Unexpected radical generation on γ-irradiating metastable forms of water at 77 K

Abstract

The effects of γ-irradiating three metastable forms of water at 77 K have been studied by electron spin resonance spectroscopy and are compared with those of hexagonal ice. Two amorphous forms of water were made by so-called hyperquenching of liquid water droplets (hyperquenched glassy water, HGW) and by deposition of water vapours (amorphous solid water, ASW) at 77 K. Metastable cubic ice was made either by heating HGW, or by hyperquenching liquid water droplets at 170 K. Whereas γ-irradiation of hexagonal ice at 77 K produces only OH radicals as primary radiation product, on γ-irradiation of HGW and ASW comparable yields of OH and HO2 radicals were observed, and their ratio did not depend on radiation dose. Mainly OH radicals were formed on γ-irradiating cubic ice made directly from liquid water. In contrast, comparable amounts of OH and HO2 radicals were formed on irradiation of cubic ice made by heating HGW, and their relative yields depend strongly on the irradiation dose. HO2 radicals are formed as primary products of radiation, as are OH radicals, and their formation is attributed to the abundant presence of defects, with two oxygens facing each other without hydrogen in between, in HGW, in ASW and in disordered regions of cubic ice. When small amounts of thymine were added to HGW, thymine radicals were formed by addition of atomic hydrogen on γ-irradiation already at 77 K at the expense of the relative contribution of HO2 radicals. This is not due to scavenging of once-formed HO2 radicals by thymine, but it occurs concurrently with HO2 radical formation. Therefore, it is rationalized by a concerted process involving the primary intermediate H2O+. The radiation effect on thymine must be indirect because of the large water-to-thymine ratio of ≈104 in homogeneous glassy solution. Possible implications of these studies are discussed for our understanding of (i) defects in metastable forms of water and their detection via HO2 radical formation, (ii) radiation damage to biomolecules in aqueous solution, and (iii) radical generation on high-energy irradiation of ASW in outer space and the secondary formation of stable molecules on decay of the radicals.