Investigation of DNA binding modes for a symmetrical cyanine dye trication: effect of DNA sequence and structure

The DNA binding behavior of a tricationic cyanine dye (DiSC3+(5)) was studied using the [Poly(dA-dT)]2, [Poly(dI-dC)]2 and Poly(dA) x Poly(dT) duplex sequences and the Poly(dA) x 2Poly(dT) triplex. Optical spectroscopy and viscometry results indicate that the dye binds to the triplex structure by intercalation, to the nonalternating Poly(dA) x Poly(dT) duplex through minor groove binding and to the alternating [Poly(dA-dT)]2 duplex by a combination of two binding modes: intercalation at low concentration and dimerization within the minor groove at higher concentration. Dimerization occurs at lower dye concentrations for the [Poly(dI-dC)]2 sequence, consistent with our previous investigations on an analogous monocationic cyanine dye. [Seifert, J.L., et al. (1999) J. Am. Chem. Soc. 121, 2987-2995] These studies illustrate the diversity of DNA binding modes that are available to a given ligand structure.