Self-assembly of Pure Nanotubes from a Single-Chain Diacetylene Amine Salt

Introduction The discovery that complex chiral diacetylene derivatives can self-assemble into nanostructures 1 has led to a search for simpler molecules that retain that activity. The search for novel materials is complicated because the factors governing self-assembly are complex and until the simple system is identified the underlying mechanisms are difficult to discover. Unlike spherical liposomes, lipid tubules are thought to require chiral molecules in their formation and are expected to reflect the chiral nature of the lipids used. This chirality in molecular packing is reflected in helical markings often visible in electron micrographs of tubules and in large peaks observed in their circular dichroism spectra, both of which change handedness when the opposite enantiomer lipid is used . Lack of chirality does not in and of itself prevent self-organization since both chiral and achiral diacetylenic amphiphiles have been shown to form cylindrical microstructures from aqueous solution under quite specific conditions . The majority of work, however, has been concentrated on relatively complex chiral, multi-chain lipids as precursors for microstructured materials. We attempted to synthesize the simple quaternary ammonium salt by direct quaternization of the diacetylene amine compound 2 (Figure 1). The reaction product was found to be a remarkably versatile nanostructure-forming moiety that when self-assembled could induce a visible color change. Surprisingly, a detailed analysis of the reaction product revealed a family of amine salt derivatives. Synthesis of seven individual amine, amine salt, and quaternary ammonium salt derivatives of diacetylene demonstrates that the only precursor that has the potential to form nanotubes (in a single 100% yield and uniform manner) is the secondary amine salt (compound 3). To our knowledge the remarkable self-assembly of this inexpensive and simple lipid is unprecedented and represents a real step toward the rational design of bioactive monodisperse nanostructures.