Ion induced formation of complex HCN-species in solid-phase adenine

Introduction

The pathways from simple organic molecules to prebiotics and complex life on Earth remain poorly understood. Adenine and other organic compounds involved in biochemistry have been detected in carbonaceous chondrites and laboratory analogs of Titan aerosols (Hayatsu et al., 1964; Martins et al., 2008; Pilling et al., 2009). Laboratory experiments propose abiotic nucleic base formation in various space environments (Sebree et al., 2018; Kimura and Kitadai, 2015; Gupta et al., 2011). Radiation exposure in space raises questions about adenine's stability under ionizing radiation, as it is an essential component of DNA. Previous studies investigated solid adenine's survivability under different radiation sources such as high energy representatives of galactic cosmic rays (Huang et al., 2014; Peeters et al., 2003; Poch et al., 2014; Evans et al., 2011). Here, ions are more representative of the solar wind. Higher energy radiation leads to adenine destruction and formation of smaller molecules or ions (Peeters et al., 2003). However, adenine exhibits higher photostability compared to other compounds under low-energy UV-radiation, indicating the formation of more complex molecules protecting it from further destruction (Poch et al., 2014).  This study explores the effects of irradiating solid adenine thin films with 30 to 70 keV ¹⁸O⁺- and ²⁰Ne⁺-ions, an energy range not thoroughly investigated previously (Huang et al., 2014). In-situ IR-spectroscopy and ex-situ ultra-high-resolution mass spectrometry (HRMS) provide insight into spectral and chemical properties, focusing on the chemical analysis of the higher mass ranges.

Experimental Methodology

Two methods were used for the sample synthesis. Firstly, at Institut de Planétologie et d'Astrophysique de Grenoble (IPAG), adenine powder was dissolved in an ethanol-water solution and deposited onto MgF₂ windows, resulting in "grainy" films. Secondly, at Laboratoire Interuniversitaire des Systèmes Atmospheriques (LISA), sublimation/ recondensation reactor produced homogeneous "thin-films" on MgF₂ windows at 130 °C under 2.0x10^-3 mbar pressure. The irradiation experiments were conducted at GANIL (Caen) and IJCLab (Orsay) using ¹⁸O⁺ and ²⁰Ne⁺-ion beams. In total, 17 adenine samples underwent irradiation with energies ranging from 30 to 70 keV. Ion flux and total fluence varied, with some samples cooled to 150 K for comparison purposes. IR spectra were recorded during irradiation. Finally, in the ex-situ analysis phase, the chemical composition was determined using the ultra-high-resolution mass spectrometer Orbitrap. The soluble parts of samples were analyzed via Electrospray Ionization (ESI) in negative mode. The mass spectra were interpreted using an in-house program (ATTRIBUTOR). There, molecular ions are attributed to each individual mass peak.

Results and Conclusions

The results show that low energy ion irradiation of adenine leads not only to its destruction but also to the formation of larger molecules following (HCN)𝑥R, where x ≤ 14. These HCN-molecules show a great chemical diversity and complexity with R being C𝑥, H𝑥, N𝑥, N𝑥H𝑦, C𝑥H𝑦 or C𝑥N𝑦, with the latter being the most versatile. New appearing bands in the IR-spectra indicate the formation of nitriles, primary amines and CN ring bonds. The aromaticity equivalent (Xc) of the HCN-molecules reveals their structure:

Xc = ((C#-((H#-N#)-C#))/DBE)+1    (1)

where C#, H# and N# is the number of carbon, hydrogen and nitrogen atoms in the molecule and DBE is the double bond equivalent. Yassine et al. (2014) propose a threshold value of Xc ≥ 2.5 for the presence of aromatic structures and Xc ≥ 2.7143 for condensed aromatic structures or polycyclic aromatic nitrogen bearing hydrocarbons (PANH) (s. Figure 1). Due to the degree of aromatization, a repeating ring structure as shown in Figure 1 is likely.

The free parameters (ion, energy, temperature) of the irradiation experiment do not have a noticeable influence on the chemical complexity of the samples. Furthermore, nitrogen is very efficiently incorporated into the new forming molecules of the irradiated adenine samples.

Figure 1: The aromaticity equivalent (Xc) as a function of number of carbon atoms (C#) for the molecules in the irradiated adenine samples. The size of the circles corresponds to the normalized intensity of the molecule in the mass spectrum (left). Schematic chemical structure versions of HCN-polymer from Völker (1960) (right).

 

Acknowledgements

We thank the Programme National de Planétologie (PNP) for supporting this work. This work is supported by the French National Research Agency in the framework of the "Investissements d’avenir” program (ANR-15-IDEX-02) and the generic call for proposals (ANR-22-CE49- 0017). The experiments were performed at the Grand Accélérateur National d’Ions Lourds (GANIL) by means of the CIRIL Interdisciplinary Platform, part of CIMAP laboratory, Caen, France. We thank the staff of CIMAP-CIRIL and GANIL for their invaluable support. We acknowledge the fundings from ANR IGLIAS grant ANR-13-BS05-0004 of the French Agence Nationale de la Recherche.

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