Property of the M-DNA probed by a minor groove binding dye 4′,6-diamidino-2-phenylindole

Spectral properties including circular and linear dichroism (CD and LD) of M-DNA, a molecular electric wire, formed at a high Zn²⁺ concentration have been studied using a minor groove binding drug 4′,6-diamidino-2-phenylindole (DAPI) as a probe. As the Zn²⁺ concentration increased, the magnitude of LD in the DNA absorption region decreased at pH 8.5, implying the aggregation of DNA, which is in contrast with the retained LD magnitude at pH 7.0. As the M-DNA formed, change in the secondary structure of DNA was observed by CD spectrum, which resembles that of the C-form DNA, although overall structure seemed to remain as a right handed double helix. The DAPI-DNA complex in the presence of high concentration of Zn²⁺ ions at pH 7.0 exhibited the similar CD spectrum with that in the absence of Zn²⁺ ion, consisting of type I, II and III. In contrast, at pH 8.5 at a high Zn²⁺ concentration in which DNA is in its M-form, DNA bound DAPI produced only the type III CD, suggesting that DAPI binds at the surface of M-DNA: the presence of Zn²⁺ ions prevents the minor groove binding of DAPI.