Identification of DNA Bases and Their Cations in Astrochemical Environments: Computational Spectroscopy of Thymine as a Test Case

Yage Zhao,Majdi Hochlaf,Malgorzata Biczysko

doi:10.3389/fspas.2021.757007

Abstract

Increased importance of vibrational fingerprints in the identification of molecular systems, can be highlighted by the upcoming interstellar medium (ISM) observations by the James Webb Space Telescope, or in a context of other astrochemical environments as meteorites or exoplanets, Mars robotic missions, such as instruments on board of Perseverance rover. These observations can be supported by combination of laboratory experiments and theoretical calculations, essential to verify and predict the spectral assignments. Astrochemical laboratory simulations have shown that complex organic molecules (COMs) can be formed from simple species by vacuum ultraviolet or X-ray irradiation expanding interest in searching for organic biological and prebiotic compounds. In this work an example of nucleobase, thymine, is selected as a test case for highlighting the utility of computational spectroscopic methods in astrochemical studies. We consider mid-infrared (MIR) and near-infrared (NIR) vibrational spectra of neutral (T) and cationic (T+) thymine ground states, and vibrationally-resolved photoelectron (PE) spectra in the far UV range from 8.7 to 9.4 eV. The theoretical framework is based on anharmonic calculations including overtones and combination bands. The same anharmonic wavenumbers are applied into the simulations of vibrationally-resolved photoelectron spectra based on Franck-Condon computations. The infrared and vibrationally-resolved photoelectron spectra are compared with the available experimental counterparts to verify their accuracy and provide assignment of the observed transitions. Finally, reliable predictions of spectra, going beyond currently available experimental data, either dealing with energy ranges, resolution or temperature, which can support astrochemistry studies are provided.

Highlights

Before the middle of the last century, it was widely believed that the environment in the cosmic space was not suitable to the formation and survival of molecules, and that most matter in space would exist as atoms or amorphous dust grains (Herzberg, 1988)
We focus on the only stable cis conformer of thymine, for which a highly accurate structural parameters have been obtained by ab initio methodologies and semi-experimental approaches (Vogt et al, 2014)
For neutral thymine accurate reference equilibrium structure has been obtained by two complementary techniques (Vogt et al, 2014), namely ab initio, re structural parameters from the composite approach based on the all-electron CCSD (T) (Raghavachari et al, 1989) geometry optimizations employing the cc-pwCVTZ basis set (Peterson and Dunning, 2002) and the semi-experimental (Pulay et al, 1978) method, based on the fit of equilibrium rotation constants, which agree with each other to within 0.002 Å and 0.2°, further confirming their high accuracy

Summary

Introduction

Before the middle of the last century, it was widely believed that the environment in the cosmic space was not suitable to the formation and survival of molecules, and that most matter in space would exist as atoms or amorphous dust grains (Herzberg, 1988). Dense interstellar clouds are the regions where most of the chemical reactions involved occur. These are the environments where new planetary systems are formed, so, some of the chemical species produced in these environments are expected to be transported to small objects such as asteroids and comets. This implies that newly formed planetary systems likely possess an inventory of complex prebiotic organics, which can be delivered to and seed planets, as was the case for the primitive Earth (Sandford et al, 2020). Photochemistry with high-energy processing of the interstellar ice mantles, planetary atmospheres, or the planetary soil is a key mechanism for extraterrestrial synthesis of prebiotic molecules (Arumainayagam et al, 2021), but photochemistry can lead to the molecules modification or destruction (SerranoAndrés and Merchán, 2009)

Methods

Results

Conclusion