Prebiotic Chemistry at Ice-Mineral Interfaces
FT-IR Study of UV Irradiation of Formamide Ices on Grain Analog Surfaces
Chemical species occurring in natural environments, such as molecules on primitive planets or moons, and accreted on dust grains in presolar or interstellar media, exist in close contact with heterogeneous mineral surfaces. The nature of the chemical interaction of specific surfaces with specific compounds often leads to enormous catalytic enhancement of reaction rates with high chemical specificity. Reactions with formamide to produce prebiotic heterocyclic organics have been shown to enhance yield and product specificity. Chemical heterogeneity can also be induced by phase segregation occurring from condensation/evaporation or freeze thaw cycling. Porosity in low temperature ice mixtures also has an important effect on local concentrations of simple organic precursors important to the build up of prebiotic organic compounds. Studies of the fundamental interactions of simple small molecules (e.g. methane, carbon monoxide, ammonia) are used to probe the accumulation of pockets of hydrophobic or hydrophilic precursors. Electron and photon irradiation is used to stimulate reactions within pores. Product distributions of codeposited mixtures and loaded porous ices are measured by FTIR spectroscopy and post-irradiation thermal desorption mass spectrometry.
Surface Chemistry on Titan
Titan, the largest satellite orbiting Saturn, is one of the most viable environments for the synthesis of molecular precursors to life in our Solar System. The basic requirements for life (i.e., a solvent, a nutrient source, an energy source, and a suitable environment) are present on this icy moon to some extent. Prebiotic evolution on Titan hinges on whether the energy sources and environmental conditions on the surface, where nutrients are abundant and a solvent is available, are actually sufficient to support a unique biochemistry. Since life on Titan would necessarily be different from life on Earth, understanding the abiogenic synthesis of prebiotic molecules under conditions reminiscent of Titan's surface is vital to determining whether "weird life" might exist on Titan and similar planetary bodies throughout the universe. Numerous laboratory [1-4], theoretical [5-8], and mission [9-12] studies have been performed to explore the abundant chemistry of Titan's atmosphere. However, our understanding of the chemistry on Titan remains incomplete since surprisingly few studies have been conducted to investigate reactions of the organic inventory on the surface [13-15] where the water-ice bedrock is sprinkled with mineral particles left behind by impact events and blanketed by a film of tholins (i.e., polymeric CxHyNz species ) and liquid alkanes.
Our research is striving to determine whether abiogenic synthesis of prebiotic molecules (i.e., polyatomic molecules containing C-N, C-O and C-P bonds) is possible on the surface of Titan given the presence of potentially catalytic minerals and low fluxes of nonthermal radiation (e.g., sunlight, solar wind and cosmic rays). Surface sensitive techniques such as electron/photon stimulated desorption - time-of-flight mass spectrometry (ESD/PSD-TOF MS), temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRRAS) is being employed to investigate systems of known composition and structure that are analogous to Titan&$39s surface. We hope to answer the questions:
Studies of Terrestrial Mineral Grain Prebiotic Chemistry
Our investigation of the surface chemistry involved in the formation of nucleic acids begins by first studying the condensation of formamide (NH2CHO) using mineral catalysts to produce nucleobases. Because formamide chemistry allows for much versatility in phosphorylation of nucleosides as well as having favorable physiochemical and thermodynamic conditions for the formation of such informational polymers, we are focusing on the surface reactions of formamide on phosphate and other minerals. Attenuated Total Reflection and Infrared Reflection Absorption Spectroscopy (IRAS) are being used to study the reactions on the surface of minerals when introduced to formamide. Thus far, we have studied the condensation of formamide using: kaolinite (Al2O3SiO2(OH)4), synthetic libethenite (Cu2(OH)PO4), monobasic sodium phosphate (NaH2PO4·H2O), and tribasic sodium phosphate (Na3PO4). Examples of our studies are given below.
The catalysis of formamide with Cu2OHPO4 (synthetic libethenite)
The vibrational spectrum of the initial Cu2OHPO4, which by its chemical composition is identical to the mineral libethenite, has been studied. The main absorption bands are associated with the hydroxyl and phosphate groups. For example, the intensive absorption at 3450-3470 cm-1 corresponds to OH stretching (3454 and 3471 cm-1), the 812 cm-1 feature to OH bending vibrations and the intense absorption between 900-1050 cm-1 to unresolved PO4 symmetric stretching vibration. The interaction of copper phosphate with liquid formamide at elevated temperatures results in complete restructuring of the solid phase. The color and dispersion of the powder changes significantly. FTIR spectra of the solid phase after separation from formamide and drying shows that it is no longer Cu2OHPO4. Characteristic bands of OH at 3470 cm-1 and 812 cm-1 are no longer observed. This means that not only the first surface layers but also the bulk of the salt is converted into a new substance with a number of new absorption bands. These new bands are localized close to the range of characteristic vibrations of formamide functional groups and probably belong to some organo-metallic complex. Some minor changes of the FTIR absorption are observed also in the liquid phase of formamide.
Reaction model: Catalysis of formamide with kaolinite (Al2O3SiO2(OH)4) to study the surface formation of nucleobases
The phyllosilicate clay mineral, kaolinite, catalyzes reactions with formamide at a mean temperature of 160ºC. According to prior studies, kaolin selectively produces the products purine, adenine, and cytosine after only 48 hrs. In our studies, samples are taken of both the liquid and solid layers at regular time intervals during the reaction process. Infrared Reflection Absorption Spectroscopy (IRAS) are taken of the liquid and its residue as well as the wet and dried solid. Multiple spectra are also taken after the solid is dried at increasingly higher temperatures to monitor reductions in peak intensity.
When studying the spectrum of the dried white solid after reaction of formamide with kaolinite after 48 hrs at 160ºC, four peaks emerge that are not seen in the initial or 3.5 hr dried solid. The peak at 1723 cm-1 could correspond to a CO stretching vibration. The broader spectral feature at 1580 cm-1 could be NH2 deformation which is also seen in formamide around 1600 cm-1 or it could represent a product having a shifted NH2 deformation feature. A small peak at 1343 cm-1 and a more intense peak at 1372 cm-1 are in similar positions as the NH2 scissors mode seen in formamide. However, the proportions of the two peaks are different from those seen in formamide. The solid wet initial mixture of formamide and kaolinite was compared to the solid wet product after 48hrs at 160ºC. A peak grows in at around 1214 cm-1. This spectral feature is interesting because it is not present in either formamide or kaolin itself and may indicate some intermediate or product forming. The spectral features of diaminomaleonitrile (DAMN) are also being studied because it could be an intermediate in the reaction. The ATR of DAMN neat shows a peak around 1238 cm-1, which could represent the shifted peak in the mixture.
- Saladino, R.; Crestini, C.; Negri, V.; Cicirielli, F.; Costanzo, G.; Di Mauro, E. ChemBioChem. 2006. 7(11), 1707.
- Saladino, R.; Crestini, C.; Costanzo, G.; Negri, R.; Di Mauro, E. Bioorganic & Medicinal Chemistry. 2001. 9, 1249.