Impact of disorder on formation of free radicals by gamma-irradiation: Multi-frequency EPR studies of trehalose polymorphs

Abstract

Electron paramagnetic resonance (EPR) studies of radiation-induced radicals in two anhydrous trehalose polymorphs, beta-crystalline (TRE c ) and glassy (TRE g ), were conducted with the aim to resolve whether different types of free radicals are induced in a differently disordered environment. The multifrequency approach (9.5 GHz, 94 GHz, and 244 GHz) was 2 applied to improve the g-tensor resolution of the complex EPR spectra. In addition, thermal stability of EPR spectra and respective decay kinetics were analyzed in a series of thermal annealing studies in the temperature interval from 333 K to 363 K. It was found that in the crystalline matrix a more complex transformation process of induced radicals is taking place than in the glassy host matrix. Qualitative decomposition of the experimental spectra reproduced essential EPR spectral features in both matrices when 4 contributing radical species were assumed. These were interpreted as carbon-centered radicals while the possibility of formation of alkoxy radicals due to the abstraction of a hydrogen atom was ruled out. Only in one case, tentative assignment of EPR spectral components revealed the formation of the same radical species in both TRE c and TRE g . Furthermore, thermal annealing of TRE g lost one of the radical species, whereas in TRE c all 4 radical species pertained irrespectively of the treatment. Therefore, the here presented results provide experimental evidence that the extent of disorder present in the material strongly affects the type and stability of radicals induced by ionizing radiation.