Morphology control between twisted ribbon, helical ribbon, and nanotube self-assemblies with his-containing helical peptides in response to pH change

Dual-Responsive Supramolecular Chiral Assemblies from Amphiphilic Dendronized Tetraphenylethylenes

Supramolecular assembly of amphiphilic molecules in aqueous solutions to form stimuli-responsive entities is attractive for developing intelligent supramolecular materials for bioapplications. Here we report on the supramolecular chiral assembly of amphiphilic dendronized tetraphenylethylenes (TPEs) in aqueous solutions. Hydrophobic TPE moieties were connected to the hydrophilic three-fold dendritic oligoethylene glycols (OEGs) through a tripeptide proline-hydroxyproline-glycol (POG) to afford the characteristic topological structural effects of dendritic OEGs and the peptide linker. Both ethoxyl- and methoxyl-terminated dendritic OEGs were used to modulate the overall hydrophilicity of the dendronized TPEs. Their supramolecular aggregates exhibited thermoresponsive behavior that originated from the dehydration and collapse of the dendritic OEGs, and their cloud point temperatures (Tcps) were tailored by solution pH conditions. Furthermore, aggregation-induced fluorescent emission (AIE) from TPE moieties was used as an indicator to follow the assembly, which was reversibly tuned by temperature variation at different pH conditions. Supramolecular assemblies from these dendronized amphiphiles exhibited enhanced supramolecular chirality, which was dominated mainly by the interaction balance between TPE with dendritic OEG and TPE with POG moieties and was modulated through different solvation by changing solution temperature or pH conditions. More interestingly, ethoxyl-terminated dendritic OEG provided a much stronger shielding effect than its methoxyl-terminated counterpart to prevent amino groups within the peptide from protonation, even in strong acidic conditions, resulting in different responsive behavior to the solution temperature and pH conditions for these supramolecular aggregates.

Self-Sorting in Diastereomeric Mixtures of Functionalized Dipeptides

Self-sorting in functionalized dipeptide systems can be driven by the chirality of a single amino acid, both at a high pH in the micellar state and at a low pH in the gel state. The structures formed are affected to some degree by the relative concentrations of each component showing the complexity of such an approach. The structures underpinning the gel network are predefined by the micellar structures at a high pH. Here, we describe the systems prepared from two dipeptide-based gelators that differ only by the chirality of one of the amino acids. We provide firm evidence for self-sorting in the micellar and gel phases using small-angle neutron scattering and cryo-transmission electron microscopy (cryo-TEM), showing that complete self-sorting occurs across a range of relative concentrations.

Control of the stepwise self-assembly process of a pH-responsive amphiphilic 4-aminoquinoline-tetraphenylethene conjugate

Controlling the kinetic processes of self-assembly and switching their kinetic properties according to the changes in external environments are crucial concepts in the field of supramolecular polymers in water for biological and biomedical applications. Here we report a new self-assembling amphiphilic 4-aminoquinoline (4-AQ)-tetraphenylethene (TPE) conjugate that exhibits kinetically controllable stepwise self-assembly and has the ability of switching its kinetic nature in response to pH. The self-assembly process of the 4-AQ amphiphile comprises the formation of sphere-like nanoparticles, a transition to short nanofibers, and their growth to long nanofibers with ∼1 μm length scale at room temperature (RT). The timescale of the self-assembly process differs according to the pH-responsivity of the 4-AQ moiety in a weakly acidic to neutral pH range. Therefore, after aging for 24 h at RT, the 4-AQ amphiphile forms metastable short nanofibers at pH 5.5, while it forms thermodynamically favored long nanofibers at pH 7.4. Moreover, the modulation of nanofiber growth proceeding spontaneously at RT was achieved by switching the kinetic pathway through changing the pH between 7.4 and 5.5.

