Self-assembly of diphenylalanine peptides into microtubes with "turn on" fluorescence using an aggregation-induced emission molecule

Cystine-cored diphenylalanine appended peptide-based self-assembled fluorescent nanostructures direct redox-responsive drug delivery

Fabrication of stimuli-responsive superstructure capable of delivering chemotherapeutics directly to the cancer cell by sparing healthy cells is crucial. Herein, we developed redox-responsive hollow spherical assemblies through self-assembly of disulfide-linked cysteine-diphenylalanine (SN). These fluorescent hollow spheres display intrinsic green fluorescence, are proteolytically stable and biocompatible, and allow for real-time monitoring of their intracellular entry. The disulfide bond facilitates selective degradation in the presence of high glutathione (GSH) concentrations, prevalent in cancer cells. We achieved efficient encapsulation (68.72%) of the anticancer drug doxorubicin (Dox) and demonstrated GSH-dependent, redox-responsive drug release within cancerous cells. SN-Dox exhibited a 20-fold lower effective concentration (2.5 μM) for compromising breast cancer cell viability compared to non-malignant cells (50 μM). The ability of SN-Dox to initiate DNA damage signaling and trigger apoptosis was comparable to that of the unencapsulated drug. Our findings highlight the potential of SN for creating site-specific drug delivery vehicles for sustained therapeutic release.

RAFT-synthesis and self-assembly-induced emission of pendant diphenylalanine-tetraphenylethylene copolymers

Manipulation of the properties of aggregation-induced emission luminogens (AIEgens) by combining self-assembling motifs has attracted significant interest as a promising approach to developing various advanced materials. In this study, pendant diphenylalanine-tetraphenylethylene (TPE) copolymers exhibiting the ability for self-assembly and AIE properties were synthesized via reversible addition-fragmentation chain-transfer (RAFT) copolymerization. The resulting anionic and non-ionic amphiphilic copolymers with a carbon-carbon main chain bearing diphenylalanine-TPE through-space interactions self-assembled into nanorods and nanofibers, showing blue emissions originating from the aggregation of TPE side chains in the assembled structures. Suitable tuning of the comonomer composition, monomer structure, and environmental conditions (e.g., solvent polarity) enables manipulation of the self-assembled structures, AIE properties, and aggregation-induced circular dichroism by achiral TPE units via through-space interactions with diphenylalanine moieties.

Effect of the spacer on the structure and self-assembly of FF peptide mimetics

We have designed and synthesized a series of FF peptide mimetics with conformationally rigid and flexible spacers to study the effect of spacers on their structure and self-assembly. The results help in understanding biomolecular aggregation and provide a strategy to obtain fractal pattern materials. From X-ray single crystal analysis, the m-diaminobenzene appended FF peptide mimetic adopts a duplex structure stabilized by multiple intermolecular hydrogen bonds. There is also a water molecule bridging between two strands of the duplex. Moreover, the duplex is stabilized by three face-to-face, face-to-edge and edge-to-edge π-π interactions. The duplex formation is also supported by mass spectrometry. In higher order packing, the dimeric subunits further self-assembled to form a complex sheet-like structure stabilized by multiple intermolecular hydrogen bonding and π-π stacking interactions. Moreover, the 1,4-butadiene and m-xylylenediamine appended FF peptide mimetics form stimuli-responsive organogels in a wide range of solvents including methanol. The rheology data of FF peptide mimetic gels as a function of angular frequency and oscillatory strain also supported the formation of strong physically crosslinked gels. The FE-SEM images of the xerogels obtained from different organic solvents show that the network morphology of FF peptide mimetics varies depending on the nature of the solvents.

Functionalized Fluorescent Nanostructures Generated from Self-Assembly of a Cationic Tripeptide Direct Cell-Selective Chemotherapeutic Drug Delivery

