the nucleobases adenine, cytosine, uracil and hypoxanthine on magnesium oxide were investigated, determining the equilibrium adsorption isotherms. Langmuir- type isotherms were fitted to data, assuming a rapid reversible equilibration of adsorption, demonstrated effectively through desorption experiments. The Langmuir equilibrium adsorption constant K and the amount of the solute per unit of adsorbent mass necessary to complete the monolayer b were calculated. The results indicate that magnesium oxide is a good adsorbent for nucleobases (adenine > uracil > hypoxantine > cytosine), suggesting a role of metal oxides in concentrating biomolecules in prebiotic conditions that might have favored the passage from geochemistry to biochemistry. Then, to characterize at molecular level the nature of the interaction between nucleobases and the surface of the minerals magnesium oxide and forsterite, which is of interest in prebiotic processes, vibrational spectroscopy studies were carried out, using the diffuse reflectance infrared Fourier transform spectroscopy technique (DRIFTS). Finally, the photostability of nucleobases adsorbed on such minerals was investigated through in situ UV irradiation experiments. Experimental results confirm the high intrinsic photostability of such molecules, because a rather low probability of interaction between UV radiation and nucleobases was estimated, and furthermore indicate that cytosine and hypoxanthine have a greater photostability compared to adenine and uracil, both pure and adsorbed onto magnesium oxide and forsterite. Such minerals neither show protective effects against the UV radiation, nor seem to promote the formation of new species.