Photoluminescent Peptide-Based Nanostructures as FRET Donor for Fluorophore Dye

The Characterization of Multifaceted PREP1 Peptides Provides Insights into Correlations between Spectroscopic and Structural Properties of Amyloid-like Assemblies

The widespread ability of proteins and peptides to self-assemble by forming cross-β structure is one of the most significant discoveries in structural biology. Intriguingly, the cross-β association of proteins/peptides may generate intricate supramolecular architectures with uncommon spectroscopic properties. We have recently characterized self-assembled peptides extracted from the PREP1 protein that are endowed with interesting structural/spectroscopic properties. We here demonstrate that the green fluorescence emission of the peptide PREP1[117-132] (λem ~520 nm), can be induced by excitation with UV radiation. The associated unusually large Stokes shift (Δλ ~150 nm) represents, to the best of our knowledge, the first evidence of an internal resonance energy transfer in amyloid-like structures, where the blue emission of some assemblies becomes the excitation radiation for others. Moreover, the characterization of PREP1[117-132] variants provides insights into the sequence/structure and structure/spectroscopic properties relationships. Our data suggests that the green fluorescence is plausibly associated with antiparallel β-sheet states of the peptide whereas parallel β-sheet assemblies are only endowed with blue fluorescence. Notably, the different PREP1[117-132] variants also form assemblies characterized by distinct morphologies. Indeed, the parent peptide and single mutants form compact but structured aggregates whereas most of the double mutants exhibit elongated and highly extended fibers.

Influence of Urea and Dimethyl Sulfoxide on K-Peptide Fibrillation

Protein fibrillation leads to formation of amyloids-linear aggregates that are hallmarks of many serious diseases, including Alzheimer's and Parkinson's diseases. In this work, we investigate the fibrillation of a short peptide (K-peptide) from the amyloidogenic core of hen egg white lysozyme in the presence of dimethyl sulfoxide or urea. During the studies, a variety of spectroscopic methods were used: fluorescence spectroscopy and the Thioflavin T assay, circular dichroism, Fourier-transform infrared spectroscopy, optical density measurements, dynamic light scattering and intrinsic fluorescence. Additionally, the presence of amyloids was confirmed by atomic force microscopy. The obtained results show that the K-peptide is highly prone to form fibrillar aggregates. The measurements also confirm the weak impact of dimethyl sulfoxide on peptide fibrillation and distinct influence of urea. We believe that the K-peptide has higher amyloidogenic propensity than the whole protein, i.e., hen egg white lysozyme, most likely due to the lack of the first step of amyloidogenesis-partial unfolding of the native structure. Urea influences the second step of K-peptide amyloidogenesis, i.e., folding into amyloids.

Fluorescence Emission of Self-assembling Amyloid-like Peptides: Solution versus Solid State

Analysis of the intrinsic UV-visible fluorescence exhibited by self-assembling amyloid-like peptides in solution and in solid the state highlights that their physical state has a profound impact on the optical properties. In the solid state, a linear dependence of the fluorescence emission peaks as a function of excitation wavelength is detected. On the contrary, an excitation-independent emission is observed in solution. The present findings constitute a valuable benchmark for current and future explanations of the fluorescence emission by amyloids.

Amplified spontaneous emission and gain in highly concentrated Rhodamine-doped peptide derivative

Bioinspired fluorescence, being widely explored for imaging purposes, faces challenges in delivering bright biocompatible sources. While quite a few techniques have been developed to reach this goal, encapsulation of high-quantum yield fluorescent dyes in natural biological forms suggest achieving superior light-emitting characteristics, approaching amplified spontaneous emission and even lasing. Here we compare gain capabilities of highly concentrated Rhodamine B solutions with a newly synthesized biocompatible peptide derivative hybrid polymer/peptide material, RhoB-PEG1300-F6, which contains the fluorescent covalently bound dye. While concentration quenching effects limit the maximal achievable gain of dissolved Rhodamine B, biocompatible conjugation allows elevating amplification coefficients towards moderately high values. In particular, Rhodamine B, anchored to the peptide derivative material, demonstrates gain of 22–23 cm⁻¹ for a 10⁻² M solution, while a pure dye solution possesses 25% smaller values at the same concentration. New biocompatible fluorescent agents pave ways to demonstrate lasing in living organisms and can be further introduced to therapeutic applications, if proper solvents are found.

