Non-covalent versus covalent control of self-assembly and chirality of Nile red-modified nucleoside and DNA.

A DNA-based covalent versus a non-covalent approach is demonstrated to control the optical, chirooptical and higher order structures of Nile red (Nr) aggregation. Dynamic light scattering and TEM studies revealed that in aqueous media Nr-modified 2'-deoxyuridine aggregates through the co-operative effect of various non-covalent interactions including the hydrogen bonding ability of the nucleoside and sugar moieties and the pi-stacking tendency of the highly hydrophobic dye. This results in the formation of optically active nanovesicles. A left-handed helically twisted H-type packing of the dye is observed in the bilayer of the vesicle as evidenced from the optical and chirooptical studies. On the other hand, a left-handed helically twisted J-type packing in vesicles was obtained from a non-polar solvent (toluene). Even though the primary stacking interaction of the dye aggregates transformed from H-->J while going from aqueous to non-polar media, the induced supramolecular chirality of the aggregates remained the same (left-handed). Circular dichroism studies of DNA that contained several synthetically incorporated and covalently attached Nr-modified nucleosides revealed the formation of helically stacked H-aggregates of Nr but--in comparison to the noncovalent aggregates--an inversed chirality (right-handed). This self-assembly propensity difference can, in principle, be applied to other hydrophobic dyes and chromophores and thus open a DNA-based approach to modulate the primary stacking interactions and supramolecular chirality of dye aggregates.