Purpose: To determine the possible effects of (125)I-to-DNA distance on the magnitude and mechanism of Auger-electron induced-double-strand break (DSB) production.
Materials and methods: We have synthesized a series of (125)I-labeled Hoechst (H) derivatives ((125)IE-H, (125)IB-H, (125)I-C(8)-H and (125)I-C(12)-H). While all four molecules share a common DNA minor groove binding bis-benzimidazole motif, they are designed to position (125)I at varying distances from the DNA helix. Each Hoechst derivative was incubated at 4°C in phosphate buffered saline (PBS) together with supercoiled (SC) (3)H-pUC19 plasmid DNA (ratio 3:1) ± the •OH scavenger dimethyl sulfoxide (DMSO) (0.2 M). Aliquots were analyzed on agarose gels over time and DSB yields per decay of (125)I atom were determined. Docking of the iodinated compounds on a DNA molecule was carried out to determine the distance between the iodine atom and the central axis of DNA.
Results: In the absence of DMSO, the results show that the DSB yields decrease monotonically as the (125)I atom is distanced - by 10.5 Å to 13.9 Å - from the DNA helix ((125)IEH: 0.52 ± 0.01; (125)IB-H: 0.24 ± 0.03; (125)I-C(8)-H: 0.18 ± 0.02; (125)I-C(12)-H: 0.10 ± 0.00). In the presence of DMSO, DSB yields for (125)IEH (0.49 ± 0.02) and (125)IB-H (0.26 ± 0.04) remain largely unchanged indicating that DSB are entirely produced by direct effects. Strikingly, (125)I-C(8)-H or (125)I-C(12)-H, did not produce detectable DSB in the presence of DMSO under similar conditions suggesting when (125)I atom is positioned > 12 Å from the DNA, DSB are entirely produced by indirect effects.
Conclusion: These results suggest that at a critical distance between the (125)I atom and the DNA helix, DSB production switches from an 'all' direct to an 'all' indirect mechanism, the latter situation being comparable to the decay of (125)I free in solution. These experimental findings were correlated with theoretical expectations based on microdosimetry.