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 e lectrochemistry of intercalated methylene blue has been investigated a t these modified electrodes. Chronocoulometry, cyclic voltammetry, ell ipsometry, and quantitation via P-32 labeling are all consistent with a surface coverage of greater than or equal to 75% with the DNA helice s stacked at an angle from the electrode surface. Cyclic voltammetry a t low methylene blue/duplex stoichiometries yields well-behaved surfac e waves with E degrees = -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 gav e an affinity constant equal to 3.8(5) x 10(6) M(-1) with a saturation value of 1.4(2) methylene blue intercalators per DNA duplex. Taken to gether, these experiments support a model for the surface morphology i n which DNA duplexes are densely packed; methylene blue therefore reve rsibly binds to sites in the DNA that are close to the bulk solution. Electrochemistry at DNA-derivatized electrodes provides a valuable met hodology to examine DNA-bound redox reactions and may offer new insigh t into DNA-mediated electron transfers.