Synthesis, DNA binding, and photonuclease activity of new tetraaza macrocyclic constrained isoxazole rings as subunit in metal complexes

The DNA-binding and photonuclease activity of newly synthesized tetra-azamacrocyclic ligand L (C(32)H(32)N(8)O(4)) and its complexes of type [MLCl(2)] and [ML]Cl(2) (where M = Co(II), Fe(II) and Cu(II); L = N,N'-[3-(4-{5-[(2-amino-ethylamino)-methyl]-isoxazol-3yl}-phenyl)-isoxazol-5-yl methyl-ethane-1,2-diamine] are specified. An octahedral geometry has been proposed for Fe(II) and Co(II) complexes, while the Cu(II) complex has a square planar environment. The absorption spectral results indicate that the complexes bind with the base pairs of DNA, with an intrinsic binding constant K(b) of Fe(II), Co(II), and Cu(II) complexes found to be 3.2 × 10(4) M(-1), 5.3 × 10(4) M(-1), and 4.2 × 10(4) M(-1), respectively, in 5 mM Tris-HCl/50 mM NaCl buffer at pH 7.2. The large enhancement in the relative viscosity of DNA on binding to the complexes supports the proposed DNA binding modes. The viscosity and thermal denaturation studies sustain the effective intercalation with DNA. The DNA photocleavage studies demonstrated that compounds exhibit significant photonuclease activity by a concentration dependent on singlet oxygen mediated mechanism.