Dicarboxylic oligopeptide bolaamphiphiles: Proton-triggered self-assembly of microtubes with loose solid surfaces

A new family of oligopeptide-based bolaamphiphiles, glycylglycine- (1a-h), glycylglycylglycine- (2a-b), sarcosylsarcosine- (3), L-prolyl-L-proline- (4), glycylsarcosylsarcosine- (5), and glycyl-L-prolyl-L-proline (6)-based bolaamphiphiles with a dicarboxylic headgroup at each end, has been synthesized. The oligopeptide fragments were linked via amide bond to a long-chain alpha,omega-dicarboxylic acid as a hydrocarbon spacer. Self-assembling properties of these bolaamphiphiles in water have been studied by light and cryogenic temperature transmission electron microscopy, infrared spectroscopy, and pH titration. Only sodium or potassium salts (acid soap) of the bolaamphiphiles 1a, 1c, 1e, 2a, and 2b produced well-defined microtubes of 1-3-mu m diameter with closed ends. All the tubes encapsulated a number of vesicular assemblies inside the aqueous compartment. The tube formation strongly depends on the connecting alkylene chain length, the alkylene even-odd carbon numbers, and constituent amino acid residues. Vectorial formation of acid-anion dimers and loose interpeptide hydrogen-bond networks are responsible for the microtube self-assembly. The atomic force microscopic observation of the microtube made of 1e revealed a distorted hexagonal arrangement of the headgroups on the surface. A self-assembling model and the tube formation mechanism are also discussed from the viewpoint of proton-triggered self-assembly.