We utilised a simple bidirectional (N→C and C→N) solid phase synthesis strategy entailing conventional solid phase peptide synthesis and fragment condensation with a water-soluble carbodiimide to synthesise a model anionic glycylglycine bolaamphiphile containing a suberic acid linker moiety, namely N,N'-suberoyldiglycylglycine. The synthetic suberoyldiglycylglycine was purified using its inherent ability to rapidly self-assemble in an aqueous acidic solution (0.1% trifluoroacetic acid). Monitoring of the rapid assembly process corroborated our visual observation and confirmed packing-directed self-assembly rather than non-specific aggregation or precipitation. The progress of suberoyldiglycylglycine self-assembly was observed to be via the formation of oligomers in the solution, which then self-assembled to form layered β-sheet type macrostructures. Within 24 h, nanotubes grew from these macrostructures and eventually combined to formed microtubes, which we isolated after 5-7 days.

In keeping with recent efforts to generate compounds for antibiotic and microbicide development, we focused on the creation of non-natural organo-peptide hybrids of antimicrobial peptide amides (KLK(L)n KLK-NH2 ) derived from sapecin B and a self-assembling oligoglycine organo-peptide bolaphile containing an ω-amino fatty acid residue. The hybrid organo-peptide bolaphiles with two cationic KLK tripeptide motifs linked with an ω-amino acid residue (penta-, octa- or undecamethylene chain) maintained the self-assembling properties of the root oligoglycine bolaphile. Electron microscopy clearly revealed complex supramolecular architectures for both sapecin B-derived peptides and the hybrid analogues. FT-IR spectroscopy indicated that the supramolecular structures were composed primarily of β-sheets. CD revealed that the hybrid bolaphiles did not share the same secondary structures as the sapecin B peptides in solution. However, although secondary structures of antimicrobial peptides are central in the activity, the organo-peptide bolaphiles also retained the potent antimicrobial activity of the leader sapecin B-derived peptide against both Gram-positive and Gram-negative bacteria. In general, the hybrids were more selective than the sapecin B peptides, as they displayed little or no appreciable haemolytic activity. The results obtained herald a new approach for the design of purpose-built hybrid organo-peptide bolaphiles.

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