Helical aggregation of cyanine dyes on DNA templates: Effect of dye structure on formation of homo- and heteroaggregates

UV-visible and circular dichroism (CD) spectroscopic experiments were used to characterize DNA-templated helical aggregates formed from a variety of symmetrical, cationic cyanine dyes. Two types of electronic couplings can be observed in these aggregates: face-to-face interactions between dye monomers and end-to-end interactions between dye dimers. The face-to-face interactions yield approximately 4-fold stronger couplings due to greater orbital overlap. Variation of the heterocyclic groups on the dye reveals that aggregation on DNA follows the following trend: quinoline > benzothiazole > benzoxazole > dimethylindole. This trend follows the tendency of these dyes to aggregate in aqueous solution. Within the benzothiazole class of dyes, a dicarbocyanine dye (pentamethine bridge) aggregates much more readily than the carbocyanine (trimethine bridge) analogue. However, a tricarbocyanine dye (heptamethine bridge) aggregates less readily than the dicarbocyanine, in contrast to their relative tendency to aggregate in water. In addition, heteroaggregates in which two different dyes associate cofacially on the DNA template can be synthesized.