THE EFFECTS OF LATTICE WATER ON FREE-RADICAL YIELDS IN X-IRRADIATED CRYSTALLINE PYRIMIDINES AND PURINES - A LOW-TEMPERATURE ELECTRON-PARAMAGNETIC-RESONANCE INVESTIGATION
Ne. Mroczka et al., THE EFFECTS OF LATTICE WATER ON FREE-RADICAL YIELDS IN X-IRRADIATED CRYSTALLINE PYRIMIDINES AND PURINES - A LOW-TEMPERATURE ELECTRON-PARAMAGNETIC-RESONANCE INVESTIGATION, Radiation research, 147(5), 1997, pp. 560-568
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.