Comparison of Strand Breaks in Plasmid DNA after Positional Changes of Auger Electron-Emitting Iodine-125

To elucidate the kinetics of the induction of DNA strand breaks by low-energy Auger electron emitters, we compared the yields of DNA breaks in supercoiled pUC19 DNA after the decay of 125 I (1) in proximity to DNA after minor-groove binding (^{125}{\rm I}\text{-iodoHoechst}$ 33342, ^{125}{\rm IH}$) and (2) at a distance from DNA (^{125}{\rm I}\text{-iodoantipyrine}$, ^{125}{\rm IAP}$). Iodine-125 bound to the minor groove in DNA or free in solution is equally effective per decay in producing single-strand breaks (SSBs), while 125 I bound to the minor groove is 6.7-fold more efficient than 125 I free in solution in producing double-strand breaks (DSBs) (1.08 ± 0.13 compared to 0.16 ± 0.01 DSB/decay). Consequently, SSB to DSB ratios for ^{125}{\rm IAP}$ and γ radiation (20.7 ± 2.9 and 43.8 ± 1.5, respectively) are greater than that for ^{125}{\rm IH}$ (2.9 ± 0.4). Finally, the decay of ^{125}{\rm IH}$ leads to fragmentation of plasmid DNA beyond SSBs and DSBs.