Compounds based on (1,4,7-trimethyl-1,4,7-triazacyclononane)iron(III) chloride have been synthesized. Exceedingly low concentrations (approximately 0.5 mu M) of these reagents are required to effect single-stranded oxidative cleavage of plasmid DNA at physiological pH and temperature. Approximately 3 breaks per plasmid per micromolar of reagent occur in 1 h at 37 degrees C. The addition of dithiothreitol dramatically increased the effectiveness of these compounds; only 0.05 mu M of reagent was required for DNA cleavage. When psoralen (a DNA photocross-linking agent) was attached to the iron complex, irradiation further increased the cleavage efficiency. The DNA cleaving abilities rival those of the cytotoxic antitumor drug bleomycin. Unlike bleomycin, the synthetic agents cut DNA with little sequence specificity. The lability of the chloride ligands, the hard acid character of iron(III), and the absence of base specificity in the DNA cleaving reaction suggest that a cationic iron species binds to the phosphate backbone of DNA. The reaction’s dependence on reductants and dissolved oxygen suggests that it proceeds by a redox mechanism. Crystals of (1,4,7-trimethyl-1,4,7-triazacyclononane)FeCl3) (L’FeCl3) belong to the monoclinic space group P2(1)/c, with a = 12.321(2)Angstrom, b = 7.3220(10) Angstrom, c = 15.903(3) Angstrom, V = 1434.7(5) Angstrom(3), and Z = 4 at 293 K. Refinement of 145 least squares parameters for 2613 independent reflections with F > 4.0 sigma(F) converged to R = 3.43% and R(W) = 6.45%. The coordination geometry around iron(III) approximates a trigonally distorted octahedron. The N-Fe-N bond angles (77.8 degrees-78.8 degrees) are compressed, while the Cl-Fe-Cl angles (96.7 degrees-97.0 degrees) are expanded from the octahedral value.