Electrostatic Control of Shape Selection and Nanoscale Structure in Chiral Molecular Assemblies

How molecular chirality manifests at the nano- to macroscale has been a scientific puzzle since Louis Pasteur discovered biochirality. Chiral molecules assemble into meso-shapes such as twisted and helical ribbons, helicoidal scrolls (cochleates), or möbius strips (closed twisted ribbons). Here we analyze self-assembly for a series of amphiphiles, C _n -K, consisting of an ionizable amino acid [lysine (K)] coupled to alkyl tails with n = 12, 14, or 16 carbons. This simple system allows us to probe the effects of electrostatic and van der Waals interactions in chiral assemblies. Small/wide-angle X-ray scattering (SAXS/WAXS) reveals that at low pH, where the headgroups are ionized (+1), C₁₆-K forms high aspect ratio, planar crystalline bilayers. Molecular dynamics (MD) simulations reveal that tilted tails of the bilayer leaflets are interdigitated. SAXS shows that, with increasing salt concentration, C₁₆-K molecules assemble into cochleates, whereas at elevated pH (reduced degree of ionization), helices are observed for all C _n -K assemblies. The shape selection between helices and scrolls is explained by a membrane energetics model. The nano- to meso-scale structure of the chiral assemblies can be continuously controlled by solution ionic conditions. Overall, our study represents a step toward an electrostatics-based approach for shape selection and nanoscale structure control in chiral assemblies.

Mechanistic process understanding of the self-assembling behaviour of asymmetric bolaamphiphilic short-peptides and their templating for silica and titania nanomaterials

Investigation of the self-assembly of peptides is critically important to clarify certain biophysical phenomena, fulfill some biological functions, and construct functional materials. However, it is still a challenge to precisely predict the self-assembled structures of peptides because of their complicated driving forces and various assembling pathways. In this work, to elucidate the effects of noncovalent interactions including hydrogen bonding, molecular geometry, and hydrophobic and electrostatic interactions on the peptide self-assembly, a series of asymmetric bolaamphiphilic short peptides consisting of Ac-EI₃K-NH₂ (EI₃K), Ac-EI₄K-NH₂ (EI₄K), Ac-KI₃E-NH₂ (KI₃E) and Ac-KI₄E-NH₂ (KI₄E) were designed and their self-assembling behaviors at different solution pH values were investigated systematically. The peptides self-assembled into twisted nanofibers under most conditions except for EI₄K in a strongly alkaline solution and KI₄E under a strongly acidic condition, in which they self-assembled into nanotubes via helical monolayer nanosheet intermediates. In particular, KI₄E nanotubes are formed under acidic conditions, and its diameters are ∼500 nm much greater than most of the self-assembled structures from bolaamphiphilic peptides. Moreover, reversible morphological transition between the nanotubes and twisted nanofibers was observed with the change in solution pH. Such tunable self-assembled structures and switchable surface properties of the asymmetric bolaamphiphilic short-peptides allow them to be used as templates to construct advanced materials. Silica and titania nanomaterials faithful to the peptide templates in morphology were prepared at ambient temperature. This work clearly elucidates the effects of noncovalent interactions on the peptide self-assembly and also provides new insights into the design and preparation of complicated inorganic materials from tunable organic templates.

Control of Peptide Amphiphile Supramolecular Nanostructures by Isosteric Replacements

Supramolecular nanostructures with tunable properties can have applications in medicine, pharmacy, and biotechnology. In this work, we show that the self-assembly behavior of peptide amphiphiles (PAs) can be effectively tuned by replacing the carboxylic acids exposed to the aqueous media with isosteres, functionalities that share key physical or chemical properties with another chemical group. Transmission electron microscopy, atomic force microscopy, and small-angle X-ray scattering studies indicated that the nanostructure's morphologies are responsive to the ionization states of the side chains, which are related to their pK_a values. Circular dichroism studies revealed the effect of the isosteres on the internal arrangement of the nanostructures. The interactions between diverse surfaces and the nanostructures and the effect of salt concentration and temperature were assessed to further understand the properties of these self-assembled systems. These results indicate that isosteric replacements allow the pH control of supramolecular morphology by manipulating the pK_a of the charged groups located on the nanostructure's surface. Theoretical studies were performed to understand the morphological transitions that the nanostructures underwent in response to pH changes, suggesting that the transitions result from alterations in the Coulomb forces between PA molecules. This work provides a strategy for designing biomaterials that can maintain or change behaviors based on the pH differences found within cells and tissues.

