Strong Electro-Optic Effect and Spontaneous Domain Formation in Self-Assembled Peptide Structures

Geometric Transformations Afforded by Rotational Freedom in Aramid Amphiphile Nanostructures

Molecular self-assembly in water leads to nanostructure geometries that can be tuned owing to the highly dynamic nature of amphiphiles. There is growing interest in strongly interacting amphiphiles with suppressed dynamics, as they exhibit ultrastability in extreme environments. However, such amphiphiles tend to assume a limited range of geometries upon self-assembly due to the specific spatial packing induced by their strong intermolecular interactions. To overcome this limitation while maintaining structural robustness, we incorporate rotational freedom into the aramid amphiphile molecular design by introducing a diacetylene moiety between two aramid units, resulting in diacetylene aramid amphiphiles (D-AAs). This design strategy enables rotations along the carbon-carbon sp hybridized bonds of an otherwise fixed aramid domain. We show that varying concentrations and equilibration temperatures of D-AA in water lead to self-assembly into four different nanoribbon geometries: short, extended, helical, and twisted nanoribbons, all while maintaining robust structure with thermodynamic stability. We use advanced microscopy, X-ray scattering, spectroscopic techniques, and two-dimensional (2D) NMR to understand the relationship between conformational freedom within strongly interacting amphiphiles and their self-assembly pathways.

Triboelectric Generator Based on Oriented Self-Assembled Peptide Microbelts

Along with piezoelectric nanogenerators, triboelectric nanogenerators (TENGs) collecting energy from mechanical vibrations proved to be simple, low-cost, and efficient sources of electricity for various applications. In view of possible biomedical applications, the search for TENGs made of biomolecular and biocompatible materials is demanding. Diphenylalanine (FF) microstructures are promising for these applications due to their unique characteristics and ability to form various morphologies (microribbons, spherical vesicles, fibrils, micro- and nanotubes, nanorods, etc.). In this work, we developed a contact-separate mode TENG based on arrays of oriented FF microbelts deposited by dip-coating technique and studied their performance under various temperature treatments. We show that these TENGs outperform piezoelectric nanogenerators based on FF microbelts in terms of short-circuit current (ISC), open-circuit voltage (VOC), and output power. It was found that bound water captured in FF nanochannels mainly affects VOC, whereas mobile water increases ISC. We also found that the cyclization of FF molecules increases the performance of TENG likely due to an increase in surface energy and surface flattening.

Structural and photoactive properties of self-assembled peptide-based nanostructures and their optical bioapplication in food analysis

BACKGROUND

Food processing plays an important role in the modern industry because food quality and security directly affect human health, life safety, and social and economic development. Accurate, efficient, and sensitive detection technology is the basis for ensuring food quality and security. Optosensor-based technology with the advantage of fast and visual real-time detection can be used to detect pesticides, metal ions, antibiotics, and nutrients in food. As excellent optical centres, self-assembled peptide-based nanostructures possess attractive advantages, such as simple preparation methods, controllable morphology, tunable functionality, and inherent biocompatibility.

AIM OF REVIEW

Self-assembled peptide nanostructures with good fabrication yield, stability, dispersity in a complex sample matrix, biocompatibility, and environmental friendliness are ideal development goals in the future. Owing to its flexible and unique optical properties, some short peptide self-assemblies can possibly be used to achieve the purpose of rapid and sensitive detection of composition in food, agriculture, and the environment, expanding the understanding and application of peptide-based optics in analytical chemistry.

KEY SCIENTIFIC CONCEPT OF REVIEW

The self-assembly process of peptides is driven by noncovalent interactions, including hydrogen bonding, electrostatic interactions, hydrophobic interactions, and π-π stacking, which are the key factors for obtaining stable self-assembled peptide nanostructures with peptides serving as assembly units. Controllable morphology of self-assembled peptide nanostructures can be achieved through adjustment in the type, concentration, and pH of organic solvents and peptides. The highly ordered nanostructures formed by the self-assembly of peptides have been proven to be novel biological structures and can be used for the construction of optosensing platforms in biological or other systems. Optosensing platforms make use of signal changes, including optical signals and electrical signals caused by specific reactions between analytes and active substances, to determine the content or concentration of an analyte.

