Structural Characterization of PEGylated Hexaphenylalanine Nanostructures Exhibiting Green Photoluminescence Emission

Peptides containing aromatic residues are known to exhibit spontaneous phenomena of supramolecular organization into ordered nanostructures (NSs). In this work we studied the structural behavior and optoelectronic properties of new biocompatible materials obtained by the self-assembly of a series of hexaphenylalanines (F6) modified at the N terminus by a PEG chain of different lengths. PEG(12)-F6, PEG(18)-F6, and PEG(24)-F6 peptides were synthesized by coupling sequentially two, three, or four units of amino-carboxy-PEG(6) blocks, each one containing six oxyethylene repetitions. Changes in the length and composition of the PEG chain were found to modulate the structural organization of the phenylalanine-based nanostructures. An increase in the self-aggregation tendency was observed with longer PEG chains, whereas, independently of the PEG length, the peptide NSs display cross-beta-like secondary structures with an antiparallel beta-strand arrangement. WAXS/GIWAXS diffraction patterns indicate a progressive decrease in fiber order along the series. All the PEG-F6 derivatives present blue photoluminescent (PL) emission at 460 nm, with the adduct with the longest PEG chain (PEG(24)-F6) showing an additional green emission at 530 nm.