Mechanisms of photoinitiated cleavage of DNA by 1,8-naphthalimide derivatives

Using water-soluble 1,8-naphthalimide derivatives, the mechanisms of photos ensitized DNA damage have been elucidated. Specifically, a comparison of ra te constants for the photoinduced relaxation of supercoiled to circular DNA , as a function of dissolved halide, oxygen and naphthalimide concentration , has been carried out. The singlet excited states of the naphthalimide der ivatives were quenched by chloride, bromide and iodide. In all cases the qu enching products were naphthalimide triplet states, produced by induced int ersystem crossing within the collision complex. Similarly, the halides were found to quench the triplet excited state of the 1,8-naphthalimide derivat ives by an electron transfer mechanism. Bimolecular rate constants were <10 (5) M-1 s(-1) for quenching by bromide and chloride. As expected from therm odynamic considerations quenching by iodide was 6.7 X 10(9) and 8.8 X 10(9) M-1 s(-1) for the two 1,8-naphthalimide derivatives employed. At sufficien tly high ground-state concentration self-quenching of the naphthalimide tri plet excited state also occurs. The photosensitized conversion of supercoil ed to circular DNA is fastest when self-quenching reactions are favored. Th e results suggest that, in the case of 1,8-naphthalimide derivatives, radic als derived from quenching of the triplet state by ground-state chromophore s are more effective in cleaving DNA than reactive oxygen species or radica ls derived from halogen atoms.