Nanodrug delivery systems (NDDs) capable of conveying chemotherapeutics directly into malignant cells without harming healthy ones are of significant interest in the field of cancer therapy. However, the development of nanostructures with the requisite biocompatibility, inherent optical properties, cellular penetration ability, encapsulation capability, and target selectivity has remained elusive. In an effort to develop cell-selective NDDs, we have synthesized a cationic tripeptide Boc-Arg-Trp-Phe-OMe (PA1), which self-assembles into well-ordered spheres in 100% aqueous medium. The inherent fluorescence properties of the peptide PA1 were shifted from the ultraviolet to the visible region by the self-assembly. These fluorescent nanostructures are proteolytically stable, photostable, and biocompatible, with characteristic blue fluorescence signals that permit us to monitor their intracellular entry in real time. We also demonstrate that these tripeptide spherical structures (TPSS) have the capacity to entrap the chemotherapeutic drug doxorubicin (Dox), shuttle the encapsulated drug within cancerous cells, and initiate the DNA damage signaling cascade, which culminates in apoptosis. Next, we functionalized the TPSS with an epithelial-cell-specific epithelial cell adhesion molecule aptamer. Aptamer-conjugated PA1 (PA1-Apt) facilitated efficient Dox delivery into the breast cancer epithelial cell line MCF7, resulting in cell death. However, cells of the human cardiomyocyte cell line AC16 were resistant to the cell killing actions of PA1-Apt. Together, these data demonstrate that not only can the self-assembly of cationic tripeptides like PA1 be exploited for efficient drug encapsulation and delivery but their unique chemistry also allows for functional modifications, which can improve the selectivity of these versatile NDDs.

Dipeptide nanoparticle and aptamer-based hybrid fluorescence platform for enrofloxacin determination

A novel fluorescence platform was fabricated for enrofloxacin determination by using cDNA-modified dipeptide fluorescence nanoparticles (FDNP-cDNA) and aptamer-modified magnetic Fe₃O₄ nanoparticles (Fe₃O₄-Apt). The FDNP were prepared via tryptophan-phenylalanine self-assembling. When magnetic Fe₃O₄-Apt incubated with standard solution or sample extracts, the target enrofloxacin was selectively captured by the aptamer on the surface of the Fe₃O₄ nanoparticles. After removing interference by washing with phosphate-buffered saline, the FDNP-cDNA was added, which can bind to the aptamer on the surface of the Fe₃O₄ nanoparticles not occupied by the analyte. The higher the concentration of the target enrofloxacin in the standard or sample solution is, the less the FDNP-cDNA can be bound with the Fe₃O₄ nanoparticles, and the more the FDNP-cDNA can be observed in the supernatant. Fluorescence intensity (E_x/E_m = 310/380 nm) increased linearly in the enrofloxacin concentration range 0.70 to 10.0 ng/mL with a detection limit of 0.26 ng/mL (S/N = 3). Good recoveries (88.17-99.30%) were obtained in spiked lake water, chicken, and eel samples with relative standard deviation of 2.7-6.2% (n = 3).

Fabrication of self-assembled nanostructures for intracellular drug delivery from diphenylalanine analogues with rigid or flexible chemical linkers

Self-assembly of molecular building blocks is a simple and useful approach to generate supramolecular structures with varied morphologies and functions. By studying the chemical properties of the building blocks and tuning the parameters of their self-assembly process, the resultant supramolecular assemblies can be optimized for the required downstream applications. To this end, in the present study we have designed and synthesized three different molecular building blocks composed of two diphenylalanine (FF) units connected to each other through three different linkers: ethylenediamine, succinic acid, or terephthalaldehyde. Under identical conditions, all the three building blocks self-assemble into supramolecular architectures with distinct morphologies. However, by varying the polarity of the self-assembly medium, the nature of the non-covalent interactions changes in such a way as to generate additional self-assembled structures unique to each building block. Utilizing microscopic and spectroscopic techniques, we characterized the morphological variety generated by each building block/linker combination. These data represent the first report analysing the diversity of nanostructures that can be generated from identical dipeptide-based molecular backbones simply by varying the chemical linker. We also demonstrate that the spherical assemblies and nanorod structures fabricated from these dipeptide/linker pairs can act as drug delivery systems. More specifically, the spherical assembly generated by two FF dipeptides linked via ethylenediamine and nanorods fabricated from terephthalaldehyde linked FF dipeptides were able to encapsulate the cancer chemotherapeutic agent doxorubicin (DOX) and chaperone the drug into cells. Thus, these supramolecular assemblies represent a new platform for the development of efficient and effective intracellular drug delivery systems.