Solvent Effects on the Self-Assembly of an Amphiphilic Polypeptide Incorporating α-Helical Hydrophobic Blocks

The self-assembly of biological molecules is an important pathway to understanding the molecular basis of complex metabolic events. The presence of a cosolvent in an aqueous solution during the self-assembly process can promote the formation of kinetically trapped metastable intermediates. In nature, a category of cosolvents termed osmolytes can work to strengthen the hydrogen-bond network of water such that the native states of certain proteins are favored, thus modulating their function and stability. However, identifying cosolvents that act as osmolytes in biomimetic applications, such as the self-assembly of soft materials, remains challenging. The present work examined the effects of ethanol (EtOH) and acetonitrile (ACN) as cosolvents on the self-assembly of the amphiphilic polypeptide PSar₃₀-(l-Leu-Aib)₆ (S30L12), which incorporates α-helical hydrophobic blocks, in aqueous solution. The results provided a direct observation of morphological behavior of S30L12 as a function of solvent composition. Morphological transitions were investigated using transmission electron microscopy, while the packing of peptide molecules was assessed using circular dichroism analyses and evaluations of membrane fluidity. In the EtOH/H₂O mixtures, the EtOH strengthened the hydrogen-bond network of the water, thus limiting the hydrophobic hydration of S30L12 assemblies and enhancing hydrophobic interactions between assemblies. In contrast, ACN formed self-associated nanoclusters in water and at the hydrophobic cores of peptide assemblies to stabilize the edges exposed to bulk water and enhance the assembly kinetics. Fourier transform infrared (FT-IR) analysis indicated that both EtOH and ACN can modify the self-assembly of biomaterials in the same manner as osmolyte protectants or denaturants.

Self-Assembly of Peptide Chiral Nanostructures with Sequence-Encoded Enantioseparation Capability

Biopolymers such as polysaccharides and proteins have been widely used for the chiral separation of various components due to the intrinsic chirality of the polymers. Amyloid-like short peptides can also self-assemble into diverse chiral supramolecular nanostructures or polymers with precisely tailored architectures driving by noncovalent interactions. However, the use of such supramolecular nanostructures for the resolution and separation of chiral components remains largely unexplored. Here, we report that the self-assembled peptide supramolecular nanostructures can be used for the highly efficient chiral separation of various enantiomers. By rationally designing the constituent amino acid sequence of the peptides and the self-assembling environment, we can fabricate supramolecular polymers with distinct surface charges and architectures, including nanohelices, nanoribbons, nanosheets, nanofibrils, and nanospheres. The various supramolecular nanostructures were then used to resolve the racemic mixtures of α-methylbenzylamine, 2-phenylpropionic acid, and 1-phenylethanol. The results indicated that the self-assembled peptide polymers showed excellent enantioselective separation efficiency for different chiral molecules. The enantioselective separation efficiency of the peptide nanostructures can be tailored by changing their surface charges, morphology, and the constituent amino acid sequences of the peptides.

Protamine-induced condensation of peptide nanofilaments into twisted bundles with controlled helical geometry

Chiral self-assembly of peptides is of fundamental interest in the field of biology and material science. Protamine, an alkaline biomacromolecule which is ubiquitous in fish and mammalian, plays crucial roles in directing the helical twisting of DNA. Inspired by this, we reported a bioinspired pathway to direct the hierarchical chiral self-assembly of a short synthetic dipeptide. The peptide could self-assemble into negatively charged chiral micelles in water that spontaneously formed a nematic liquid crystalline phase. By incorporation with protamine, the micelles condensed with the protamine into large helical bundles with precisely controlled diameter. Furthermore, to simulate the intracellular environments, we investigated macromolecular crowding on the coassembly of peptide and protamine, which leads to the formation of much thinner helical structures. The results highlight the roles of highly charged biomacromolecules and macromolecular crowding on peptide self-assembly, which are beneficial for the practical applications of self-assembling peptides in biomedicine and sensing.

