A series of half-sandwich Ru(II) arene complexes of the type [Ru(η(6)-arene)(L)Cl](PF6) 1-4, where arene is benzene (1, 2) or p-cymene (3, 4) and L is N-methylhomopiperazine (L1) or 1-(anthracen-10-ylmethyl)-4-methylhomopiperazine (L2), has been isolated and characterized by using spectral methods. The X-ray crystal structures of 2, 3 and 4 reveal that the compounds possess a pseudo-octahedral "piano-stool" structure equipped with the arene ligand as the seat and the bidentate ligand and the chloride ion as the legs of the stool. The DNA binding affinity determined using absorption spectral titrations with CT DNA and competitive DNA binding studies varies as 4 > 2 > 3 > 1, depending upon both the arene and diazacycloalkane ligands. Complexes 2 and 4 with higher DNA binding affinities show strong hypochromism (56%) and a large red-shift (2, 10; 4, 11 nm), which reveals that the anthracenyl moiety of the ligand is stacked into the DNA base pairs and that the arene ligand hydrophobicity also dictates the DNA binding affinity. In contrast, the monocationic complexes 1 and 3 are involved in electrostatic binding in the minor groove of DNA. The enhancement in viscosities of CT DNA upon binding to 2 and 4 are higher than those for 1 and 3 supporting the DNA binding modes of interaction inferred. All the complexes cleave DNA effectively even in the absence of an external agent and the cleavage ability is enhanced in the presence of an activator like H2O2. Tryptophan quenching measurements suggest that the protein binding affinity of the complexes varies as 4 > 2 > 3 > 1, which is the same as that for DNA binding and that the fluorescence quenching of BSA occurs through a static mechanism. The positive ΔH(0) and ΔS(0) values for BSA binding of complexes indicate that the interaction between the complexes and BSA is mainly hydrophobic in nature and the energy transfer efficiency has been analysed according to the Förster non-radiative energy transfer theory. The variation in the ability of complexes to cleave BSA in the presence of H2O2, namely, 4 > 2 > 3 > 1, as revealed from SDS-PAGE is consistent with their strong hydrophobic interaction with the protein. The IC50 values of 1-4 (IC50: 1, 28.1; 2, 23.1; 3, 26.2; 4, 16.8 μM at 24 h; IC50: 1, 19.0; 2, 15.9; 3, 18.1; 4, 9.7 μM at 48 h) obtained for MCF 7 breast cancer cells indicate that they have the potency to kill cancer cells in a time dependent manner, which is similar to cisplatin. The anticancer activity of complexes has been studied by employing various biochemical methods involving different staining agents, AO/EB and Hoechst 33258, which reveal that complexes 1-4 establish a specific mode of cell death in MCF 7 breast cancer cells. The comet assay has been employed to determine the extent of DNA fragmentation in cancer cells.

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...

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The complexes [Co(diimine)(3)](ClO(4))(2)1-3 and [Ni(diimine)(3)](ClO(4))(2)4-6, where diimine = 1,10-phenanthroline (phen) (1,4), 5,6-dimethyl-1,10-phenanthroline (5,6-dmp) (2,5) and dipyrido[3,2-d:2',3'-f]quinoxaline (dpq) (3,6), have been isolated, characterized and their interaction with CT DNA studied by using a host of physical methods. The X-ray crystal structures of rac-[Co(5,6-dmp)(3)](ClO(4))(2)2 and rac-[Ni(5,6-dmp)(3)](ClO(4))(2)5 have been determined and the isostructural and also isomorphous complex cations possess distorted octahedral coordination geometries. The absorption spectral titrations of the complexes with DNA reveal that the CT DNA binding affinity (K(b)) of the complexes varies as 3>2>1; 6>5>4. The Ni(II) complexes display DNA binding stronger than the corresponding Co(II) analogues, which is expected of their bigger sizes. The higher DNA binding affinity of 3 and 6 is due to the involvement in partial insertion of the extended phen ring in between the DNA base pairs. In contrast, 2 and 5 interact with DNA in the major groove through hydrophobic forces involving the methyl groups on the 5,6 positions of phen ring. An enhancement in relative viscosities of DNA upon binding to 1-6 is consistent with the DNA binding affinities. The CD spectral studies show only an induced CD band on the characteristic positive band of CT DNA for both the phen (1,4) complexes. In contrast, the 5,6-dmp (2,5) and dpq (3,6) complexes bound to CT DNA exhibit biphasic CD signals in place of the positive CD band and the negative helicity band disappears. This reveals that the complexes bind to DNA enantiopreferentially and effect changes in secondary structure of DNA. The CV and DPV responses indicate that the DNA-bound dpq complexes are stabilized in the lower oxidation state of Co(II) more than in the Co(III) oxidation state. The prominent DNA cleavage abilities of 1-3 observed in the presence of H(2)O(2) (200 μM) follows the order 2>1>3 with efficiencies of more than 90% even at 10 μM complex concentration. Interestingly, Ni(II) complexes 4-6 exhibit higher cytotoxicity (IC(50): 1, 28.0; 2, 15.0; 3, 20.0; 4, 8.0; 5, 2.0; 6, 2.0 μM at 48 h; IC(50): 1, 30.0; 2, 20.0; 3, 25.0; 4, 10.0; 5, 3.0; 6, 3.0 μM at 24 h) against human breast cancer (MCF 7) cell lines than the Co(II) complexes 1-3 as well as cisplatin in spite of their inability to cleave DNA. Also, the 5,6-dmp complex 5 shows cytotoxicity higher than the dpq complex 6 at 24 h incubation time and both 5 and 6 display apoptotic and necrotic modes of cell death.

