Mixed Ligand Copper(II) Complexes of N,N-Bis(benzimidazol-2-ylmethyl)amine (BBA) with Diimine Co-Ligands: Efficient Chemical Nuclease and Protease Activities and Cytotoxicity

Abstract

A series of mononuclear mixed ligand copper(II) complexes [Cu(bba)(diimine)](ClO(4))(2)1-4, where bba is N,N-bis(benzimidazol-2-ylmethyl)amine and diimine is 2,2'-bipyridine (bpy) (1), 1,10-phenanthroline (phen) (2), 5,6-dimethyl-1,10-phenanthroline (5,6-dmp) (3), or dipyrido[3,2-d:2',3'-f]quinoxaline (dpq) (4), have been isolated and characterized by analytical and spectral methods. The coordination geometry around copper(II) in 2 is described as square pyramidal with the two benzimidazole nitrogen atoms of the primary ligand bba and the two nitrogen atoms of phen (2) co-ligand constituting the equatorial plane and the amine nitrogen atom of bba occupying the apical position. In contrast, the two benzimidazole nitrogen atoms and the amine nitrogen atom of bba ligand and one of the two nitrogen atoms of 5,6-dmp constitute the equatorial plane of the trigonal bipyramidal distorted square based pyramidal (TBDSBP) coordination geometry of 3 with the other nitrogen atom of 5,6-dmp occupying the apical position. The structures of 1-4 have been optimized by using the density functional theory (DFT) method at the B3LYP/6-31G(d,p) level. Absorption spectral titrations with Calf Thymus (CT) DNA reveal that the intrinsic DNA binding affinity of the complexes depends upon the diimine co-ligand, dpq (4) > 5,6-dmp (3) > phen (2) > bpy (1). The DNA binding affinity of 4 is higher than 2 revealing that the π-stacking interaction of the dpq ring in between the DNA base pairs with the two bzim moieties of the bba ligand stacked along the DNA surface is more intimate than that of phen. The complex 3 is bound to DNA more strongly than 1 and 2 through strong hydrophobic interaction of the methyl groups on 5,6-positions of the phen ring in the DNA grooves. The extent of the decrease in relative emission intensities of DNA-bound ethidium bromide (EB) upon adding the complexes parallels the trend in DNA binding affinities. The large enhancement in relative viscosity of DNA upon binding to 3 and 4 supports the DNA binding modes proposed. Interestingly, the 5,6-dmp complex 3 is selective in exhibiting a positive induced CD band (ICD) upon binding to DNA suggesting that it induces a B to A conformational change. In contrast, 2 and 4 show induced CD responses indicating their involvement in strong DNA binding. Interestingly, only the dpq complex 4, which displays the strongest DNA binding affinity and is efficient in cleaving DNA in the absence of an activator with a rate constant of 5.8 ± 0.1 h(-1), which is higher than the uncatalyzed rate of DNA cleavage. All the complexes exhibit oxidative DNA cleavage ability, which varies as 4 > 2 > 3 > 1 (ascorbic acid) and 3 > 2 > 4 > 1 (H(2)O(2)). Also, the complexes cleave the protein bovine serum albumin in the presence of H(2)O(2) as an activator with the cleavage ability varying in the order 3 > 4 > 2 > 1. The highest efficiency of 3 to cleave both DNA and protein in the presence of H(2)O(2) is consistent with its strong hydrophobic interaction with the biopolymers. The IC(50) values of 1-4 ag...

DNA damage and induction of apoptosis in pancreatic cancer cells by a new dinuclear bis(triazacyclonane) copper complex

“…It is worthy of note that no external reducing agent was added. This does not necessarily suggest a hydrolytic cleavage mechanism, as several self-activating oxidative Cu metallonucleases have been described [39][40][41][42][43][44][45]. In fact, Cu complexes that actually cleave DNA by a hydrolytic mechanism are less numerous [46][47][48].…”

Synthesis of a DNA-targeting nickel (II) complex with testosterone thiosemicarbazone which exhibits selective cytotoxicity towards human prostate cancer cells (LNCaP)

“…A solution of CT-DNA in the buffer solution gave the ratio of UV absorbance at 260 and 280 nm (A 260 /A 280 ) of about 1.75-1.90, indicating that the DNA was sufficiently free from protein contamination [27]. The concentration of DNA was determined spectrophotometrically by employing a molar absorption coefficient by monitoring the UV absorbance at 260 nm using e 260 = 6600 mol À1 cm 2 .…”

Synthesis and the characterization of Schiff-base copper complexes: Reactivity with DNA, 4-NPP and BNPP

“…Several copper complexes have been found to exhibit these modes of interaction. Furthermore, a number of the copper complexes have been found to exhibit cytotoxicity levels that are comparable to or greater than that of cisplatin [26][27][28][29][30][31][32][33][34][35][36][37][38][39].…”