Electrochemical investigations of anthraquinone tagged oligonucleotides and electrodes

by (Adam Nathanial), 1974- Mullenix

The DNA double helix is one of the most studied structures in modern science. These molecules provide the basic blueprint for all life as it is presently known to exist. Continuing studies of this structure, and the implications that come with them, currently yield a constantly growing base of knowledge for the study of drug interactions, specifics in the role of the sequence of bases in the DNA chain, and an understanding of the small structural abnormalities and the resulting benefits and problems in the human genome. The research presented herein attempts to examine such interactions using anthraquinone-based modifications for the development of sensor systems for detection of the hybridization event. The anthraquinone modifications described were applied using modified electrodes and modified nucleotides included in the DNA sequences. The first study examines the use of an anthraquinone-modified uridine nucleotide. This nucleotide was included in the synthesis of a DNA pentamer and a set of 20mers varying in the positioning and number of the anthraquione moiety. The free adsorption of these compounds at a hanging mercury drop electrode obeyed the Langmuir adsorption isotherm. The molecular footprint and adsorption parameters scaled with the size of the molecule. These adsorbed layers were examined by cyclic voltammetry to determine the effects of competitive adsorption and hybridization upon the measured signal. Results in these studies indicated a detectable change in the standard potential and signal size with hybridization. This study was limited, however, with the use of a mercury electrode making it impracticable in mass use sensor systems. The other two studies involved the application of thiol modifications for the formation of self-assembled monolayers on gold electrodes. The first approach used a molecular beacon type system to examine the effect electrode positioning played in the measured signal. Here oligodeoxynucleotides (ODNs) were modified on one end with a thiol group for electrode attachment. The other end -iv- -vof the ODN was substituted with the anti-cancer drug, daunomycin, for its electrochemical activity. Addition of a complementary DNA strand changed the position of this drug in relation to the electrode surface and in turn affected the kinetics of the electrode reaction under study. The preliminary results indicated a disappearance in the detectable signal resulting from the hybridization event prompting future examination. The final study applied an anthraquinone to a gold electrode using a thiol modification. Interaction of DNA with this immobilized layer was examined for any effects in the reduction/oxidation kinetics of the anthraquinone redox couple in the presence of DNA. The kinetics were measured by two methods: potential step chronoamperometry and AC voltammetry. The chronoamperometric data were fit to a two-stepped EE reduction mechanism. While no effect of DNA on the AC voltammetry data was seen, a slight decrease in the rate constants derived from the anodic chronoamperometric data was seen.