Electrochemistry of methylene blue bound to a DNA-modified electrode

Gold surfaces have been derivatized with 15-base-pair double-stranded DNA oligonucleotides containing a pendant 5' hexanethiol linker. The electrochemistry of intercalated methylene blue has been investigated at these modified electrodes. Chronocoulometry, cyclic voltammetry, ellipsometry, and quantitation via ³²P labeling are all consistent with a surface coverage of ≤75% with the DNA helices stacked at an angle from the electrode surface. Cyclic voltammetry at low methylene blue/duplex stoichiometries yields well-behaved surface waves with E° = -0.25 V (vs SCE), a value 0.03 V negative of that in aqueous solution. A binding isotherm for methylene blue at an electrode derivatized with the double-stranded sequence 5' SH-(CH₂)₆-p-AGTACAGTCATCGCG 3' was obtained from coulometric titrations and gave an affinity constant equal to 3.8(5) x 10⁶ M^-1 with a saturation value of 1.4(2) methylene blue intercalators per DNA duplex. Taken together, these experiments support a model for the surface morphology in which DNA duplexes are densely packed; methylene blue therefore reversibly binds to sites in the DNA that are close to the bulk solution. Electrochemistry at DNA-derivatized electrodes provides a valuable methodology to examine DNA-bound redox reactions and may offer new insight into DNA-mediated electron transfers.