New cyclam based Zn(II) complexes: effect of flexibility and para substitution on DNA binding, in vitro cytotoxic studies and antimicrobial activities

A series of macrobicyclic binuclear Zn(II) complexes were synthesized from their macrocyclic ligand (L¹ and L²) by Schiff base condensation with appropriate aliphatic or aromatic diamines, ZnCl₂.4H₂O, and triethylamine. All the complexes were characterized by elemental and spectral analysis. The enhanced DNA binding affinity of complexes [Zn₂L^1a], [Zn₂L^1c], and [Zn₂L^1d] is due to the existence of the electron-donating CH₃ group, which leads to hydrophobic interaction with the hydrophobic DNA surface. The existence of electron-withdrawing Br atom in complex [Zn₂L^2a], [Zn₂L^2c], and [Zn₂L^2e] leads to lesser DNA binding affinity. The fluorescence quenching of the Zn(II) complex at 370 nm indicates the strong coordination of metal ions with N and O atoms of the ligand. The 3D fluorescence spectrum of [Zn₂L^2d] complex has been quenched more compared to [Zn₂L^2a] due to the planarity of aromatic system. DNA cleavage of Zn(II) complexes begins at a low concentration (25 μM) and reaches the maximum cleavage with a successive increase in concentration (100 μM). All the complexes were screened for antimicrobial and anticancer activity.

Graphical abstract

Macrobicyclic binuclear zinc(II) complexes synthesized from their macrocyclic ligands by Schiff base condensation were characterized by elemental and spectral analysis. The enhanced DNA binding affinity (CH₃ group) and lesser DNA binding affinity (Br group) have been explained. The 3D fluorescence spectrum shows quenching due to the planarity of aromatic system. DNA cleavage and in vitro activities have been discussed.