Smart Thixotropic Hydrogels by Disulfide-Linked Short Peptides for Effective Three-Dimensional Cell Proliferation

Supramolecular assembly of short peptides is a crucial process and has shown numerous potential applications as biomaterials. In the present work, the hydrogelation process of short peptides containing C-terminal "Lys-Cys" (KC) residues have been studied in detail. The N-terminal capping is found to be essential for effective gelation. Out of 12 peptides we studied, two of them could form hydrogels efficiently: Ac-VVKC-NH₂ and Ac-FFKC-NH₂. In both cases, the monomer-to-dimer formation through disulfide linkages by Cys residues controls the aggregation process. Interestingly, the presence of H₂O₂ facilitated the dimerization and thereby reduced the gelation time but could not impart much effect on the mechanical properties of the gels. Detailed rheological study revealed that both hydrogels are thixotropic in nature. Moreover, they are responsive to glutathione (GSH) due to the presence of disulfide linkages. However, the hydrogel of Ac-FFKC-NH₂ is found to be stronger and more effective for biological applications. The thixotropic nature as well as a model drug release study in response to varying GSH concentration indicates the possible use of the hydrogel as an injectable local drug delivery vehicle. The hydrogel of Ac-FFKC-NH₂ is noncytotoxic in nature. Three-dimensional cell proliferation has been found to be more effective than 2D, as it mimics the in vivo situation more closely if not exactly. In the present study, we have shown that both differentiated RAW macrophages and undifferentiated THP-1 monocytes could proliferate significantly within the 3D matrix of the hydrogel, without depicting any apparent cytotoxicity. Thus, the hydrogel of Ac-FFKC-NH₂ has potential for application in localized drug administration and as a supporting biomaterial to study basic phenomena involving cell behavior.

Green fluorescent protein inspired fluorophores

Green fluorescence proteins (GFP) are appealing to a variety of biomedical and biotechnology applications, such as protein fusion, subcellular localizations, cell visualization, protein-protein interaction, and genetically encoded sensors. To mimic the fluorescence of GFP, various compounds, such as GFP chromophores analogs, hydrogen bond-rich proteins, and aromatic peptidyl nanostructures that preclude free rotation of the aryl-alkene bond, have been developed to adapt them for a fantastic range of applications. Herein, we firstly summarize the structure and luminescent mechanism of GFP. Based on this, the design strategy, fluorescent properties, and the advanced applications of GFP-inspired fluorophores are then carefully discussed. The diverse advantages of bioinspired fluorophores, such as biocompatibility, structural simplicity, and capacity to form a variety of functional nanostructures, endow them potential candidates as the next-generation bio-organic optical materials.

Soft-Matter Nanotubes: A Platform for Diverse Functions and Applications

Self-assembled organic nanotubes made of single or multiple molecular components can be classified into soft-matter nanotubes (SMNTs) by contrast with hard-matter nanotubes, such as carbon and other inorganic nanotubes. To date, diverse self-assembly processes and elaborate template procedures using rationally designed organic molecules have produced suitable tubular architectures with definite dimensions, structural complexity, and hierarchy for expected functions and applications. Herein, we comprehensively discuss every functions and possible applications of a wide range of SMNTs as bulk materials or single components. This Review highlights valuable contributions mainly in the past decade. Fifteen different families of SMNTs are discussed from the viewpoints of chemical, physical, biological, and medical applications, as well as action fields (e.g., interior, wall, exterior, whole structure, and ensemble of nanotubes). Chemical applications of the SMNTs are associated with encapsulating materials and sensors. SMNTs also behave, while sometimes undergoing morphological transformation, as a catalyst, template, liquid crystal, hydro-/organogel, superhydrophobic surface, and micron size engine. Physical functions pertain to ferro-/piezoelectricity and energy migration/storage, leading to the applications to electrodes or supercapacitors, and mechanical reinforcement. Biological functions involve artificial chaperone, transmembrane transport, nanochannels, and channel reactors. Finally, medical functions range over drug delivery, nonviral gene transfer vector, and virus trap.

Molecular motifs encoding self-assembly of peptide fibers into molecular gels

Peptides are a promising class of gelators, due to their structural simplicity, biocompatibility and versatility. Peptides were synthesized based on four amino acids: leucine, phenylalanine, tyrosine and tryptophan. These peptide gelators, with systematic structural variances in side chain structure and chain length, were investigated using Hansen solubility parameters to clarify molecular features that promote gelation in a wide array of solvents. It is of utmost importance to combine both changes to structural motifs and solvent in simultaneous studies to obtain a global perspective of molecular gelation. It was found that cyclization of symmetric dipeptides, into 2,5-diketopiperazines, drastically altered the gelation ability of the dipeptides. C-l-LL and C-l-YY, which are among the smallest peptide LMOGs reported to date, are robust gelators with a large radius of gelation (13.44 MPa^1/2 and 13.90 MPa^1/2, respectively), and even outperformed l-FF (5.61 MPa^1/2). Interestingly, both linear dipeptides (l-FF and l-LL) gelled similar solvents, yet when cyclized only cyclo-dityrosine was a robust gelator, while cyclo-diphenylalanine was not. Changes in the side chains drastically affected the crystal morphology of the resultant gels. Symmetric cyclo dipeptides of leucine and tyrosine were capable of forming extremely high aspect ratio fibers in numerous solvents, which represent new molecular motifs capable of driving self-assembly.

