Valine Radiolysis by H⁺, He⁺, N⁺, and S¹⁵⁺ MeV Ions

Radiolysis of biomolecules by fast ions has interest in medical applications and astrobiology. The radiolysis of solid D-valine (0.2-2 μm thick) was performed at room temperature by 1.5 MeV H⁺, He⁺, N⁺, and 230 MeV S¹⁵⁺ ion beams. The samples were prepared by spraying/dropping valine-water-ethanol solution on ZnSe substrate. Radiolysis was monitored by infrared spectroscopy (FTIR) through the evolution of the intensity of the valine infrared 2900, 1329, 1271, 948, and 716 cm^-1 bands as a function of projectile fluence. At the end of sample irradiation, residues (tholins) presenting a brownish color are observed. The dependence of the apparent (sputtering + radiolysis) destruction cross section, σ_d, on the beam stopping power in valine is found to follow the power law σ_d = aS_eⁿ, with n close to 1. Thus, σ_d is approximately proportional to the absorbed dose. Destruction rates due to the main galactic cosmic ray species are calculated, yielding a million year half-life for solid valine in space. Data obtained in this work aim a better understanding on the radioresistance of complex organic molecules and formation of radioproducts.