CobaltIII polypyridyl complexes

Over the past decades, the potential application of polypyridyl transition metal complexes as non-radioactive nucleic acid probes, which can recognize and cleave DNA, has stimulated intensive research.8,40 43 In comparison to other complexes, the octahedral polypyridyl complexes are particularly suitable for these applications because they are coordinatively saturated, inert to substitution, rigid and structurally well defined. In addition, these complexes can interact non-covalently with nucleic acids by intercalation, groove binding or electrostatic binding. When the size, shape and chirality of the complexes are suited to the DNA structure, the complexes may approach closely to and intercalate the base pairs of DNA; and the spectroscopic properties, such as electronic absorption and steady-state emission, and the thermodynamic properties, such as viscosity, will display change. On the other hand, upon irradiation, these complexes can induce single- or double-stranded cleavage of plasmid DNA. Although they have not received as much attention as the ruthenium system, cobalt complexes have attracted some interest due to their unique electronic structure and redox properties.

Maiya et al. reported the interaction between DNA and cobalt(III) complexes-containingdipyrido[3,2-a:2',3'-c]phenazine (dppz) and its derivatives (Figure 11.3).44,45 This study demonstrated that these cobalt(III) complexes bind to DNA in an intercalative mode and affect the photocleavage of DNA (super-coiled pBR 322 DNA). Ji and co-workers discovered that cobalt(III) complexes-containing imidazo[4,5-f][1,10]-phenanthroline derivatives are avid binders of calf-thymus DNA (Figure 11.4).46 49 Cobalt(III) complexes have been found to promote the single-stranded cleavage of plasmid pBR 322 DNA upon irradiation. Mechanisms for photocleavage were also investigated. Recently, n3

N^CN

[Co(phen)2(qdppz)]3+ Figure 11.3 Cobalt(III) complexes-containing dppz and its derivatives

Figure 11.4 Cobalt(III) complexes with imidazo[4,5-/][1,10]phenanthroline derivatives

"N

M II

"N

H Cl NN

H Cl NN

Figure 11.4 (Continued)

Figure 11.5 Chemical structure of [Co(tfa)2(happ)]

Cheng and co-workers have synthesized a novel octahedral cobalt(II) complex [Co(tfa)2(happ)] (tfa = trifluoroacetate; happ = hexaazaphenanthroline-cyclophane) (Figure 11.5). This complex exhibits low affinity toward double-stranded DNA and low cleavage reactivity toward single-stranded DNA. However, this complex was found to act as a specific probe for the DNA bulge with single-stranded DNA cleavage activity in the presence of H2O2.50,51

In the last few years, some other ligands have been complexed to cobalt(III) complexes.52,53 Nonetheless, no significant improvements have been obtained with respect to the above-mentioned examples.

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