Kinetic and Thermodynamic Driving Factors in the Assembly of Phenylalanine-Based Modules

The formation of ordered protein and peptide assemblies is a phenomenon related to a wide range of human diseases. However, the mechanism of assembly at the molecular level remains largely unknown. Minimal models enable the exploration of the underlying interactions that are at the core of such self-assembly processes. In particular, the ability of phenylalanine, a single aromatic amino acid, to form an amyloid-like structure has challenged the previous dogma viewing a peptide backbone as a prerequisite for assembly. The driving forces controlling the nucleation and assembly in the absence of a peptide backbone remain to be identified. Here, aiming to unravel these forces, we explored the kinetics and thermodynamics of three phenylalanine-containing molecules during their assembly process: the amino acid phenylalanine, which accumulates in phenylketonuria patients, the diphenylalanine core-motif of the amyloid β peptide related to Alzheimer's disease, and the extended triphenylalanine peptide which forms a range of distinct nanostructures in vitro. We found that the aggregation propensity, regarding the critical monomer concentration, strongly increases with size, with triphenylalanine being the most aggregation-prone species under our experimental conditions. In the context of classical nucleation theory, this increase in aggregation propensity can be attributed to the larger free energy decrease upon aggregation of larger peptides and is not due to the presence/absence of a peptide bond per se. Taken together, this work provides insights into the aggregation processes of chemically simple systems and suggests that both backbone-containing peptides and backbone-lacking amino acids assemble through a similar mechanism, thus supporting the classification of amino acids in the continuum of amyloid-forming building blocks.

Guided Assembly and Patterning of Intrinsically Fluorescent Amyloid Fibers with Long-Range Order

Fibrillar amyloids exhibit a fascinating range of mechanical, optical, and electronic properties originating from their characteristic β-sheet-rich structure. Harnessing these functionalities in practical applications has so far been hampered by a limited ability to control the amyloid self-assembly process at the macroscopic scale. Here, we use core-shell electrospinning with microconfinement to assemble amyloid-hybrid fibers, consisting of densely aggregated fibrillar amyloids stabilized by a polymer shell. Up to centimeter-long hybrid fibers with micrometer diameter can be arranged into aligned and ordered arrays and deposited onto substrates or produced as free-standing networks. Properties that are characteristic of amyloids, including their high elastic moduli and intrinsic fluorescence signature, are retained in the hybrid fiber cores, and we show that they fully persist through the macroscopic fiber patterns. Our findings suggest that microlevel confinement is key for the guided assembly of amyloids from monomeric proteins.

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).

Efficient Water Self-Diffusion in Diphenylalanine Peptide Nanotubes

Nanotubes of self-assembled dipeptides exemplified by diphenylalanine (FF) demonstrate a wide range of useful functional properties, such as high Young's moduli, strong photoluminescence, remarkable piezoelectricity and pyroelectricity, optical waveguiding, etc., and became the object of intensive research due to their ability to combine electronic and biological functions in the same material. Two types of nanoconfined water molecules (bound water directly interacting with the peptide backbone and free water located inside nanochannels) are known to play a key role in the self-assembly of FF. Bound water provides its structural integrity, whereas movable free water influences its functional response. However, the intrinsic mechanism of water motion in FF nanotubes remained elusive. In this work, we study the sorption properties of FF nanotubes directly considering them as a microporous material and analyze the free water self-diffusion at different temperatures. We found a change in the regime of free water diffusion, which is attributed to water cluster size in the nanochannels. Small clusters of less than five molecules per unit cell exhibit ballistic diffusion, whereas, for larger clusters, Fickian diffusion occurs. External conditions of around 40% relative humidity at 30 °C enable the formation of such large clusters, for which the diffusion coefficient reaches 1.3 × 10^-10 m² s^-1 with an activation energy of 20 kJ mol^-1, which increases to attain 3 × 10^-10 m² s^-1 at 65 °C. The observed peculiarities of water self-diffusion along the narrow FF nanochannels endow this class of materials with a new functionality. Possible applications of FF nanotubes in nanofluidic devices are discussed.