A series of water soluble copper(II) complexes of the types [Cu(L)Cl] 1-2, where LH is 2-(2-(1H-benzimidazol-2-yl)ethyliminomethyl)phenol (H(L1)), and 2-(2-(1H-benzimidazol-2-yl)-ethyliminomethyl)-4-methylphenol (H(L2)), and [Cu(L)Cl2] 3-6, where L is (2-pyridin-2-yl-ethyl)pyridin-2-ylmethyleneamine (L3), 2-(1H-benzimidazol-2-yl)ethylpyridin-2-yl-methyleneamine (L4), 2-(1H-benzimidazol-2-yl)ethyl(1H-imidazol-2-ylmethylene)amine (L5), and 2-(1H-benzimidazol-2-yl)ethyl-(4,4a-dihydroquinolin-2-ylmethylene)amine (L6), have been isolated and characterized by elemental analysis, electronic absorption, ESI-MS and EPR spectral techniques and the electrochemical method. The single crystal X-ray structures of [Cu(L1)Cl] 1 and [Cu(L2)Cl] 2 possess a distorted square-based coordination geometry while [Cu(L4)Cl2] 4 and [Cu(L6)Cl2] 6 possess a distorted trigonal bipyramidal coordination geometry. Both absorption spectral titration and an EthBr displacement assay reveal that all the complexes bind with calf thymus (CT) DNA through covalent mode of DNA interaction involving the replacement of an easily removable chloride ion with DNA nucleobases. All the complexes exhibit oxidative cleavage of supercoiled (SC) plasmid DNA in the presence of hydrogen peroxide as an activator. It is remarkable that at 50 μM concentration 5 and 6 completely degrade SC DNA into undetectable minor fragments and thus they act as efficient chemical nucleases. All the complexes are remarkable in displaying cytotoxicity against the HBL-100 human breast cancer cell line with potency more than that of the widely used drug cisplatin and hence they have the potential to act as promising anticancer drugs. Interestingly, they are non-toxic to normal cell lymphocytes isolated from human blood samples, revealing that they are selective in killing only the cancer cells.

