Minor groove binding of a novel tetracationic diviologen

Novel tetracationic diviologen compounds of the general formula CH ₃(CH₂)_nV²⁺(CH₂) ₆V²⁺(CH₂)_nCH₃ (where V²⁺ = 4,4′-bipyridinium and n = 5 or 11) were investigated as electrochemical reporters of DNA duplex formation. These compounds bind to both single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA) when the DNA is either present in solution or immobilized at electrode surfaces. Binding to thiolated ssDNA and dsDNA immobilized at Au electrodes was characterized using the electrochemical response for the reduction of the V²⁺ state to the V⁺ (viologen radical cation) state. An analysis of the charge for this reduction provided isotherms and binding constants for binding of these diviologens to both forms of immobilized DNA. Saturation of the binding is achieved at solution concentrations near 20 μM. For both the n = 5 and 11 diviologens, binding to ssDNA is driven by electrostatic charge neutralization. For the n = 11 case, the binding is cooperative. In the presence of dsDNA, the n = 11 diviologen exhibits a unique reduction potential for the V^2+/+ redox couple that is shifted approximately 100 mV negative of that in the presence of ssDNA. This new electrochemical signature is attributed to the reduction of viologen groups bound in the minor groove of the DNA duplex. For dsDNA in solution, an increase in the thermal denaturation temperature (T _m) from 60 to 66°C as a function of the n = 11 diviologen concentration confirmed its interaction with the duplex. Circular dichroism (CD) spectroscopy also was used to investigate the binding of both the V ²⁺ and V⁺ redox states of the n = 11 diviologen to dsDNA in solution. For the V⁺ state, a CD signal was observed that is consistent with the presence of face-to-face π dimers of the viologen groups. This unambiguously demonstrates the binding of this redox state of the diviologen in the dsDNA minor groove and the formation of such π dimers in the minor groove.