Structural Characterization of PEGylated Hexaphenylalanine Nanostructures Exhibiting Green Photoluminescence Emission

Abstract

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 -F6, PEG -F6, and PEG -F6 peptides were synthesized by coupling sequentially two, three, or four units of amino-carboxy-PEG 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-β-like secondary structures with an antiparallel β-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 -F6) showing an additional green emission at 530 nm.

Fluorescence and Morphology of Self‐Assembled Nucleobases and Their Diphenylalanine Hybrid Aggregates

“…Inspection of the very simple motif of these β structures, that is, the assertion modes of the β strands, indicates that the intrinsic preference of the F‐rich peptides to the antiparallel sheet is perturbed by the presence of a single PNA bearing a purine base or a PNA dimer. As schematically shown in Figure b, the parallel association of strands favors base–base interactions that somehow compete with the factors that favor the antiparallel state . As a result of these competing factors, only tFF and cFF present a uniform antiparallel state, whereas the others are likely to be made of a mixture of parallel and antiparallel states (Figure b, c).…”

“…The appearance of two bands at around 1640 and 1680 cm −1 in aggregates formed by tFF and cFF supports the hypothesis of antiparallel stacks. Similar results have been reported in the literature for hexaphenylalanine aggregates . On the other hand, the profiles observed for the other samples are characterized by a very large band, with a maximum at 1640 cm −1 .…”

“…In contrast to FF, which forms nanotubulars tructures,t he conjugates designed and characterized herein assume cross-b structures that are somehow modulated by the nature and number of bases attached to the FF moiety.I nspection of the very simplem otif of these b structures,t hat is, the assertion modes of the b strands, indicates that the intrinsic preference of the F-rich peptides to the antiparallel sheet is perturbed by the presence of as inglePNA bearing ap urine base or aP NA dimer.A ss chematically shown in Figure 4b,t he parallel association of strandsf avors base-base interactions that somehow compete with the factors that favor the antiparallel state. [22] As ar esult of these competing factors, only tFF andc FF present auniform antiparallel state, whereas the others are likely to be made of a mixtureo fp arallel and antiparallel states (Figure 4b,c). This is in line with the observation that pyrimidine-pyrimidine stacking associations are weaker than that of purine-purine ones.…”

“…The appearance of two bands at around1 640 and 1680 cm À1 in aggregates formed by tFF and cFF supports the hypothesis of antiparallel stacks.Similarresults have been reported in the literature for hexaphenylalanine aggregates. [22] On the other hand, the profiles observed for the other samples are characterized by av ery large band, with am aximum at 1640cm À1 .I nt hese cases, it is possible that residual trifluoroacetic acid( TFA) results in broadening of the signals. Ac omparativea nalysiso f these FTIR data suggestst hat the size of the bases of the PNA moiety has as ignificant impact on the structure of the basic motif of thesea ggregates.…”

Light waveguiding in bioinspired peptide nanostructures

“…Figure F to H demonstrates blue and green FL from β‐sheet FFF nanospheres and FFF nanoplates excited by 360 and 410‐nm light, respectively. Found in ultrashort dipeptide and tripeptide nanofibers, visible FL effect has the similar physical origin as FL effect revealed in PEGylated peptide (PEG‐F6) and its derivatives, nonaromatic biogenic and synthetic peptides, amyloid fibrils, and native silk fibrils . This common biophotonic phenomenon is considered as an optical signature of β‐sheet peptide/protein biological secondary structure .…”