On the proton transfer behavior of the primary oxidation product in irradiated DNA

Results are surveyed from investigations of six adenine and six guanine crystal systems X-irradiated and studied at temperatures near that of liquid helium (T less than 10 K). Basic conclusions from the overall results are that the primary oxidation products deprotonate rapidly, and that the specific site of deprotonation is environment-dependent. The following systematic behaviors were identified: (1) in no case was there deprotonation at a site hydrogen-bonded to a halide ion (three examples with guanine and two with adenine); (2) likewise, in no case was there deprotonation at a site hydrogen-bonded to a phosphate group (three examples, all with guanine); (3) in no case was there deprotonation at a site involving an greater than N-H . . . N less than hydrogen bond; (4) in all cases except one, the site of net deprotonation involved greater than N-H . . . O hydrogen bonds. In the remaining case, the leaving proton was uninvolved in hydrogen bonding. A review of results obtained previously from DNA leads to the conclusion that the actual protonation states of DNA oxidation products are unknown at the present time. The results presented here predict with high probability that such DNA products also will deprotonate, but the environment dependence makes it difficult to predict the specific sites. Thus the importance of obtaining this information from direct experimental evidence is increased.