Multiple binding modes for dicationic Hoechst 33258 to DNA.

The binding of dicationic Hoechst 33258 (ligand) to DNA was characterized by means of the fluorescence spectra, fluorescence intensity titration, time-resolved fluorescence decay, light scattering, circular dichroism, and fluorescence thermal denaturation measurements, and two binding modes were distinguished by the experimental results. Type 1 binding has the stoichiometry of one ligand to more than 12 base pairs, and it is defined as quasi-minor groove binding which has the typical prolonged fluorescence lifetime of about 4.4 ns. In type 1 binding, planar conformation of the ligand is favorable. Type 2 binding with phosphate to ligand ratio (P/L) < 2.5 has the stoichiometry of one ligand to two phosphates. It is defined as a highly dense and orderly stacked binding with DNA backbone as the template. Electrostatic interactions between doubly protonated ligands and negatively charged DNA backbone play a predominant role in the type 2 binding mode. The characteristics of this type of binding result in a twisted conformation of the ligand that has a fluorescence lifetime of less than 1 ns. The results also indicate that the binding is in a cooperative manner primarily by stacking of the aromatic rings of the neighboring ligands. Type 1 binding is only observed for double-stranded DNA (dsDNA) with affinity constant of 1.83 x 10(7) M-1. In the type 2 binding mode, the binding affinity constants are 4.9 x 10(6) and 4.3 x 10(6) M-1 for dsDNA and single-stranded DNA (ssDNA), respectively. The type 2 binding is base pair independent while the type 1 binding is base pair related. The experiments described in this paper revealed that the dication bindings are different from the monocation bindings reported by previous study. The dication binding leads to stronger aggregation at low ligand concentration and results in orderly arrangements of the ligands along DNA chains. Furthermore the dication binding is demonstrated to be beneficial for enhancing the DNA's stability.