The Effects of Lattice Water on Free Radical Yields in X-Irradiated Crystalline Pyrimidines and Purines: A Low-Temperature Electron Paramagnetic Resonance Investigation

The hydration layer of DNA increases the target size of DNA with respect to the formation of direct-type damage by ionizing radiation. The mechanisms that give rise to this increase are being investigated by EPR spectroscopy. To determine these mechanisms, it is necessary to distinguish between the change in sample mass and changes in packing/conformation brought about by the change in the level of hydration. Certain model compounds that crystallize as hydrates provide a system where the effects of mass and packing can be discerned. Three such hydrate crystals were used in this work: barbituric acid dihydrate (${\rm BA}\colon 2{\rm H}_{2}{\rm O}$), inosine dihydrate (${\rm IR}\colon 2{\rm H}_{2}{\rm O}$) and thymine monohydrate (${\rm T}\colon {\rm H}_{2}{\rm O}$). The free radical yields (±25%) in the native crystals at 7-11 K are 0.08, 0.03 and 0.02, respectively. Removal of the lattice water leaves behind an ordered lattice and results in free radical yields of 0.08, 0.03 and