Reconstructive Phase Transition in Ultrashort Peptide Nanostructures and Induced Visible Photoluminescence

A reconstructive phase transition has been found and studied in ultrashort di- and tripeptide nanostructures, self-assembled from biomolecules of different compositions and origin such as aromatic, aliphatic, linear, and cyclic (linear FF-diphenylalanine, linear LL-dileucine, FFF-triphenylalanine, and cyclic FF-diphenylalanine). The native linear aromatic FF, FFF and aliphatic LL peptide nanoensembles of various shapes (nanotubes and nanospheres) have asymmetric elementary structure and demonstrate nonlinear optical and piezoelectric effects. At elevated temperature, 140-180 °C, these native supramolecular structures (except for native Cyc-FF nanofibers) undergo an irreversible thermally induced transformation via reassembling into a completely new thermodynamically stable phase having nanowire morphology similar to those of amyloid fibrils. This reconstruction process is followed by deep and similar modification at all levels: macroscopic (morphology), molecular, peptide secondary, and electronic structures. However, original Cyc-FF nanofibers preserve their native physical properties. The self-fabricated supramolecular fibrillar ensembles exhibit the FTIR and CD signatures of new antiparallel β-sheet secondary folding with intermolecular hydrogen bonds and centrosymmetric structure. In this phase, the β-sheet nanofibers, irrespective of their native biomolecular origin, do not reveal nonlinear optical and piezoelectric effects, but do exhibit similar profound modification of optoelectronic properties followed by the appearance of visible (blue and green) photoluminescence (PL), which is not observed in the original peptides and their native nanostructures. The observed visible PL effect, ascribed to hydrogen bonds of thermally induced β-sheet secondary structures, has the same physical origin as that of the fluorescence found recently in amyloid fibrils and can be considered to be an optical signature of β-sheet structures in both biological and bioinspired materials. Such PL centers represent a new class of self-assembled dyes and can be used as intrinsic optical labels in biomedical microscopy as well as for a new generation of novel optoelectronic nanomaterials for emerging nanophotonic applications, such as biolasers, biocompatible markers, and integrated optics.