Structural characterization of PEGylated hexaphenylalanine nanostructures exhibiting green photoluminescence emission.

Peptides containing aromatic residues are known to give spontaneous phenomena of supramolecular organization into ordered nanostructures (NSs). Here we studied the structural behavior and the optoelectronic properties of new biocompatible materials obtained by self-assembling of a series of hexaphenylalanines (F6) modified at the N-terminus with a PEG chain of different length. PEG12-F6, PEG18-F6 and PEG24-F6 peptides were synthesized by coupling sequentially two, three or four units of amino-carboxy PEG6 blocks, each one containing six ethoxylic repetitions. Changes in length and composition of the PEG chain were found able to modulate the structural organization of phenylalanine based nanostructures. An increase in the self-aggregation tendency with longer PEG is observed; while, independently from PEG length, the peptide NSs display cross-beta like secondary structure with an antiparallel beta-strand arrangement. WAXS/GIWAXS diffraction patterns indicate a progressive decrease of the fiber order along the series. All the PEG-F6 derivatives present blue photoluminescent (PL) emission at 460 nm, while the adduct with longest PEG (PEG24-F6) shows an additional green emission at 530 nm.