Aromatic Oligomers that Form Hetero Duplexes in Aqueous Solution

The electron-deficient 1,4,5,8-naphthalenetetracarboxylic diimide (Ndi) and electron-rich 1,5-dialkoxynaphthalene (Dan) have been shown to complex strongly with each other in water due to the hydrophobic effect as modulated through the electrostatic complementarity of the stacked dimer. Previously, oligomers of alternating Ndi and Dan units, termed aedamers, were the first foldamers to employ intramolecular aromatic stacking to effect the formation of secondary structure of nonnatural chains in aqueous solution. Described here is the use of this aromatic-aromatic (or pi-pi) interaction, this time in an intermolecular format, to demonstrate the self-assembly of stable hetero duplexes from a set of molecular strands (1a-4a) and (1b-4b) incorporating Ndi and Dan units, respectively. A 1-to-1 binding stoichiometry was determined from NMR and isothermal titration calorimetry (ITC) investigations, and these experiments indicated that association is enthalpically favored with the tetra-Ndi (4a) and tetra-Dan (4b) strands forming hetero duplexes (4a:4b) with a stability constant of 350 000 M-1 at T = 318 K. Polyacrylamide gel electrophoresis (PAGE) also illustrated the strong interaction between 4a and 4b and support a 1-to-1 binding mode even when one component is in slight excess. Overall, this system is the first to utilize complementary aromatic units to drive discrete self-assembly in aqueous solution. This new approach for designing assemblies is encouraging for future development of duplex systems with highly programmable modes of binding in solution or on surfaces.