Intrinsically Fluorescent Nano-Scaled Peptide Aggregates Upon Arginine to Citrulline Swap

The supramolecular assembly of short peptides into ordered structures offers promise for developing bio-nanomaterials with diverse applications in drug delivery, electronics, and optical engineering. Intrinsic fluorescence of polypeptide aggregates is typically associated with delocalization of electron densities in dense hydrogen bonding networks, dipolar coupling of aromatic amino acid residues, and possibly by the 'cluster-derived luminescence' associated with supramolecular structures. In a handful of examples, self-assembly of short peptides has provided ordered intrinsically fluorescent nanostructures. In this study, replacement of a single arginine residue with citrulline in a macrocyclic peptide has led to intrinsic fluorescence. The parent arginine-containing peptide macrocycle was previously shown to adopt a β-sheet conformation that aggregated into nonfluorescent spherical particles. The change from the electrostatic positive charge of the guanidine side chain to a hydrogen bonding neutral urea caused the β-sheet peptide to aggregate into larger-sized intrinsically fluorescent rods by a phenomenon that is ascribed to electron delocalization through π-π stacking interactions.