Electrochemical detection of interactions between DNA and various ligands
Antibodies specific for DNA, with varying degrees of sequence specificity, are common in many autoimmune diseases including systemic lupus erythematosus. The presence of anti-DNA antibodies is a useful determinant in arriving at a prognosis in these conditions. Given the prevalence of these diseases in both the developing and developed world and the difficulty that often accompanies diagnosis of autoimmune diseases, it is desirable to have sensitive, rapid, and inexpensive diagnostic tools for these diseases. Because of the great sensitivity of electrochemical techniques and their potential utility in characterizing interactions between macromolecules, electrochemistry has great potential as a diagnostic tool for any disease involving antibodies. Anti-DNA antibodies are present in many autoimmune diseases, notably systemic lupus erythematosus. Since DNA is a stable and well-characterized antigen, an electrochemical-based assay is particularly useful for diagnosis of these diseases. The impedance of a gold surface was measured in the presence and absence of single- and double-stranded DNA monolayers. The DNA monolayer was diluted with butanethiol in order to provide a surface with more accessible binding sites than an undiluted monolayer. The change in impedance of the DNA monolayer following exposure to various small molecules and macromolecules was assessed. The molecules used included polyamines that induce conformational changes in DNA, proteins which bind DNA specifically, proteins which bind DNA non-specifically, and proteins which do not bind DNA. The presence of a DNA monolayer, whether single- or double-stranded, increased the impedance of the gold surface and dilution of the monolayers by butanethiol decreased the impedance, as expected. When exposed to polyamines, the impedance of the DNA monolayer decreased further. This could be due to lowered charge repulsion, to DNA condensation, or to a combination of both. When methylated bovine serum albumin was exposed to the monolayer, there was an increase in impedance. Conversely, when bovine serum albumin was exposed to the monolayer, the impedance was only increased at very high concentrations of protein. The increase following exposure to high concentrations of bovine serum albumin was likely due to deposition of protein on to the monolayer. The specificity of these interactions was illustrated by experiments with the antibody Hed 10, which binds single-stranded but not double-stranded DNA. Exposure to Hed 10 only caused a significant change in impedance when exposed to monolayers of single-stranded DNA.The decreased impedance of the DNA monolayer caused by the presence of polyamines is consistent with the known structural perturbations induced by these molecules as measured with other methods. Similarly, the increase in impedance caused by the presence of proteins which bind DNA is consistent with increased steric interference by the protein-DNA complex. The failure of proteins which do not bind DNA to affect the impedance of the monolayer indicated that the effects in the experiments with DNA-binding proteins were due to protein binding and not other factors. The specificity of the assay as demonstrated by the results of the experiments with Hed 10 suggest that impedance-based measurements may provide the basis for a reliable, sensitive, and inexpensive assay for detecting the presence of anti-DNA antibodies in the serum of autoimmune disease patients.
DNA-binding drugs, DNA-binding proteins, electrochemistry
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