Photoluminescent Peptide-Based Nanostructures as FRET Donor for Fluorophore Dye

A great interest has been recently generated by the discovery that peptide-based nanostructures (NSs) endowed with cross-beta structure may show interesting photoluminescent (PL) properties. It was shown that NSs formed by PEGylated hexaphenylalanine (PEG(8)-F6, PEG=polyethylene glycol) are able to emit at 460 nm when excited at 370 or 410 nm. Here, the possibility to transfer the fluorescence of these PEG(8)-F6-based NSs by foster resonance electron transfer (FRET) phenomenon to a fluorescent dye was explored. To achieve this aim, the 4-chloro-7-nitrobenzofurazan (NBD) dye was encapsulated in these NSs. Structural data in solution and in solid state, obtained by a variety of techniques (circular dichroism, Fourier-transform infrared spectroscopy, wide-angle X-ray scattering, and small-angle X-ray scattering), indicated that the organization of the peptide spine of PEG(8)-F6 NS, which consists of anti-parallel beta-sheets separated by a dry interface made of interacting phenylalanine side chains, was maintained upon NBD encapsulation. The spectroscopic characterization of these NSs clearly showed a redshift of the emission fluorescence peak both in solution and in solid state. This shift from 460 to 530 nm indicated that a FRET phenomenon from the peptide-based to the fluorophore-encapsulated NS occurred. FRET could also be detected in the PEG(8)-F6 conjugate, in which the NBD was covalently bound to the amine of the compound. On the basis of these results, it is suggested that the red-shift of the intrinsic PL of NSs may be exploited in the bio-imaging field.