Stimuli-Responsive, Pentapeptide, Nanofiber Hydrogel for Tissue Engineering

Short peptides are uniquely versatile building blocks for self-assembly. Supramolecular peptide assemblies can be used to construct functional hydrogel biomaterials-an attractive approach for neural tissue engineering. Here, we report a new class of short, five-residue peptides that form hydrogels with nanofiber structures. Using rheology and spectroscopy, we describe how sequence variations, pH, and peptide concentration alter the mechanical properties of our pentapeptide hydrogels. We find that this class of seven unmodified peptides forms robust hydrogels from 0.2-20 kPa at low weight percent (less than 3 wt %) in cell culture media and undergoes shear-thinning and rapid self-healing. The peptides self-assemble into long fibrils with sequence-dependent fibrillar morphologies. These fibrils exhibit a unique twisted ribbon shape, as visualized by transmission electron microscopy (TEM) and Cryo-EM imaging, with diameters in the low tens of nanometers and periodicities similar to amyloid fibrils. Experimental gelation behavior corroborates our molecular dynamics simulations, which demonstrate peptide assembly behavior, an increase in β-sheet content, and patterns of variation in solvent accessibility. Our rapidly assembling pentapeptides for injectable delivery (RAPID) hydrogels are syringe-injectable and support cytocompatible encapsulation of oligodendrocyte progenitor cells (OPCs), as well as their proliferation and three-dimensional process extension. Furthermore, RAPID gels protect OPCs from mechanical membrane disruption and acute loss of viability when ejected from a syringe needle, highlighting the protective capability of the hydrogel as potential cell carriers for transplantation therapies. The tunable mechanical and structural properties of these supramolecular assemblies are shown to be permissive to cell expansion and remodeling, making this hydrogel system suitable as an injectable material for cell delivery and tissue engineering applications.

Self-Assembled Helical and Twisted Nanostructures of a Preferred Handedness from Achiral π-Conjugated Oligo( p-phenylenevinylene) Derivatives

The formation of chiral nanostructures from the self-assembly of achiral building blocks without external symmetry breaking inducing factors is believed to associate with the origin of chirality. Herein, we reported the synthesis and self-assembly of oligo( p-phenylenevinylene)- b-poly(ethylene glycol) (OPV₃- b-PEG₁₇, the subscripts represent the number of repeat unit of each block) in solution. We systematically examined the influence of solvent, heating temperature, and concentration of OPV₃- b-PEG₁₇ on the self-assembly of OPV₃- b-PEG₁₇ by UV/vis absorption and fluorescence spectrometry, circular dichroism technique, and transmission electron and atomic force microscopy. Interestingly, helical and twisted nanoribbons and nanotubes of a preferred handedness can be formed from achiral OPV₃- b-PEG₁₇ in the mixture of water/ethanol (v/v = 1/1) and the solution showed an obvious exciton-coupled bisignated signal, which indicated that symmetry breaking occurred during the formation of these nanostructures without external inducing factors. Our results showed that the occurrence of symmetry breaking is subtle to the experimental factors including solvent, heating temperature, and concentration of OPV₃- b-PEG₁₇. The directional π-π stacking along with steric repulsion between PEG domains should be the driving force for the formation of these chiral nanostructures. The occurrence of statistical fluctuations in the initial stage of self-assembly led to an accidental excess of helical or/and twisted structures, that is, symmetry breaking. Subsequently, the autocatalysis effect resulted in the formation of helical or/and twisted nanoribbons with a preferred handedness.

Aromatic Motifs Dictate Nanohelix Handedness of Tripeptides

Self-assembly of peptides and amyloid fibrils offers an appealing approach for creating chiral nanostructures, which has promising applications in the fields of biology and materials science. Although numerous self-assembled chiral materials have been designed, the precise control of their twisting tendency and their handedness is still a challenge. Herein, we report the self-assembly of chiral nanostructures with precisely tailored architectures by changing the amino acid sequences of the peptides. We designed a series of self-assembling tripeptides bearing different l-amino acid sequences. The peptide with l-Phe-l-Phe sequence preferred to self-assemble into left-handed nanohelices, while with l-Phe-l-Trp right-handed nanohelices would be formed. Moreover, the diameter of the self-assembled nanohelices could be tailored by changing the terminal amino acids (His, Arg, Ser, Glu, and Asp). Circular dichroism (CD) and molecular dynamics simulations (MDSs) revealed that both of the right- and left-handed nanohelices formed by the tripeptides showed negative Cotton effects in the peptide adsorption region but exhibited nearly opposite CD Cotton effects in the aromatic regions. These results indicated that the handedness of the self-assembled helical nanofibers was not only determined by the chirality of the peptide backbone but also closely related to the aromatic stacking, hydrogen bonding and steric interactions induced by the side chains. The findings deepen our understanding on the chiral self-assembly of peptide and offer opportunities for the creation of highly functional chiral nanomaterials.

