ELECTROCHEMICAL DETERMINATION OF NUCLEIC-ACID DIFFUSION-COEFFICIENTS THROUGH NONCOVALENT ASSOCIATION OF A REDOX-ACTIVE PROBE

The diffusion coefficients of sonicated calf-thymus DNA and an oligonucleotide fragment have been measured via an electrochemical strategy. The complex [Os(bpy)(2)(dppz)](2+) (bpy = 2, 2'-bipyridine, dppz = dipyridophenazine) has been synthesized as a simple, single-electron intercalating redox couple. The free complex undergoes a kinetically reversible single-electron oxidation to the corresponding Os(III) complex at an applied potential of 0.72 V vs Ag/AgCl, which is insufficient to achieve electrocatalytic DNA oxidation. Upon addition of DNA, the complex binds via intercalation to a two-base-pair site with binding constant K-b = 4 x 10(6) M(-1), as reflected in the changes in the photophysics of the Os(II)-dppz chromophore. Changes in the voltammetric current upon binding reveal a slight limitation on heterogeneous charge-transfer kinetics (k = 5 x 10(-4) cm s(-1)) and a dramatic limitation on mass transfer of bound complexes. Normal pulse voltammetry gives diffusion coefficients of sonicated calf-thymus DNA and (dA)(20).(dT)(20) equal to 2.0 x 10(-6) and 1.2 x 10(-6) cm(2) s(-1), respectively; each agrees with values reported for light-scattering experiments and theoretical calculation.