Effects of metal ions on radical formation in gamma-irradiated DNA in the solid state.

Abstract

The effects of metal ions on the formation of radicals produced by $gamma$ radiolysis of DNA in vacuo were studied by esr and hydrogen-transfer measurements. At metal to nucleotide ratios of 1 : 10, a three- to tenfold decrease of the esr signal due to carbon was observed for transition metals such as Cr$sup 3+$, Fe$sup 3+$, Fe$sup 2+$, Ni$sup 2+$, and Co$sup 2+$, and smaller decreases (by 25 to 40 percent) were seen for paramagnetic rare earths (Eu$sup 3+$, Yb$sup 3+$, and Sm$sup 3+$). These effects were independent of dose and temperature (77 to 298$sup 0$K). The diamagnetic ions Mg$sup 2+$, Zn$sup 2+$, Cd$sup 2+$, Ag$sup +$, La$sup 3+$, and Lu$sup 3+$ had no effect; however, Ce$sup 4+$, Pb$sup 2+$, Sn$sup 4+$, and Hg$sup 2+$ decreased the radical yield by 10 to 60 percent. At metal to nucleotide ratios of 1:100, the radical yield was decreased twofold for Cr$sup 3-$, Cu$sup 2+$, and Fe$sup 2+$, about 30 percent for Mn$sup 2+$, Ni$sup 2+$, and Fe$sup 3+$ and less than 10 percent for Co$sup 2+$. From measurements of the power-saturation of the carbon signal for the 1:100 metal-DNA complexes, it can be inferred that the decrease of the esr signalsmore » is not due to spin-spin interactions between carbon radicals and paramagnetic ions. For the 1:10 metal-DNA complexes, spin-spin interactions may contribute to signal reduction. However, these results correlate with measurements of the radiation-induced hydrogen transfer, which can be explained by assuming a decreased radical yield; 1:10 complexes of Ni$sup 2+$ with adenine, polyadenylic acid, dextran, poly-DL-alanine, and poly-L-glutamic acid decreased the radical yield by 60 to 70 percent, but no effect was observed for thymine and thymidine. It appears that the metal ions act by quenching excited states and/or by charge scavenging. (auth)« less