Tailoring a Tyrosine-Rich Peptide into Size- and Thickness-Controllable Nanofilms

Self-assembled nanostructures of tyrosine-rich peptides have a number of potential applications such as biocatalysts, organic conducting films, and ion-selective membranes. In modulating a self-assembly process of peptides, the interfacial force is an important factor for kinetic control. Here, we present the formation of large-sized and thickness-controllable nanofilms from the YYACAYY peptide sequence (Tyr-C7mer peptide) using Langmuir-Blodgett and Langmuir-Schaefer deposition methods. The Tyr-C7mer peptide showed typical surfactant-like properties, which were demonstrated via the isotherm test (surface pressure-area) by spreading the Tyr-C7mer peptide solution onto an air/water interface. Uniform and flat peptide nanofilms were successfully fabricated and characterized. The redox activity of densely packed tyrosine moieties on the peptide nanofilm was also evaluated by assembling silver nanoparticles on the nanofilm without requiring any additives.

Solvent Dielectricity-Modulated Helical Assembly and Morphologic Transformation of Achiral Surfactant-Inorganic Cluster Ionic Complexes

Ionic complexes comprising single/double chain cationic surfactant and Lindqvist-type polyoxomolybdate anionic cluster were used for controlled self-assembly in organic solutions. In the solvent with low dielectric constant the complexes self-assembled into flat ribbon like lamellar aggregations with an inverse bilayer substructure where the cluster located at the middle. Under the condition of increased dielectric constant, the solvent triggered the formation of helical self-assemblies, which finally transformed from helical ribbons to the flower-like assemblies due to the bilayer becoming excessively twisted. The self-assembled morphology and the substructure were characterized by SEM, TEM, and XRD. The solvent dielectricity-controlled morphologic transformations modulated by the variation of electrostatic interactions between organic cations and inorganic polyanions were demonstrated by ¹H NMR and IR spectra. The strategy in this work represents an effective route in targeting the chirality-directed functionalization of inorganic clusters by combining controllable and helical assemblies of achiral polyoxometalate complexes in one system.

Self-assembling peptide and protein amyloids: from structure to tailored function in nanotechnology

Self-assembled peptide and protein amyloid nanostructures have traditionally been considered only as pathological aggregates implicated in human neurodegenerative diseases. In more recent times, these nanostructures have found interesting applications as advanced materials in biomedicine, tissue engineering, renewable energy, environmental science, nanotechnology and material science, to name only a few fields. In all these applications, the final function depends on: (i) the specific mechanisms of protein aggregation, (ii) the hierarchical structure of the protein and peptide amyloids from the atomistic to mesoscopic length scales and (iii) the physical properties of the amyloids in the context of their surrounding environment (biological or artificial). In this review, we will discuss recent progress made in the field of functional and artificial amyloids and highlight connections between protein/peptide folding, unfolding and aggregation mechanisms, with the resulting amyloid structure and functionality. We also highlight current advances in the design and synthesis of amyloid-based biological and functional materials and identify new potential fields in which amyloid-based structures promise new breakthroughs.

