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 respe ct to the formation of direct-type damage by ionizing radiation. The m echanisms that give rise to this increase are being investigated by EP R spectroscopy. To determine these mechanisms, it is necessary to dist inguish between the change in sample mass and changes in packing/confo rmation brought about by the change in the level of hydration. Certain model compounds that crystallize as hydrates provide a system where t he effects of mass and packing can be discerned. Three such hydrate cr ystals were used in this work: barbituric acid dihydrate (BA:2H(2)O), inosine dihydrate (IR:2H(2)O) and thymine monohydrate (T:H2O). The fre e 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 a nd <0.004, respectively. Thus removal of the lattice water does not af fect the free radical yield in BA:2H(2)O or IR:2H(2)O but decreases th e free radical yield in T:H2O by an order of magnitude. Based on these observations and the known crystal packing, we conclude that the hydr ogen bonding network is a major factor in determining the distribution and yield of trapped free radicals. We ascribe this to the importance of proton transfer processes which act to reduce the probability of r adical combination. Consistent with this conclusion are the types of f ree radicals trapped in these crystalline materials before and after d ehydration. From these results, we argue that a major determinant of f ree radical yields in solid-state samples of DNA constituents is molec ular packing. In addition, the absence of HO. radicals trapped in sing le crystals of BA:2H(2)O provides an upper limit for the yield of trap ped HO. of less than 10(-4) mu mol/J. This supports the thesis that at <77 K direct ionization of those waters associated directly with a py rimidine or purine results in hole transfer to that molecule. Hydroxyl radical formation on a water adjacent to a DNA base is predicted to b e negligible. (C) 1997 by Radiation Research Society.