Interaction with DNA of a heteronuclear [Na₂Cu₄] coordination cluster obtained from the assembly of two hydroxo-bridged [Cu^II₂] units by a dimeric sodium nitrate template

The heteronuclear [Na₂Cu₄(bemp)₂(OH)₂(NO₃)₂(OH₂)₄]·5H₂O (1·5H₂O; H₃bemp: 2,6-bis-[(2-hydroxyethylimino)-methyl]-4-methyl-phenol) cluster has been synthesized in aqueous-methanol at room temperature and structurally characterized. The water soluble complex is obtained from the template assembly of two [Cu₂(bemp)(OH)] neutral fragments through their weak oxophillic interactions with two interconnected NaNO₃ units as core. Four [Na₂Cu₄] units form a metal–organic cage arrangement in the crystal lattice that traps a (H₂O)₇ cluster. Variable-temperature magnetic susceptibility measurements (2–300 K) reveals a strong antiferromagnetic coupling between the Cu^II ions within the dimers with J = −124.1 cm⁻¹ (in the H = −2JS₁S₂ convention). The interaction of complex 1·3H₂O with calf thymus DNA (binding constant K_b, 4.6 × 10⁴ M⁻¹) in Tris buffer was studied by UV-visible and emission titration, and cyclic voltammetry. The hexanuclear Na₂Cu₄ complex also binds double-stranded supercoiled plasmid pBR322 DNA and displays efficient hydrolytic cleavage. The hydrolytic mechanism is supported by evidence from DNA relegation employing T4 ligase assay and reactive oxygen species (ROS) quenching cleavage experiments.