Tuning the Viscoelasticity of Peptide Vesicles by Adjusting Hydrophobic Helical Blocks Comprising Amphiphilic Polypeptides

Amphiphilic block polypeptides of poly(sarcosine)-b-(l-Val-Aib)₆ and poly(sarcosine)-b-(l-Leu-Aib)₆ and their stereoisomers were self-assembled in water. Three kinds of binary systems of poly(sarcosine)-b-(l-Leu-Aib)₆ with poly(sarcosine)-b-poly(d-Leu-Aib)₆, poly(sarcosine)-b-poly(l-Val-Aib)₆, or poly(sarcosine)-b-(d-Val-Aib)₆ generated vesicles of ca. 200 nm diameter. The viscoelasticity of the vesicle membranes was evaluated by the nanoindentation method using AFM in water. The elasticity of the poly(sarcosine)-b-(l-Leu-Aib)₆/poly(sarcosine)-b-poly(d-Leu-Aib)₆ vesicle was 11-fold higher than that of the egg yolk liposome but decreased in combinations of the Leu- and Val-based amphiphilic polypeptides. The membrane elasticity is found to be adjustable by a suitable combination of helical blocks in terms of stereocomplex formation and the interdigitation of side chains among helices in the molecular assemblies.

A rational molecular design of triazine-containing alkynylplatinum(ii) terpyridine complexes and the formation of helical ribbons via Pt⋯Pt, π–π stacking and hydrophobic–hydrophobic interactions

The self-assembly of strategically designed triazine-containing alkynylplatinum(ii) terpyridine complexes yielded sophisticated helical ribbons through a balance of multiple non-covalent interactions.

Peptide-Directed Assembly of Single-Helical Gold Nanoparticle Superstructures Exhibiting Intense Chiroptical Activity

Chiral nanoparticle assemblies are an interesting class of materials whose chiroptical properties make them attractive for a variety of applications. Here, C18-(PEPAuM-ox)2 (PEPAuM-ox = AYSSGAPPMoxPPF) is shown to direct the assembly of single-helical gold nanoparticle superstructures that exhibit exceptionally strong chiroptical activity at the plasmon frequency with absolute g-factor values up to 0.04. Transmission electron microscopy (TEM) and cryogenic electron tomography (cryo-ET) results indicate that the single helices have a periodic pitch of approximately 100 nm and consist of oblong gold nanoparticles. The morphology and assembled structure of C18-(PEPAuM-ox)2 are studied using TEM, atomic force microscopy (AFM), Fourier transform infrared (FTIR) spectroscopy, circular dichroism (CD) spectroscopy, X-ray diffraction (XRD), and solid-state nuclear magnetic resonance (ssNMR) spectroscopy. TEM and AFM reveal that C18-(PEPAuM-ox)2 assembles into linear amyloid-like 1D helical ribbons having structural parameters that correlate to those of the single-helical gold nanoparticle superstructures. FTIR, CD, XRD, and ssNMR indicate the presence of cross-β and polyproline II secondary structures. A molecular assembly model is presented that takes into account all experimental observations and that supports the single-helical nanoparticle assembly architecture. This model provides the basis for the design of future nanoparticle assemblies having programmable structures and properties.

Water-Floating Giant Nanosheets from Helical Peptide Pentamers

One of the important challenges in the development of protein-mimetic materials is understanding the sequence-specific assembly behavior and dynamic folding change. Conventional strategies for constructing two-dimensional (2D) nanostructures from peptides have been limited to using β-sheet forming sequences as building blocks due to their natural tendency to form sheet-like aggregations. We have identified a peptide sequence (YFCFY) that can form dimers via a disulfide bridge, fold into a helix, and assemble into macroscopic flat sheets at the air/water interface. Due to the large driving force for 2D assembly and high elastic modulus of the resulting sheet, the peptide assembly induces flattening of the initially round water droplet. Additionally, we found that stabilization of the helix by dimerization is a key determinant for maintaining macroscopic flatness over a few tens of centimeters even with a uniform thickness of <10 nm. Furthermore, the ability to transfer the sheets from a water droplet to another substrate allows for multiple stacking of 2D peptide nanostructures, suggesting possible applications in biomimetic catalysis, biosensors, and 2D related electronic devices.

