Structure and spectral performance of Co(II) and Co(III) complexes for DNA-binding: Synthesis, characterization and comparison

Abstract

Abstract There have been many literatures reporting the binding performance of transition metal complexes towards DNA, aiming at anti-cancer and anti-tumor drugs. However, most of these references focused on divalent transition metal cations, such as Co(II), Ni(II), Cu(II) and Zn(II). Only a few efforts have been devoted to high-valent transition metal cations. In this paper, we synthesized two Co(II) and Co(III) complexes, tris{2-(pyridin-2-yl)benzo[d]imidazole-1-ide}-cobalt(III) [denoted as Co(L)3] and bis{2-(pyridin-2-yl)benzo[d]imidazole-1-ide}-{2-(pyridin-2-yl)-1H-benzo[d]imidazole}-cobalt(II) [denoted as Co(HL)(L)2], using a diamine ligand 2-(pyridin-2-yl)-1H-benzo[d]imidazole (denoted as HL). Their molecular structure was identified with their single crystals, MS and NMR spectra. Co(L)3 and Co(HL)(L)2 share the same crystal system (monoclinic) and space group (P21/c). Their crystal cell parameters were found rather similar to each other owing to their rather similar ligands, showing a slightly distorted octahedron coordination geometry. No intramolecular interaction, superamolecular interactions and π-π staking between neighboring Co(L)3 molecules or neighboring Co(HL)(L)2 molecules were found, forming a clean coordination environment. Density functional theory calculation on their single crystals suggested that they had similar onset electronic transitions. Their thermal stability, absorption and emission were compared. Owing to their rather similar molecular structures, most of their features were similar. Their binding performance for calf thymus DNA (CT-DNA) was then analyzed and compared. The binding mode between two Co complexes and CT-DNA was analyzed and confirmed by means of viscosity monitoring, circular dichroism analysis, SEM and XRD comparison. Their binding constants were determined as 974.89\xa0M-1 for Co(L)3 and 829.25\xa0M-1 for Co(HL)(L)2, respectively. We found a hydrogen bond between Co(L)3 and CH3OH. No hydrogen bonds were found in Co(HL)(L)2. This result suggested that the Co(L)3 interact with Ct-DNA via the combination of hydrogen bond and weak intermolecular interaction with DNA, while Co(HL)(L)2 interacts with Ct-DNA via only weak intermolecular interaction with DNA. These values were two fold higher than those of literature values (