Photo- and Aromatic Stacking-Induced Green Emissive Peptidyl Nanoparticles for Cell Imaging and Monitoring of Nucleic Acid Delivery

Owing to their potential applications in biomedicine and biotechnology, peptide nanostructures that exhibit stable intrinsic fluorescence in the visible range are highly desired. This research proposes a facile strategy to construct peptidyl virus-like nanoparticles (NVPs) that show green luminescence by coassembly of two bioactive ferrocene-diphenylalanine-based (Fc-FF) peptides. The green fluorescence of NVPs was originated from the highly ordered structures assembled by the amphiphilic Fc-FF-based peptides via strong π-π stacking interactions. In the assemblies, Fc-FF chromophore can be hydrolyzed under the natural light irradiation, which eliminates the fluorophore quenching effect of Fc and increases the aromatic stacking interactions, thereby giving rise to strong fluorescent nanoparticles. The NVPs could cross cytomembrane barriers by virtue of the HIV V3 peptide and the nuclear localization signal, and could thus be used for long-term cell imaging with excellent photostability and biocompatibility in physiological condition. In addition, NVPs could package DNA and be used to monitor the delivery of DNA, indicating great potential in the tracking and monitoring of genetic biological processes.

A novel aggregation-induced emission enhancement triggered by the assembly of a chiral gelator: from non-emissive nanofibers to emissive micro-loops

A novel aggregation-induced emission enhancement triggered by chiral self-assembled superstructures has been developed in this work.

Water ordering controls the dynamic equilibrium of micelle-fibre formation in self-assembly of peptide amphiphiles

Understanding the role of water in governing the kinetics of the self-assembly processes of amphiphilic peptides remains elusive. Here, we use a multistage atomistic-coarse-grained approach, complemented by circular dichroism/infrared spectroscopy and dynamic light scattering experiments to highlight the dual nature of water in driving the self-assembly of peptide amphiphiles (PAs). We show computationally that water cage formation and breakage near the hydrophobic groups control the fusion dynamics and aggregation of PAs in the micellar stage. Simulations also suggest that enhanced structural ordering of vicinal water near the hydrophilic amino acids shifts the equilibrium towards the fibre phase and stimulates structure and order during the PA assembly into nanofibres. Experiments validate our simulation findings; the measured infrared O–H bond stretching frequency is reminiscent of an ice-like bond which suggests that the solvated water becomes increasingly ordered with time in the assembled peptide network, thus shedding light on the role of water in a self-assembly process.

The role of water in the kinetics of the self-assembly process of amphiphilic peptides still remains unknown. Sankaranarayanan et al. have shown through computational study that water has a dual nature when dictating the mechanism and dynamics of self-assembly of peptide amphiphiles.

Photo-responsive modulation of hybrid peptide assembly, charge transfer complex formation and gelation

Light has been used as trigger for modulation of supramolecular assembly of hybrid peptides followed by charge transfer complex formation and organogelation.

Functional architectures based on self-assembly of bio-inspired dipeptides: Structure modulation and its photoelectronic applications

Getting inspiration from nature and further developing functional architectures provides an effective way to design innovative materials and systems. Among bio-inspired materials, dipeptides and its self-assembled architectures with functionalities have recently been the subject of intensive studies. However, there is still a great challenge to explore its applications likely due to the lack of effective adaptation of their self-assembled structures as well as a lack of understanding of the self-assembly mechanisms. In this context, taking diphenylalanine (FF, a core recognition motif for molecular self-assembly of the Alzheimer's β-amyloid polypeptides) as a model of bio-inspired dipeptides, recent strategies on modulation of dipeptide-based architectures were introduced with regard to both covalent (architectures modulation by coupling functional groups) and non-covalent ways (controlled architectures by different assembly pathways). Then, applications are highlighted in some newly emerging fields of innovative photoelectronic devices and materials, such as artificial photosynthetic systems for renewable solar energy storage and renewable optical waveguiding materials for optoelectronic devices. At last, the challenges and future perspectives of these bio-inspired dipeptides are also addressed.

