Radiolysis of cytosine at cryogenic temperatures by swift heavy ion bombardments

We investigated the radiolysis effects on the cytosine in the solid phase irradiated by swift heavy ions as galactic cosmic ray analogues (GCRs). Infrared (IR) absorption spectroscopy was employed to monitor the physical and chemical radiolytic modifications. The targets were prepared on ZnSe in two different ways: (1) by dropping a nucleobase-water-ethanol solution on the substrate and evaporating the solvent and (2) by sublimation of nucleobase powders in an oven and condensation on the windows. Both types of samples present similar IR absorption spectra. From the exponential decrease of the areas of IR absorption bands as a function of projectile fluence, apparent destruction cross sections (σd) were determined and were found to be very similar for samples prepared using both techniques. The destruction cross section of solid cytosine at cryogenic temperatures follows an electronic stopping (Se) power law: σd = C Sen, where C is a constant and the exponential n is a dimensionless quantity. We determined σd = (3 ± 1) × 10-17 Se (1.25 ±0.06). New absorption features emerge from cytosine degradation, which can be attributed to OCN-, H2CO, and HNCO. By using the observed power law, the half-life of cytosine exposed to galactic cosmic rays was estimated in the order of Mega years. The findings reported here may help a better understanding of complex organic molecule radiostability

Swift heavy ion irradiation of thymine at cryogenic temperature

Thymine (C5H6N2O2) is a basic N-heterocyclic nucleobase in all known organisms, and this molecule is also found in meteoritic materials. This study aims to investigate thymine's physical and chemical modifications under ion irradiation in cryogenic conditions. Space radiation was simulated by exposing thymine at 27 K to 230 MeV 48Ca10+ ions. Fourier transform infrared spectroscopy (FTIR) was employed to monitor the degradation of a 2.8 μm thick sample film under irradiation. From the intensity decrease of the infrared absorptions as a function of ion fluence, the destruction cross-section (σ), required to dissociate or eject a thymine molecule, is deduced by an exponential function. The physical and chemical modifications induced by energetic projectiles can be related to the electronic stopping power Se as σ=Se/D0, where D0=9.6±0.4 eV/molecule is the effective mean dose needed to destroy the thymine molecule at 27 K. Also, new molecular species formed under irradiation are observed and, based on infrared spectra, identified as CN−, OCN−, HCNO, and CO