Interactions of Metal Cations with DNA Monolayers: Binding Kinetics and Sensing Applications

Electrostatic interactions with metal ions are essential to the structural stability and polymorphism of oligonucleotides. This thesis describes the extended applications of a simple electrochemical protocol, based on the voltammetric response of multiplycharged transition metal cations ([Ru(NH~)~]~+) bound electrostatically to DNA- modified surfaces, to explore the kinetics of metal ion-DNA interactions and potential sensing applications of DNA-modified electrodes. We first analyzed the time-dependent voltammetric behavior of the redox cation [RU(NH~)~]~' binding to and dissociating from thiolate-DNA monolayers on gold. It was found that the ion-exchange binding kinetics is dominated by the structure of the DNA monolayers, i.e., the apparent first-order rate constant decreases significantly upon increasing the surface density of DNA strands. Furthermore, it has been shown that second-order binding rate constants can be derived from the dependence of the apparent first-order rate constants on the solution concentration of the redox cations, and the dissociation rate constants were obtained by transferring the incubated electrode into a buffer solution free of redox cations. The research was then extended to the potential sensing applications of DNAmodified electrodes. After incubation in a dilute solution of redox transition metal cations, the DNA-modified electrode responds to non-electroactive metal ions, particularly those are important in biological systems. Magnesium, calcium and potassium can be detected at very low concentration levels (micromolar), and that the sensitivity decreases significantly for the monocations (e.g., K'). The derived equilibrium binding constants of the divalent metal cations to DNA-modified electrodes confirm that M ~ ~ ' is binds more strongly to DNA than ca2'. The last part of this thesis describes our first attempt to develop a chip-based electrochemical deoxyribosensor, for which we incorporated a specific nucleic acid adapter (so-called "aptamer") for ATP and adenosine into DNA monolayers on gold. The binding of molecular analytes (e.g., adenosine) induces a conformational change that can be revealed by the voltammetric response of electrostatically bound redox cations: a clear increase of the integrated charge is observed upon increasing the concentration of adenosine in solution. We have also determined the binding constant of adenosine to the deoxyribosensor based on a simple Langmuir model.