Charge dependence of Fe(ll)-catalyzed DNA cleavage

The effect of charge of the Fe(ll) reagent used to induce DNA strand cleavage reactions in the presence of a source of reducing equivalents is investigated using two oligonucleotide models. The first consists of the two strands dA₂₀ and dT₂₀, and an equimolar complex between them. The second is a short four-arm branched DNA complex composed of four 16-mer strands. In the former case, cleavage of the 1:1 complex by three reagents with different formal charge, Fe(ll).EDTA₂^-, Fe(ll).EDDA and Fe²⁺ is comparable in rate to that of the individual dT₂₀ and the dA₂₀ strands. While the three reagents show similar cleavage rates for the duplex and single stranded molecules, they give distinctive cutting patterns in the DNA tetramer, consistent with the presence of a site of excess negative charge at the branch point. Scission induced by Fe(ll).EDTA^2- shows lower reactivity at the branch site relative to duplex controls, whereas Fe(ll)²⁺shows enhanced reactivity. Formally neutral Fe(ll)EDDA shows weak loss of cutting reactivity at the branch. The position of attack by Fe(ll)²⁺ in the branched tetramer is shifted with respect to those of Fe(ll)EDTA^2- or Fe(ll)EDDA; a slower migrating species is also detected in the scission of dA_2O^.dT_2o duplex by Fe(ll) reaction. These results suggest that the Fe(ll)²⁺ reaction proceeds by a different mechanism from the other agents. The difference in cutting profiles induced by the neutral and egatively charged chelated complexes is consistent with a local electrostatic repulsion of a negatively charged source of radicals, not a positively charged one.