Amyloid-like Prep1 peptides exhibit reversible blue-green-red fluorescence in vitro and in living cells

PREP1-based peptides form amyloid-like aggregates endowed with an intrinsic blue-green-red fluorescence with an unusual sharp maximum at 520 nm upon excitation with visible light under physiological conditions. The peptide PREP1[117-132], whose sequence does not contain aromatic residues, presents a pH-dependent and reversible fluorescence, in line with its structural transition from β-sheet rich aggregates to α-helix structures. These findings further demonstrate that the non-canonical fluorescence exhibited by amyloids is an articulated phenomenon.

Fluorescence Phenomena in Amyloid and Amyloidogenic Bionanostructures

Nanoscale optical labeling is an advanced bioimaging tool. It is mostly based on fluorescence (FL) phenomena and enables the visualization of single biocells, bacteria, viruses, and biological tissues, providing monitoring of functional biosystems in vitro and in vivo, and the imaging-guided transportation of drug molecules. There is a variety of FL biolabels such as organic molecular dyes, genetically encoded fluorescent proteins (green fluorescent protein and homologs), semiconductor quantum dots, carbon dots, plasmonic metal gold-based nanostructures and more. In this review, a new generation of FL biolabels based on the recently found biophotonic effects of visible FL are described. This intrinsic FL phenomenon is observed in any peptide/protein materials folded into β-sheet secondary structures, irrespective of their composition, complexity, and origin. The FL effect has been observed both in natural amyloid fibrils, associated with neurodegenerative diseases (Alzheimer’s, Parkinson’s, and more), and diverse synthetic peptide/protein structures subjected to thermally induced biological refolding helix-like→β-sheet. This approach allowed us to develop a new generation of FL peptide/protein bionanodots radiating multicolor, tunable, visible FL, covering the entire visible spectrum in the range of 400–700 nm. Newly developed biocompatible nanoscale biomarkers are considered as a promising tool for emerging precise biomedicine and advanced medical nanotechnologies (high-resolution bioimaging, light diagnostics, therapy, optogenetics, and health monitoring).

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

Studies carried out in recent decades have revealed that the ability to self-assemble is a widespread property among biomolecules. Small nucleic acid moieties or very short peptides are able to generate intricate assemblies endowed with remarkable structural and spectroscopic properties. Herein, the structural/spectroscopic characterization of aggregates formed by nucleobases and peptide nucleic acid (PNA)-peptide conjugates are reported. At high concentration, all studied nucleobases form aggregates characterized by previously unreported fluorescence properties. The conjugation of these bases, as PNA derivatives, to the dipeptide Phe-Phe leads to the formation of novel hybrid assemblies, which are characterized by an amyloid-like association of the monomers. Although these compounds share the same basic cross-β motif, the nature and number of PNA units have an important impact on both the level of structural order and the intrinsic fluorescence of the self-assembled nanostructure.

Fmoc-diphenylalanine as a suitable building block for the preparation of hybrid materials and their potential applications

Due to its capability to self-assemble in self-supporting hydrogels (HG) under physiological conditions, Fmoc-FF is one of the most studied ultra-short peptide. The structural properties of the resulting hydrogel (mechanical rigidity, entanglement of the fibrillary network, and the thickness of the fibers) strictly depend on the experimental conditions used during the preparation. In the past few years, a broad range of applications in different fields, such as biomedical and industrial fields, have been proposed. However, the research on novel materials with enhanced mechanical properties, stability, and biocompatibility has brought about the development of novel Fmoc-FF-based hybrid systems, in which the ultra-short hydrogelator is combined with others entities such as polysaccharides, polymers, peptides, or organic molecules. The structural features and the potential applications of these novel hybrid materials, with particular attention to tissue engineering, drug delivery, and catalysis, are described here. The aim is to give the readers a tool to design new hybrid nanomaterials based on the Fmoc-FF dipeptide hydrogelator, with appropriate properties for specific applications.

