Oxidative DNA cleavage by copper complexes

The DNA cleavage activity of Cu complexes of 1,10 -phenanthroline (phen, Fig. 1) with H2O2 as coreactant was initially observed by Sigman et al. in 1979 

Formation of Cu (phen)2 is described by equilibriums summarized in Eqs. (1), (2) in Fig. 2 (9,10). Phen binds also Cu ions with equilibrium constants log Ki and log K2 of 8.8 and 6.6, respectively (10). Analysis by X-ray diffraction of monocrystals showed that the geometrical environment of the metal ion in Cu (phen)2 is tetrahedral (11 but the Cu (phen)2 complex adopts different coordination geometries as trigonal bipyramid or octahedron with one or two additional ligand(s) as H2O or chloride (12). The complex was often 

Despite the frequent use of Cu(phen)2 as a cleaving agent, the exact nature of the reactive species involved in this DNA cleavage is still unknown. 

However, reductants (always in large excess in the experiments) used for the activation of the Cu, e.g. thiols, are susceptible to compete with phen itself for copper coordination site and so decrease the quantity of Cu (phen)2 available for DNA cleavage (14). 

Kinetic studies demonstrated that the nuclease activity of Cu (phen)2 proceeds by an ordered mechanism the freely diffusing Cu (phen)2 is first reduced to the Cu complex Cu (phen)2 which binds reversibly to DNA. This non-covalently bound Cu complex is then oxidized by H2O2 to generate the reactive species responsible for DNA strand scission (19). The association constant for DNA binding of Cu (phen)2 was determined to be 4.7 x 10 (DNA base pairs) (13). However, Cu (phen)2 also binds to DNA, but the direct reduction of the complex Cu (phen)2---DNA by 0 is very slow and can be neglected (25). 

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