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

The effects of gamma-irradiating three metastable forms of water at 77 K ha ve been studied by electron spin resonance spectroscopy and are compared wi th those of hexagonal ice. Two amorphous forms of water were made by so-cal led hyperquenching of liquid water droplets (hyperquenched glassy water, HG W) 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 gamma-irradiation of hexagonal ice at 77 K produces only OH radicals as primary radiation product, on gamma-ir radiation of HGW and ASW comparable yields of OH and HO2 radicals were obse rved, and their ratio did not depend on radiation dose. Mainly OH radicals were formed on gamma-irradiating cubic ice made directly from liquid water. In contrast, comparable amounts of OH and HO2 radicals were formed on irra diation of cubic ice made by heating HGW, and their relative yields depend strongly on the irradiation dose. HO2 radicals are formed as primary produc ts of radiation, as are OH radicals, and their formation is attributed to t he 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 fo rmed by addition of atomic hydrogen on gamma-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 concurr ently with HO2 radical formation. Therefore, it is rationalized by a concer ted process involving the primary intermediate H2O+. The radiation effect o n thymine must be indirect because of the large water-to-thymine ratio of a pproximate to 10(4) in homogeneous glassy solution. Possible implications o f these studies are discussed for our understanding of (i) defects in metas table forms of water and their detection via HO2 radical formation, (ii) ra diation damage to biomolecules in aqueous solution, and (iii) radical gener ation on high-energy irradiation of ASW in outer space and the secondary fo rmation of stable molecules on decay of the radicals.