Reconstructive Phase Transition in Ultrashort Peptide
Nanostructures and Induced Visible Photoluminescence
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Abstract
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