Infrared Spectral Signatures of Nucleobases in Interstellar Ices I: Purines

Abstract

The purine nucleobases adenine and guanine are complex organic molecules that are essential for life. Despite their ubiquitous presence on Earth, purines have yet to be detected in observations of astronomical environments. This work therefore proposes to study the infrared spectra of purines linked to terrestrial biochemical processes under conditions analogous to those found in the interstellar medium. The infrared spectra of adenine and guanine, both in neat form and embedded within an ice made of H₂O:NH₃:CH₄:CO:CH₃OH (10:1:1:1:1), were analysed with the aim of determining which bands attributable to adenine and/or guanine can be observed in the infrared spectrum of an astrophysical ice analogue rich in other volatile species known to be abundant in dense molecular clouds. The spectrum of adenine and guanine mixed together was also analysed. This study has identified three purine nucleobase infrared absorption bands that do not overlap with bands attributable to the volatiles that are ubiquitous in the dense interstellar medium. Therefore, these three bands, which are located at 1255, 940, and 878 cm^-1, are proposed as an infrared spectral signature for adenine, guanine, or a mixture of these molecules in astrophysical ices. All three bands have integrated molar absorptivity values (ψ) greater than 4 km mol^-1, meaning that they should be readily observable in astronomical targets. Therefore, if these three bands were to be observed together in the same target, then it is possible to propose the presence of a purine molecule (i.e., adenine or guanine) there.

Keywords: adenine; astrobiology; astrochemistry; guanine; infrared spectroscopy; interstellar medium; nucleobases; purines.

Figure 1

Molecular structures of adenine and guanine. In each case, the pyrimidine and imidazole ring components of the molecule have been indicated using red and blue highlighting, respectively.

Figure 2

Mid-infrared absorption spectra of neat adenine, neat guanine, and the adenine–guanine mixture at 300 K.

Figure 3

Mid-infrared absorption spectra of neat adenine, neat guanine, and the adenine–guanine mixture at 20 K.

Figure 4

The fingerprint region (1800–650 cm⁻¹) of the mid-infrared absorption spectra of neat adenine, neat guanine, and the adenine–guanine mixture at 20 K. The red lines compare the positions of bands solely attributable to adenine in the spectra of neat adenine and the adenine–guanine mixture. The blue lines compare the positions of bands solely attributable to guanine in the spectra of neat guanine and the adenine–guanine mixture. The black lines compare the positions of bands in the spectra of the pure nucleobases to that of the mixture.

Figure 5

Mid-infrared absorption spectra of adenine, guanine, and the adenine–guanine mixture embedded within interstellar ice analogues at 20 K. Absorption features attributable to the purines are indicated in red.

Figure 6

The bands highlighted by red arrows indicate the three bands that are proposed as mid-infrared spectral signatures of adenine and guanine in interstellar ices.