Plasmid breakage by (125)I-labelled DNA ligands: effect of DNA-iodine atom distance on breakage efficiency.

Purpose: The aim of the study is to establish the relationship between the efficiency of DNA double-stranded breakage by (125)I-labelled DNA ligands and the distance from the decaying atom to the helical axis.

Materials And Methods: Two new iodinated minor groove binding ligands were synthesized which, on the basis of molecular modelling studies, place the iodine atom at different distances from the DNA helical axis (namely 7.4 and 11.2 A degrees ). Plasmid DNA breakage experiments, in both buffer-only and buffer + 2M dimethylsulfoxide (DMSO), were used to determine the efficiency of induction of internal double-stranded breaks (DSB) of the two new ligands, as well as that for (125)I-Hoechst 33258, which is characterized by a helical axis-iodine atom distance of 9.1 A degrees .

Results: The results showed a progressive decrease in the efficiency of DNA DSB induction with the axis-iodine atom distance, for both incubation conditions. The distance-damage relationship was somewhat steeper than previously predicted from the theoretical studies by Humm and Charlton, based on radical-mediated damage. Another distinctive trend was revealed by comparison of breakage efficiency with and without DMSO. The extent of DMSO protection increased significantly with DNA-iodine distance.

Conclusions: The steeper than predicted decrease in DSB induction with DNA-iodine distance is consistent with a substantial contribution to DNA breakage of the charge neutralization effect (arising from the transient positive charge left on the daughter Te atom), and the expectation that this contribution would be very dependent on the distance of the site of hole injection from the base-pair pi-stack. An important caveat to the results and conclusions is the need to confirm the estimated helical axis-iodine distances with X-ray crystallography studies, and for further exemplification with a more extensive collection of DNA ligands.