Synthesis, characterization, X-ray structure and DNA photocleavage by cis-dichloro bis(diimine) Co(III) complexes (2023)

Journal of Inorganic Biochemistry

Volume 100, Issue 3,

March 2006

, Pages 331-343

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Complexes of the type [Co(LL)2Cl2]Cl, where LL=N,N′-ethylenediamine (en), 2,2′-bipyridine (bpy), 1,10-phenanthroline (phen), 1,10-phenanthroline-5,6-dione (phendione) and dipyrido[3,2-a:2′,3′-c]phenazine (dppz) have been synthesized and characterized by elemental analyses, IR, UV–visible and NMR spectroscopy. Crystal structure of [Co(phendione)2Cl2]Cl·0.5HCl·3.5H2O has been solved and refined to R=0.0552. The crystal is monoclinic with space group C2/c; a=25.730(2)Å, b=12.375(1)Å, c=18.979(2)Å, β=119.925(1)° and Z=8. The DNA binding characteristics of the complexes, investigated by covalent binding assay, viscosity measurements and competitive binding fluorescence measurements show that the complexes interact with DNA covalently except the complex containing the planar dppz ligand which intercalates within the base pairs of DNA. The complexes containing en, phen and phendione cleave plasmid pBR 322 DNA upon irradiation under aerobic conditions while the complex containing the dppz ligand cleaves DNA upon irradiation under inert atmosphere. Molecular modeling studies show that the minimized structure of [Co(phendione)2Cl2]+, maintained the octahedral structure while binding to the N7 of guanines and the ligand fits into the major groove without disrupting the helical structure of the B-DNA.


Cisplatin, cis-Pt(NH3)2Cl2 and related Pt(II) complexes are used as anticancer agents in several human cancers particularly testicular and ovarian cancers [1], [2]. The action of cisplatin is dependent on the formation of cis-[Pt(NH3)2(OH2)2]2+ by sequential thermal ligand exchange resulting in the covalent binding to GpG DNA sequences forming intrastrand crosslinks disrupting cellular transcription [3], [4]. However, severe side effects especially nephrotoxicity and acquired resistance limit its widespread use in high doses [5], [6], [7]. The development of reagents that can form intrastrand crosslinks with DNA or RNA continues to be a subject of considerable interest in the areas of molecular biology and rational drug design [8]. Barton et al. initiated the binding studies of transition metal complexes with nucleic acids [9] and reported that the cis-[Ru(phen)2Cl2] (phen=1,10-phenanthroline) binds covalently to DNA and exhibits enantiomeric selectivity different from that seen on intercalation [10]. Recently, there has been an interest in the development of cisplatin analogs, which are activated by light providing a means of localizing the action thus reducing the side effects and dosage. Morrison et al. have shown that the photolysis of cis-[Cr(phen)2Cl2]+ [11], cis-[Rh(phen)2Cl2]+ [12], cis-[Rh(phen)(dppz)Cl2]+ [13] and cis-[Rh(phen)(phi)Cl2]+ [14] (phi=9,10-phenanthrolinequinonediimine, dppz=dipyrido[3,2-a:2′,3′-c]phenazine) complexes with calf-thymus DNA using long wavelength UV light result in covalent binding of the complexes to nucleic acid. These complexes are stable to hydrolysis in dark and unreactive with DNA in the absence of light. There is an evidence that binding primarily occurs to the purines with at least one adduct involving N7 of deoxyguanosine. The reaction may be thought as a prototype for the development of “photocisplatin” reagents that are light activated and could be useful for photochemotherapy of cancer. Vaidyanathan and Nair [15] have reported that cis-[Cr(dppz)2Cl2]+ complex binds to DNA by intercalation with a binding constant of 107M−1. Co (III) Xaa-Xaa-His-metallopeptides induce DNA strand scission upon photoirradiation [16] while oxygen/H2O2 activated Co(II) complex of a macrocyclic ligand based on 1,10-phenanthroline acts as a bulge-specific probe for DNA [17] and vanadyl(IV) complexes derived from N-salicylidene α-amino acids serve as efficient DNA photocleavage agents [18].

As a part of a project aimed at studying the role of ancillary ligands on DNA binding mode we report here the syntheses, characterization, covalent DNA binding and photocleavage studies of [Co(LL)2Cl2]Cl complexes, where LL=N,N′-ethylenediamine, 2,2′-bipyridine, 1,10-phenanthroline, 1,10-phenanthroline-5,6-dione and dipyrido[3,2-a:2′,3′-c]phenazine.

Section snippets

Materials and methods

Chemicals and solvents of analytical grade were used as received. Calf thymus DNA and plasmid pBR322 DNA were obtained from SRL (India) and Bangalore Genei (Bangalore, India), respectively. The enzymes used as scavengers in the DNA cleavage studies superoxide dismutase (SOD EC1.15.1.1) and catalase (EC were purchased from Sigma Chemical Co., USA and used as received. Deionised water was used for the preparation of the buffers. Sephadex G-25,150 was obtained from Aldrich Chemical Co.,


The complexes [Co(LL)2Cl2]Cl where LL=en, bpy, phen, phendione and dppz were prepared by chlorine gas oxidation resulting in green to violet crystalline solids. Single crystals of [Co(phendione)2Cl2]Cl were obtained by slow evaporation of methanol and dilute hydrochloric acid solution.

Crystal structure of [Co(phendione)2Cl2]Cl

The ORTEP diagram of the cation [Co(phendione)2Cl2]+ with the atom numbering is shown in Fig. 1. Crystal refinement and data collection is tabulated in Table 1 and selected bond lengths and angles are given in


From covalent binding assay, viscosity studies and fluorescence quenching it is observed that [Co(dppz)2Cl2]+ interacts with DNA by intercalation while other complexes bind covalently to the DNA double helix retaining the overall structure of the helix , which is supported by the molecular modeling studies. The complexes containing en, phen and phendione cleave plasmid pBR 322 DNA upon irradiation under aerobic conditions while the complex containing the dppz ligand cleaves DNA upon irradiation



A.A.K. and A.S.K. acknowledge the financial assistance from CSIR (EMR II/00(1688)), New Delhi, India. A.C.B. acknowledge financial assistance from University of Pune. The authors thank Director C-DAC, Pune for facilities and expertise available with the Bioinformatics team at C-DAC, Pune.

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