Spectroscopic studies of the multiple binding modes of a trimethine-bridged cyanine dye with DNA.

The interaction between DNA and a benzothiazole-quinoline cyanine dye with a trimethine bridge (TO-PRO-3) results in the formation of three noncovalent complexes. Unbound TO-PRO-3 has an absorption maximum (lambda(max)) of 632 nm, while the bound dyes (with calf thymus DNA) have electronic transitions with lambda(max) = 514 nm (complex I), 584 nm (complex II) and 642 nm (complex III). The blue shifts in the electronic transitions and the bisignate shape of the circular dichroism bands indicate that TO-PRO-3 aggregates with DNA.