Lighting up fluoride ions in cellular mitochondria using a highly selective and sensitive fluorescent probe

We report a highly selective and sensitive fluorescent probe () for detecting fluoride ions, for the first time, lighting up the fluoride ions in mitochondria with a strong green fluorescence. could be easily prepared as fluoride paper test strips to detect fluoride ions in aqueous solutions with a detection limit as low as 19 ppb.

Chemical reactions directed Peptide self-assembly

Fabrication of self-assembled nanostructures is one of the important aspects in nanoscience and nanotechnology. The study of self-assembled soft materials remains an area of interest due to their potential applications in biomedicine. The versatile properties of soft materials can be tuned using a bottom up approach of small molecules. Peptide based self-assembly has significant impact in biology because of its unique features such as biocompatibility, straight peptide chain and the presence of different side chain functionality. These unique features explore peptides in various self-assembly process. In this review, we briefly introduce chemical reaction-mediated peptide self-assembly. Herein, we have emphasised enzymes, native chemical ligation and photochemical reactions in the exploration of peptide self-assembly.

Controlled rod nanostructured assembly of diphenylalanine and their optical waveguide properties

Diphenylalanine (FF) microrods were obtained by manipulating the fabrication conditions. Fourier transform infrared (FTIR), circular dichroism (CD), fluorescence (FL) spectroscopy, and X-ray diffraction (XRD) measurements revealed the molecular arrangement within the FF microrods, demonstrating similar secondary structure and molecular arrangement within FF microtubes and nanofibers. Accordingly, a possible mechanism was proposed, which may provide important guidance on the design and assembly manipulation of peptides and other biomolecules. Furthermore, characterization of a single FF microrod indicates that the FF microrod can act as an active optical waveguide material, allowing locally excited photoluminescence to propagate along the length of the microrod with coupling out at the microrod tips.

Sonication-induced instant fibrillation and fluorescent labeling of tripeptide fibers

A sonication-induced instant fibril formation technique has been used for the co-assembly and fluorescent labeling of peptide nanofibers with organic dyes.

Supramolecular assembly of dipeptide functionalized benzo[ghi]perylene monoimide directs white light emission via donor–acceptor interactions

Optical and self-assembly nature of an aromatic dipeptide Phe-Phe (FF) functionalized benzo[ghi]perylene monoimide (BPI) are studied. Acceptor BPI-FF-OMe molecule shows white light emission upon energy transfer from donor pyrenebutyric acid molecule.

Adjustable nanofibers self-assembled from an irregular conformational peptide amphiphile

A peptide amphiphile adopting an irregular conformation self-assembled into dendritic nanofibers, peacock-feather-like nanofibers, and even parallel nanofibers.

Temperature-induced reversible self-assembly of diphenylalanine peptide and the structural transition from organogel to crystalline nanowires

Controlling the self-assembly of diphenylalanine peptide (FF) into various nanoarchitectures has received great amounts of attention in recent years. Here, we report the temperature-induced reversible self-assembly of diphenylalanine peptide to microtubes, nanowires, or organogel in different solvents. We also find that the organogel in isopropanol transforms into crystalline flakes or nanowires when the temperature increases. The reversible self-assembly in polar solvents may be mainly controlled by electronic and aromatic interactions between the FF molecules themselves, which is associated with the dissociation equilibrium and significantly influenced by temperature. We found that the organogel in the isopropanol solvent made a unique transition to crystalline structures, a process that is driven by temperature and may be kinetically controlled. During the heating-cooling process, FF preferentially self-assembles to metastable nanofibers and organogel. They further transform to thermodynamically stable crystal structures via molecular rearrangement after introducing an external energy, such as the increasing temperature used in this study. The strategy demonstrated in this study provides an efficient way to controllably fabricate smart, temperature-responsive peptide nanomaterials and enriches the understanding of the growth mechanism of diphenylalanine peptide nanostructures.

Peptide nanotubes

The self-assembly of different classes of peptide, including cyclic peptides, amyloid peptides and surfactant-like peptides into nanotube structures is reviewed. The modes of self-assembly are discussed. Additionally, applications in bionanotechnology and synthetic materials science are summarized.