DNA Cleavage by Copper−ATCUN Complexes. Factors Influencing Cleavage Mechanism and Linearization of dsDNA

The reactivity of two peptide-Cu complexes (GGH-Cu(-) and KGHK-Cu(+)) toward DNA cleavage has been quantitatively investigated. Neither complex promoted hydrolytic cleavage, but efficient oxidative cleavage was observed in the presence of a mild reducing agent (ascorbate) and dioxygen. Studies with scavengers of ROS confirmed hydrogen peroxide to be an obligatory diffusible intermediate. While oxidative cleavage of DNA was observed for Cu(2+)(aq) under the conditions used, the kinetics of cleavage and reaction products/pathway were distinct from those displayed by peptide-Cu complexes. DNA cleavage chemistry is mediated by the H(2)O-dependent pathway following C-4'H abstraction from the minor groove. Such a cleavage path also provides a ready explanation for the linearization reaction promoted by KGHK-Cu(+). Kinetic activities and reaction pathways are compared to published results on other chemical nucleases. Both peptide-Cu complexes were found to display second-order kinetics, with rate constants k(2) approximately 39 and 93 M(-1) s(-1) for GGH-Cu(-) and KGHK-Cu(+), respectively. Neither complex displayed enzyme-like saturation behavior, consistent with the relatively low binding affinity and residence time expected for association with dsDNA, and the absence of a prereaction complex. However, the intrinsic activity of each is superior to other catalyst systems, as determined from relative k(2) or k(cat)/K(m) values. Linearization of DNA was observed for KGHK-Cu(+) relative to GGH-Cu(-), consistent with the increased positive charge and longer residency time on dsDNA.