Light waveguiding in bioinspired peptide nanostructures

Basic optical properties of bioinspired peptide nanostructures are deeply modified by thermally mediated refolding of peptide secondary structure from α-helical to β-sheet. This conformational transition is followed by the appearance in the β-sheet structures of a wideband optical absorption and fluorescence in the visible region. We demonstrate that a new biophotonic effect of optical waveguiding recently observed in peptide/protein nanoensembles is a structure-sensitive bimodal phenomenon. In the primary α-helical structure input, light propagates via optical transmission window demonstrating conventional passive waveguiding, based on classical optics. In the β-sheet structure, fluorescent (active) light waveguiding is revealed. The latter can be attributed to completely different physical mechanism of exciton-polariton propagation, characterized by high effective refractive index, and can be observed in nanoscale fibers below diffraction limit. It has been shown that peptide material requirements for passive and active waveguiding are dissimilar. Original biocompatibility and biodegradability indicate high potential future applications of these bioinspired waveguiding materials in precise photobiomedicine towards advanced highly selective bioimaging, photon diagnostics, and optogenetics.

Fmoc-FF and hexapeptide-based multicomponent hydrogels as scaffold materials

Short peptides or single amino acids are interesting building blocks for fabrication of hydrogels, frequently used as extracellular matrix-mimicking scaffolds for cell growth in tissue engineering. The combination of two or more peptide hydrogelators could allow obtaining different materials exhibiting new architectures, tunable mechanical properties, high stability and improved biofunctionality. Here we report on the synthesis, formulation and multi-scale characterization of peptide-based mixed hydrogels formed by the low molecular weight Fmoc-FF (N^α-fluorenylmethyloxycarbonyl diphenylalanine) hydrogelator and of the PEG₈-(FY)3 hexapeptide, containing three repetitions of the Phe-Tyr motif and a PEG moiety at its N-terminus. Mixed hydrogels were also prepared by replacing PEG₈-(FY)3 with its analogue (FY)3, without the PEG moiety. Rheology analysis confirmed the improved mechanical features of the multicomponent gels prepared at two different ratios (2/1 or 1/1, v/v). However, the presence of the hydrophilic PEG polymeric moiety causes a slowing down of the gel kinetic formation (from 42 to 18 minutes) and a decrease of the gel rigidity (G' from 9 to 6 kPa). Preliminary in vitro biocompatibility and cell adhesion assays performed on Chinese hamster ovarian (CHO) cells suggest a potential employment of these multicomponent hydrogels as exogenous scaffold materials for tissue engineering.

Peptide-Based Drug-Delivery Systems in Biotechnological Applications: Recent Advances and Perspectives

Peptides of natural and synthetic sources are compounds operating in a wide range of biological interactions. They play a key role in biotechnological applications as both therapeutic and diagnostic tools. They are easily synthesized thanks to solid-phase peptide devices where the amino acid sequence can be exactly selected at molecular levels, by tuning the basic units. Recently, peptides achieved resounding success in drug delivery and in nanomedicine smart applications. These applications are the most significant challenge of recent decades: they can selectively deliver drugs to only pathological tissues whilst saving the other districts of the body. This specific feature allows a reduction in the drug side effects and increases the drug efficacy. In this context, peptide-based aggregates present many advantages, including biocompatibility, high drug loading capacities, chemical diversity, specific targeting, and stimuli responsive drug delivery. A dual behavior is observed: on the one hand they can fulfill a structural and bioactive role. In this review, we focus on the design and the characterization of drug delivery systems using peptide-based carriers; moreover, we will also highlight the peptide ability to self-assemble and to actively address nanosystems toward specific targets.

Structural Characterization of Self-Assembled Tetra-Tryptophan Based Nanostructures: Variations on a Common Theme

Over the years, a large number of multidisciplinary investigations has unveiled that the self-assembly of short peptides and even of individual amino acids can generate a variety of different biomaterials. In this framework, we have recently reported that polyethylene glycol (PEG) conjugates of short homopeptides, containing aromatic amino acids such as phenylalanine (Phe, F) and naphthylalanine (Nal), are able to form elongated fibrillary aggregates having interesting chemical and physical properties. We here extend these analyses characterizing the self-assembling propensity of PEG₆ -W4, a PEG adduct of the tetra-tryptophan (W4) sequence. A comprehensive structural characterization of PEG₆ -W4 was obtained, both in solution and at the solid state, through the combination of spectroscopic, microscopic, X-ray scattering and computational techniques. Collectively, these studies demonstrate that this peptide is able to self-assemble in fibrillary networks characterized by a cross β-structure spine. The present findings clearly demonstrate that aromatic residues display a general propensity to induce self-aggregation phenomenon, despite the significant differences in the physicochemical properties of their side chains.

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. 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 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 (PEG₂₄ -F6) showing an additional green emission at 530 nm.