Electron-Rich Oxoruthenium(IV) Cleavage Agents: A Zero-Order Rate Law for DNA Catalysis
journal contributionposted on 08.10.1997, 00:00 by Thomas W. Welch, Suzanne A. Ciftan, P. S. White, H. Holden Thorp
A new family of oxoruthenium(IV) complexes based on [(DAMP)(L)RuO]2+ have been prepared where DAMP = 2,6-bis((dimethylamino)methyl)pyridine and L = 2,2‘-bipyridine (bpy), 1,10-phenanthroline (phen), or dipyridophenazine (dppz). The structures of [(DAMP)(bpy)RuO]2+ and [(DAMP)(phen)RuO]2+ were determined by X-ray crystallography. The Ru−O bond lengths (1.805(3) and 1.814(4) Å, respectively) are indicative of multiple bonding, as expected for oxoruthenium(IV), and clear steric protection of the RuO moiety is provided by the DAMP ligand. Cyclic voltammetry shows that the tertiary amine functionalities of the DAMP ligand stabilize both the RuIVO2+ and RuIIIOH2+ redox forms relative to other (polypyridyl)oxoruthenium(IV) complexes. As a result, the oxidations of both sec-phenylethanol and trans-stilbene are ∼100 times slower for [(DAMP)(bpy)RuO]2+ than for [(bpy)2(py)RuO]2+. Accordingly, the reaction mechanisms involve oxidation of substrate only by the RuIVO2+ form with no contribution from direct oxidation by the RuIIIOH2+ intermediate, which greatly simplifies the kinetic analysis. The RuIVO2+ forms are not effective oxidants of the sugar moiety of mononucleotides; however, the base functionality of guanosine 5‘-monophosphate is oxidized at detectable rates. In contrast, cleavage of a hairpin oligonucleotide is detected at both guanine and sugar functionalities, indicating that the oligomer promotes sugar oxidation by increasing the local concentration of the metal complex. The RuIIIOH2+ form of the DAMP complexes is stable in the absence of DNA but is reduced following a zero-order rate law in the presence of calf thymus DNA. Analysis using a model that resembles Michaelis−Menten kinetics indicates that the binding domain on DNA catalyzes the disproportionation of the complex. The model yields a binding constant and a calculated first-order rate constant that are in good agreement with independent measurements.