Dual-Responsive Lipid Nanotubes: Two-Way Morphology Control by pH and Redox Effects

Lipid nanotubes are the preferred structures for many applications, especially biological ones, and thus have attracted much interest recently. However, there is still a significant need for developing more lipid nanotubes that are reversibly controllable to improve their functionality and usability. Here, we presented a two-way reversible morphology control of the nanotubes formed by the recently designed molecule AQUA (C25H29NO4). Because of its special design, the AQUA has both pH-sensitive and redox-active characters provided by the carboxylic acid and anthraquinone groups. Upon chemical reduction, the nanotubes turned into thinner ribbons, and this structural transformation was significantly reversible. The reduction of the AQUA nanotubes also switched the nanotubes from electrically conductive to insulative. Nanotube morphology can additionally be altered by decreasing the pH below the pKa value of the AQUA, at ∼4.9. Decreasing the pH caused the gradual unfolding of the nanotubes, and the interlayer distance in the nanotube's walls increased. This morphological change was fast and reversible at a wide pH range, including the physiological pH. Thus, the molecular design of the AQUA allowed for an unprecedented two-way and reversible morphology control with both redox and pH effects. These unique features make AQUA a very promising candidate for many applications, ranging from electronics to controlled drug delivery.

Self-assembly of alkynylplatinum(II) terpyridine amphiphiles into nanostructures via steric control and metal-metal interactions

A series of mono- and dinuclear alkynylplatinum(II) terpyridine complexes containing the hydrophilic oligo(para-phenylene ethynylene) with two 3,6,9-trioxadec-1-yloxy chains was designed and synthesized. The mononuclear alkynylplatinum(II) terpyridine complex was found to display a very strong tendency toward the formation of supramolecular structures. Interestingly, additional end-capping with another platinum(II) terpyridine moiety of various steric bulk at the terminal alkyne would lead to the formation of nanotubes or helical ribbons. These desirable nanostructures were found to be governed by the steric bulk on the platinum(II) terpyridine moieties, which modulates the directional metal-metal interactions and controls the formation of nanotubes or helical ribbons. Detailed analysis of temperature-dependent UV-visible absorption spectra of the nanostructured tubular aggregates also provided insights into the assembly mechanism and showed the role of metal-metal interactions in the cooperative supramolecular polymerization of the amphiphilic platinum(II) complexes.

Electrostatic and aromatic interaction-directed supramolecular self-assembly of a designed Fmoc-tripeptide into helical nanoribbons

Supramolecular self-assembly offers an efficient pathway for creating macroscopically chiral structures in biology and materials science. Here, a new peptide consisting of an N-(9-fluorenylmethoxycarbonyl) headgroup connected to an aromatic phenylalanine-tryptophan dipeptide and terminated with zwitterionic lysine (Fmoc-FWK) and its cationic form (Fmoc-FWK-NH2) were designed for self-assembly into chiral structures. It was found that the Fmoc-FWK peptide self-assembled into left-handed helical nanoribbons at pH 11.2-11.8, whereas it formed nanofibers at pH 5 and 12 and large flat ribbons composed of many nanofibers in the pH range of 6-11. However, only nanofibers were observed in the cases of Fmoc-FWK-NH2 at different values. A series of structural characterizations based on CD, FTIR, UV-vis and fluorescence spectroscopy reveal that the electrostatic and aromatic interactions and the associated hydrogen bonding direct the self-assembly into various structures. The enhanced π-π stacking and hydrogen bonding were found in the helical nanoribbons. This difference in intermolecular interactions should be derived from the ionization of carboxyl and amino groups from lysine residues at different pH values. Furthermore, we performed molecular dynamics simulations to gain insight into the assembly mechanisms. The results imply that a relatively rigid molecular conformation and the strong intramolecular aromatic interaction between Trp and Fmoc groups favor chiral self-assembly. This study is the first attempt to design a Fmoc-tripeptide for the fabrication of helical structures with macroscopic chirality, which provides a successful example and allows us to create new peptide-based chiral assembly systems.

Precise control of nanoparticle surface by host–guest chemistry for delivery to tumor

A nanocarrier with a diameter less than 30 nm was prepared here by using host–guest chemistry. A defined number of functionalized guests was incorporated in the nanocarrier, which was a nice tumor imaging probe without showing the ABC phenomenon.

Selective disruption of each part of Janus molecular assemblies by lateral diffusion of stimuli-responsive amphiphilic peptides

Stimuli-responsive Janus-type assemblies with a round-bottom flask shape are prepared from amphiphilic helical peptides using the patchwork self-assembly technique.