Synthesis, Structure, and DNA Cleavage Properties of Copper(II) Complexes of 1,4,7-Triazacyclononane Ligands Featuring Pairs of Guanidine Pendants

Two new ligands, L-1 and L-2, have been prepared via N-functionalization of 1,4,7-triazacyclononane (tacn) with pairs of ethyl- or propyl-guanidine pendants, respectively. The X-ray crystal structure of [CuL1](CIO4)(2) (C1) isolated from basic solution (pH 9) indicates that a secondary amine nitrogen from each guanidine pendants coordinates to the copper(II) center in addition to the nitrogen atoms in the tacn macrocycle, resulting in a five-coordinate complex with intermediate square-pyramidal/trigonal bipyramidal geometry. The guanidines adopt an unusual coordination mode in that their amine nitrogen nearest to the tacn macrocycle binds to the copper(II) center, forming very stable five-membered chelate rings. A spectrophotometric pH titration established the pK(app) for the deprotonation and coordination of each guanidine group to be 3.98 and 5.72, and revealed that [CuL1](2+) is the only detectable species present in solution above pH similar to 8. The solution speciation of the CuL2 complex (C2) is more complex, with at least 5 deprotonation steps over the pH range 4-12.5, and mononuclear and binuclear complexes coexisting. Analysis of the spectrophotometric data provided apparent deprotonation constants, and suggests that solutions at pH similar to 7.5 contain the maximum proportion of polynuclear complexes. Complex Cl exhibits virtually no cleavage activity toward the model phosphate diesters, bis(p-nitrophenyl)phosphate (BNPP) and 2-hydroxypropyl-p-nitrophenyl phosphate (HPNPP), while C2 exhibits moderate activity. For C2, the respective k(obs) values measured at pH 7.0 (7.24 (+/- 0.08) x 10(-5) s(-1) at 50 degrees C) and 32 (+/- 0.3) x 10(-5) s(-1) (HPNPP at 25 degrees C)) are 40- and 10-times faster than [Cu(tacn)(OH2)(2)](2+) complex. Both complexes cleave supercoiled pBR 322 plasmid DNA, indicating that the guanidine pendants of [CuL1](2+) may have been displaced from the copper coordination sphere to allow for DNA binding and subsequent cleavage. The rate of DNA cleavage by C2 is twice that measured for [Cu(tacn)(OH2)(2)](2+), suggesting some degree of cooperativity between the coppr center and guanidinium pendants in the hydrolysis of the phosphate ester linkages of DNA. A predominantly hydrolytic cleavage mechanism was confirmed through experiments performed either in the presence of various radical scavengers or under anaerobic conditions.