The complexes rac-[Fe(diimine)(3)](ClO(4))(2)1-4, where diimine = 2,2'-bipyridine (bpy) 1, 1,10-phenanthroline (phen) 2, 5,6-dimethyl-1,10-phenanthroline (5,6-dmp) 3 and dipyrido[3,2-d:2',3'-f]quinoxaline (dpq) 4, have been isolated, characterized and their interaction with calf thymus DNA studied by using a host of physical methods. The X-ray crystal structure of rac-[Fe(5,6-dmp)(3)](ClO(4))(2)3 has been determined and the packing diagram shows the presence of two enantiomeric forms of the complex cations in the same unit cell. The structures of 1-4 in solution have also been studied using UV-Visible, Cyclic Voltammetry and ESI-MS data and all data available suggests that they retain their solid state structures even in solution. The absorption spectral titrations of the iron(ii) complexes with CT DNA reveal that the DNA binding affinities of the complexes vary in the order, 4 (K(b): 9.0 × 10(3)) > 2 (6.8 × 10(3)) > 3 (4. 8 × 10(3)) > 1 (2.9 × 10(3) M(-1)). The DNA interaction of dpq complex (4) involves partial insertion of the extended phen ring in between the DNA base pairs, which is deeper than that of phen (2). The 5,6-dmp (3) complex is involved in groove binding in the major groove of DNA. The lower DNA binding affinity of 1 is due to electrostatic interaction of the cationic complexes with exterior phosphates of DNA. The EthBr displacement assay and DNA viscosity study support these DNA binding modes and the above trend in DNA binding affinities. The complexes of 1 and 2 show induced CD (ICD) upon interaction with CT DNA while 3 and 4 bound to DNA exhibit inversion in the positive band with the helicity band showing very small changes, which implies that 3 and 4 bind enantiopreferentially to DNA. The DNA cleavage abilities of 1-4 have been observed at 10 μM concentration of complexes in the presence of 100 μM H(2)O(2) and the DNA cleavage efficiency (> 90%) follows the order 3 > 1 > 2 > 4. The anticancer activity of 1-4 against human breast cancer cell line (MCF-7) has also been studied. The IC(50) values of the complexes at different incubation time intervals of 24 and 48 h follow the order, 3 (0.8, 0.6) < 4 (20.0, 17.0) < 2 (28.0, 22.0) < 1 (32.0, 29.0 μM). Interestingly, 3 exhibits anticancer activity more potent than 1, 2 and 4 and cisplatin for both 24 and 48 h. It induces cell death both through apoptosis and necrosis mechanisms, as revealed by morphological assessment data obtained by using AO/EB and Hoechst 33258 fluorescence staining methods.

The water soluble mixed ligand copper(II) complexes of the type [Cu(sal)(diimine)(ClO4)]21-5, where sal is salicylaldehyde and diimine is 2,2'-bipyridine (bpy, 1), 1,10-phenanthroline (phen, 2), 5,6-dimethyl-1,10-phenanthroline (5,6-dmp, 3), 3,4,7,8-tetramethyl-1,10-phenanthroline (3,4,7,8-tmp, 4) or dipyrido[3,2-d:2',3'-f]quinoxaline (dpq, 5), and [Cu(sal)(phen)(NO3)]2 (2a) have been successfully isolated and characterized by elemental analysis and other spectral techniques. The DNA binding and cleavage properties of 1-5 have been explored by using various physical and biochemical methods. The coordination geometry around copper(II) in the X-ray structures of 1, 2, 2a and 4 is described as an elongated octahedron. The UV-Vis and EPR spectral and ESI-MS studies reveal that in solution the dinuclear complexes dissociate into essentially mononuclear [Cu(sal)(diimine)]+ species with square-based geometry. The absorption spectral titrations and competitive DNA binding studies reveal that the intrinsic DNA binding affinity of the complexes depends upon the diimine co-ligand and is of the order of dpq (5) > 3,4,7,8-tmp (4) > 5,6-dmp (3) > phen (2) > bpy (1). The complexes 2 and 5 are involved in a partial intercalative interaction with DNA base pairs, while 3 and 4 are involved in a hydrophobic interaction with DNA and 1 is involved in an electrostatic interaction with DNA, which is supported by viscosity studies. Interestingly, only 3 and 4 are selective in exhibiting a positive induced CD band (ICD) upon binding to DNA suggesting that they induce a B to A conformational change in DNA. All the complexes exhibit an oxidative DNA cleavage ability, which varies as 5 > 4 > 3 > 2 > 1. While 4 and 5 are unique in displaying prominent double-strand DNA cleavage even in the absence of an activator, 2 and 3 display only single-strand DNA cleavage. Interestingly, all the complexes exhibit oxidative double-strand DNA cleavage in the presence of ascorbic acid, with 4 and 5 showing a DNA cleavage activity more prominent than 1 and 2. The ability of the complexes to bind and cleave the protein BSA varies in the order, 4 > 3 > 5 > 2 > 1. Interestingly, 3 and 4 cleave the protein in the presence of H2O2 as an activator in a non-specific manner suggesting that they can act as chemical proteases. It is remarkable that all the complexes exhibit cytotoxicity against human breast cancer cell lines (MCF-7) with a potency more than the widely used drug cisplatin indicating that they have the potential to act as effective anticancer drugs in a time dependent manner. The morphological assessment data obtained by using Hoechst 33258 staining reveal that 3 and 4 induce apoptosis much more effectively than the other complexes. Also, the alkaline single-cell gel electrophoresis study (comet assay) suggests that the same complexes induce DNA fragmentation more efficiently than others.