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cis-Aquahydroxo–tetraamine–cobalt(III) complexes of the general formula [Co(N4)(OH)(H2O)]2+ have been prepared (pH ∼ 7) from their corresponding well characterized carbonato or dichloro compounds, where N4 = TPA {tris(2-pyridylmethyl)amine}, 1; pmap {bis[2-(2-pyridylethyl)]-(2-pyridylmethyl)amine}, 2; 2,2,2-tet {triethylenetetraamine}, 3; 3,2,3-tet {4,7-diaza-1,10-decanediamine}, 4; tren {tris(2-aminoethyl)amine}, 5; and abap {N-(2-aminoethyl)-N,N-bis(3-aminopropyl)amine}, 6. The carbonato complex [Co(pmap)(CO3)]ClO4 was structurally characterized by X-ray crystal analysis. The interactions of the complexes 1–6 with DNA have been investigated (pH 7.0, I = 0.1 M, 37 °C). The complexes 4–6 did not show any detectable hydrolytic cleavage for DNA, whereas significant cleavages were obtained with the rest of the studied complexes (1–3). Under pseudo-Michaelis–Menten kinetic conditions, the reactivity of the complexes in promoting the hydrolytic cleavage of DNA decreases in the order 1 > 3 ≫ 2 (rates were compared at the saturation levels, i.e. V max ′ ) and the kinetic parameters k cat ′ and KM were determined: k cat ′ = 3.49 × 10−4 s−1 and KM = 159 μM for 1 and k cat ″ = 5.08 × 10−5 s−1 and KM = 296 μM for 3. The calculated values of k cat ″ correspond to an enhancement of 3.5 × 107 and 5.1 × 106 for 1 and 3, respectively, over the uncatalyzed DNA. The DNA cleavage with these two complexes was also monitored under ‘true’ Michaelis–Menten kinetic conditions using constant catalyst concentration (Co(III)) and varying substrate concentrations (DNA) but no kinetic fits were observed between kobs and [DNA] and hence the parameters kcat and KM were not calculated in this case. The reactivity of the complexes is discussed based on the structural skeletons of the tetraamine blocking ligands. |
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