Self-Assembly of C3 Symmetric Rigid Macrolactams into Very Polar and Porous Trigonal Crystals

Cyclohexane and cyclotri-β-alanyl have been used as scaffolds for the design of new C₃ -symmetric rings incorporating conjugated alkenes and dienes. All three C₃ -symmetric lactams share the same triangular shape and their crystal system is trigonal. They all belong to the R3 space group, R3m, R3 and R3c, for the increasingly large 12-, 18- and 24-membered rigid rings, respectively. All lactams stack on top of each other, through H-bonds and van der Waals noncovalent interactions, leading to endless supramolecular cylinders and tubes. The largest member of the family leads to tubes, the central pores of which is wide enough to let water in. A common feature of all the lactams is their very large dipole, of around 9 D, according to DFT calculations. Surprisingly, all the resulting cylinders and tubes pack side by side in the crystals, with all the dipoles pointing to the same direction. As a result, all three crystals are anisotropic and appear to be the first members of a new kind of highly polar crystals.

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.

Peptide Nanophotonics: From Optical Waveguiding to Precise Medicine and Multifunctional Biochips

Optical waveguiding phenomena found in bioinspired chemically synthesized peptide nanostructures are a new paradigm which can revolutionize emerging fields of precise medicine and health monitoring. A unique combination of their intrinsic biocompatibility with remarkable multifunctional optical properties and developed nanotechnology of large peptide wafers makes them highly promising for new biomedical light therapy tools and implantable optical biochips. This Review highlights a new field of peptide nanophotonics. It covers peptide nanotechnology and the fabrication process of peptide integrated optical circuits, basic studies of linear and nonlinear optical phenomena in biological and bioinspired nanostructures, and their passive and active optical waveguiding. It is shown that the optical properties of this generation of bio-optical materials are governed by fundamental biological processes. Refolding the peptide secondary structure is followed by wideband optical absorption and visible tunable fluorescence. In peptide optical waveguides, such a bio-optical effect leads to switching from passive waveguiding mode in native α-helical phase to an active one in the β-sheet phase. The found active waveguiding effect in β-sheet fiber structures below optical diffraction limit opens an avenue for the future development of new bionanophotonics in ultrathin peptide/protein fibrillar structures toward advanced biomedical nanotechnology.

Peptide Integrated Optics

Bio-nanophotonics is a wide field in which advanced optical materials, biomedicine, fundamental optics, and nanotechnology are combined and result in the development of biomedical optical chips. Silk fibers or synthetic bioabsorbable polymers are the main light-guiding components. In this work, an advanced concept of integrated bio-optics is proposed, which is based on bioinspired peptide optical materials exhibiting wide optical transparency, nonlinear and electrooptical properties, and effective passive and active waveguiding. Developed new technology combining bottom-up controlled deposition of peptide planar wafers of a large area and top-down focus ion beam lithography provides direct fabrication of peptide optical integrated circuits. Finding a deep modification of peptide optical properties by reconformation of biological secondary structure from native phase to β-sheet architecture is followed by the appearance of visible fluorescence and unexpected transition from a native passive optical waveguiding to an active one. Original biocompatibility, switchable regimes of waveguiding, and multifunctional nonlinear optical properties make these new peptide planar optical materials attractive for application in emerging technology of lab-on-biochips, combining biomedical photonic and electronic circuits toward medical diagnosis, light-activated therapy, and health monitoring.