Interaction of Hoechst 33258 with a DNA triple helix

The interaction of Hoechst 33258 molecule, a minor groove binding drug, with T-A-T triple helix and A-T double helix was studied using circular dichroism spectroscopy and thermal denaturation. The triple helix consisted of an oligonucleotide (dA)12-x-(dT)12-x-(dT)12, where x is a hexa-ethylene glycol chain bridged between the 3' phosphate of one strand and the 5' phosphate of the following strand. This oligonucleotide is able to fold back on itself to form a very stable triplex. Circular dichroism spectroscopy demonstrates that Hoechst 33258 can bind to the triple helical structure. Spectral analysis shows that the bound drug exhibits a conformation and an environment slightly different in double-stranded and in triple-stranded structure. The affinity to the triple stranded structure is found smaller than to the double stranded one. Thermal denaturation experiments demonstrate that Hoechst 33258 destabilizes the triplex whereas it stabilizes the duplex.