Free radical yields in crystalline DNA X-irradiated at 4 K

The objective of this work is to determine the extent to which various stru ctural factors influence the yield of trapped free radicals, G(tfr), in DNA irradiated at 4 K. G(tfr) was measured in a series of 13 different oligode oxynucleotides using electron paramagnetic resonance (EPR) spectroscopy. Ea ch sample consisted of crystalline duplex DNA for which the crystal structu re was verified to be that reported in the literature. We find that the G(t fr) of these samples is remarkably high, ranging from 0.55 to 0.75 mu mol/J . The standard deviation in G(tfr) for a given crystal structure is general ly small, typically less than +/-10%. Furthermore, G(tfr) does not correlat e with DNA base sequence, conformation, counterion or length of base stacki ng. Two observations point to the importance of DNA packing: (1) The radica l yields in crystalline DNA are greater than those determined previously fo r DNA films (0.2 to 0.5 mu mol/J); and (2) the variability in G(tfr) is les s in DNA crystals than in DNA films. We conclude that closely packed DNA ma ximizes radical trapping by minimizing the interhelical solvent space. Furt hermore, the high efficiency of electron and hole trapping at 4 K is not co nsistent with DNA possessing properties of a metallic conductor, In deed, i t behaves as an insulator, whether it is in A-, B-, or Z-form and whether b ase stacking is short- (8 bp) or long-range (>1000 bp). (C) 1999 by Radiati on Research Society.