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

The objective of this work is to determine the extent to which various structural 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 oligodeoxynucleotides using electron paramagnetic resonance (EPR) spectroscopy. Each sample consisted of crystalline duplex DNA for which the crystal structure was verified to be that reported in the literature. We find that the G(tfr) of these samples is remarkably high, ranging from 0.55 to 0.75 micromol/J. The standard deviation in G(tfr) for a given crystal structure is generally small, typically less than +/-10%. Furthermore, G(tfr) does not correlate with DNA base sequence, conformation, counterion or length of base stacking. Two observations point to the importance of DNA packing: (1) The radical yields in crystalline DNA are greater than those determined previously for DNA films (0.2 to 0.5 micromol/J); and (2) the variability in G(tfr) is less in DNA crystals than in DNA films. We conclude that closely packed DNA maximizes radical trapping by minimizing the interhelical solvent space. Furthermore, the high efficiency of electron and hole trapping at 4 K is not consistent with DNA possessing properties of a metallic conductor. Indeed, it behaves as an insulator, whether it is in A-, B-, or Z-form and whether base stacking is short- (8 bp) or long-range (>1000 bp).