A comparative study on the synthesis and crystal structures of Ni(<sc>ii</sc>) complexes bearing tetradentate N2O2 donor Schiff bases: biomolecular interactions, cytotoxicities and molecular docking

Schiff bases are the most widely used class of molecules and their metal complexes are utilized as medicinally important scaffolds due to their versatile biological profile. So herein, we have reported the synthesis of N2O2-type tetradentate Schiff bases {N,N '-(ethylene)bis(salicylideneimine) (1a) and N,N '-(o-phenylene)bis(salicylideneimine) (1b)} and their Ni(II) complexes, dimeric N,N '-(ethylene)bis(salicylideneiminato)nickel(II) (2a) and N,N '-(o-phenylene)bis(salicylideneiminato)nickel(II) (2b), and their characterization using CHN analysis, ESI-MS and XRD techniques. The interaction of ligands 1a-b and complexes 2a-b with the biomolecules {calf thymus DNA (CT-DNA) and bovine serum albumin (BSA)} has been investigated via electronic absorption and fluorescence spectroscopy. The results show that ligands 1a-b and complexes 2a-b can bind to CT-DNA via a minor groove mode. Moreover, the fluorescence quenching mechanism between ligands 1a-b and complexes 2a-b and BSA is a static quenching process. The results showed that the interaction between ligands 1a-b and BSA is driven by hydrogen bond and van der Waals interactions, while the hydrophobic forces stabilized the interaction between complexes 2a-b and BSA. Additionally, the r values of ligands 1a-b and complexes 2a-b (1a = 0.53 nm, 1b = 0.59 nm, 2a = 0.57 nm and 2b = 0.71 nm) calculated using FRET theory suggested that the distance between ligands 1a-b and complexes 2a-b and BSA is quite close. The radical scavenging activity of ligands 1a-b and complexes 2a-b was determined in terms of EC50, using the DPPH method. The cytotoxicities of ligands 1a-b and complexes 2a-b against human breast cancer (MCF-7) and lung cancer (A549 and H1299) cell lines and healthy lung bronchial (BEAS-2B) and breast epithelial (MCF-10A) cell lines were investigated. Molecular docking simulations were carried out to correlate the data with the experimental findings. The binding interaction of ligands 1a-b and complexes 2a-b with the B-DNA and 1M17 was generated and predicted using Autodock 4.2 software. The results indicated that ligands 1a-b and complexes 2a-b bind at the minor groove of DNA through intercalation and at the active side of the EGFR tyrosine kinase.