The effects of DNA binding on the normal pulse voltammetry of metal complexes have been investigated. Studies were performed both for oxidation of OsL 3 2+/3+ and for reduction of CoL 3 3+/2+ (L is bpy ) 2,2′bipyridine or phen ) 1,10-phenanthroline). The diffusive current obtained from voltammograms at potentials well past E 1/2 gives an accurate measure of the extent to which the complexes codiffuse with DNA or are free in solution, and this response is not affected by kinetic factors resulting from slow heterogeneous electron transfer. Analysis of the diffusion-limited current using the appropriate binding isotherm provides binding constants in good agreement with those measured by other methods. For the bpy complexes, the ionic strength dependence, the relative binding constants for the 2+ and 3+ forms, and the associated change in E 1/2 upon DNA binding are in good agreement with the predictions of polyelectrolyte theory where the 3+ ion binds more strongly. For the phen complexes, the reverse trend is observed and is consistent among the absolute binding constants, ionic strength dependence, and E 1/2 shift; this behavior is ascribed to a hydrophobic interaction. The technique is also applied to two-electron couples based on [(tpy)(L)RuOH 2 ] 2+ /[(tpy)(L)-RuO] 2+ that exhibit slow heterogeneous electron transfer; however, these kinetic complications do not prohibit accurate determination of the binding energetics using normal pulse voltammetry. Taken together, the data provide a comprehensive picture of the effects of partial DNA binding on voltammetry, which provides a basis for determining homogeneous kinetic rate constants for electrocatalytic DNA oxidation from voltammograms.

The diffusion coefficients of sonicated calf-thymus DNA and an oligonucleotide fragment have been measured via an electrochemical strategy. The complex [Os(bpy)2(dppz)12+ (bpy = 2, 2'-bipyridine, dppz = dipyridophenazine) has been synthesized as a simple, single-electron intercalating redox couple. The free complex undergoes a kinetically reversible single-electron oxidation to the corresponding Os(II1) complex at an applied potential of 0.72 V vs Ag/AgCl, which is insufficient to achieve electrocatalytic DNA oxidation.Upon addition of DNA, the complex binds via intercalation to a two-base-pair site with binding constant Kb = 4 x lo6 M-I, as reflected in the changes in the photophysics of the Os(I1)-dppz chromophore. Changes in the voltammetric current upon binding reveal a slight limitation on heterogeneous charge-transfer kinetics (k = 5 x lov4 cm s-I) and a dramatic limitation on mass transfer of bound complexes. Normal pulse voltammetry gives diffusion coefficients of sonicated calf-thymus DNA and (dA)20*(dT)20 equal to 2.0 x cm2 s-I, respectively; each agrees with values reported for light-scattering experiments and theoretical calculation. and 1.2 xThe diffusion properties of biopolymers play an important role in many physiological For example, the recognition of a specific cleavage site by restriction enzymes is thought to involve initial, nonspecific electrostatic binding of the enzyme to DNA in a mode that is weak enough that diffusion of the enzyme is still f a~i l e .~ The weakly bound enzyme then diffuses along the DNA strand until reaching the cleavage locus and effecting site-specific cleavage. The importance of these processes clearly dictates a detailed understanding of the relative diffusion properties of molecules such as DNA and DNA-binding proteins. We report here on an electrochemical system for studying the diffusion properties of nucleic acids that can be performed on small volumes of sample at relatively low nucleic acid concentrations. These studies can be performed with commercially available potentiostats and analysis software.The discovery of both natural transition-metal-based antitumor agents such as iron-bleomycin (FeBLM)4*5 and synthetic platinum drugs6 has prompted intense investigation of the interactions and reactions of synthetic transition-metal complexes with nucleic acids. In particular, extensive efforts have been made to cleave DNA oxidatively in a site-specific manner with metal complexes in either an electronic excited or electrochemically potentiated high oxidation state.I0,' Electrochemical studies have been made of electrocatalytic cleavage reactions using FeBLM,' Mn porphyrins,I0 outer-sphere oxidants,I2 and complexes based on the Ru(IV)O~+/RU(II)OH~~+ ~o u p l e . '~-'~ An advantage of electrochemical cleavage reactions is that the current measured in electrolysis or in voltammetry should provide mechanistic information on the extent or rate of the cleavage reaction. However, because currents in solution electrochemistry are governed by mass transport, reliable inte...

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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 Ru=O moiety is provided by the DAMP ligand. Cyclic voltammetry shows that the tertiary amine functionalities of the DAMP ligand stabilize both the Ru(IV)O(2+) and Ru(III)OH(2+) redox forms relative to other (polypyridyl)oxoruthenium(IV) complexes. As a result, the oxidations of both sec-phenylethanol and trans-stilbene are approximately 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 Ru(IV)O(2+) form with no contribution from direct oxidation by the Ru(III)OH(2+) intermediate, which greatly simplifies the kinetic analysis. The Ru(IV)O(2+) 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 Ru(III)OH(2+) 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.