binuclear iron(III) systems. Compared with the corresponding analogue without guanidinium groups, the new diiron(III) system led to considerable rate enhancement on DNAcleavage. The cooperativity between metal ions and guanidine groups was evidenced by the fact that no significant cleavage was observed after incubating pBR322 plasmid DNA with non-metalated ligands or free Fe3+ ion. DNA binding experiments