Transmembrane delivery of anticancer drugs through self-assembly of cyclic peptide nanotubes

Enzymatic Reaction Enhanced Diffusion Accelerates Cargo Transport through Micro/Nano-Channels

Energy conversion and transfer of enzyme-catalyzed reactions at molecular level are an interesting and challenging scientific topic that helps understanding biological processes in nature. In this study, it is demonstrated that enzyme-catalyzed reactions can enhance diffusion of surrounding molecules and thus accelerate cargo transport within 1D micro/nanochannels. Specifically, urease is immobilized on the inner walls of silica micro/nano-tubes to construct bio-catalytically active micro/nanochannels. The catalytic reaction inside the channels drives a variety of cargoes, including small dye molecules, polymers, and rigid nanoparticles (e.g., quantum dots, QDs), to pass through these micro/nanochannels much faster than they will by free diffusion. The enhanced diffusion of molecular species inside the channels is validated by direct observation of the Brownian motion of tracer particles, and further confirmed by significantly enhanced Raman intensity of reporter molecules. Finite element and Brownian dynamics simulations provide a theoretical understanding of these experimental observations. Furthermore, the effect of the channels' size on the diffusion enhancement is examined. The acceleration effect of the cargo transport through these enzymatically active micro/nanochannels can be turned on or off via chemical activators or inhibitors. This study provides valuable insights on the design of biomimetic channels capable of controlled and efficient transmembrane transport.

Single-Cell Caspase-3 Measurement Using a Biomimetic Nanochannel

Direct single-cell caspase-3 (Casp-3) analysis has remained challenging. A study of single-cell Casp-3 could contribute to revealing the fundamental pathogenic mechanisms in Casp-3-associated diseases. Here, a biomimetic nanochannel capable of single-cell sampling and ionic detection of intracellular Casp-3 is devised, which is established upon the installment of target-specific organic molecules (luc-DEVD) within the orifice of a glass nanopipette. The specific cleavage of luc-DEVD by Casp-3 could induce changes of inner-surface chemical groups and charge properties, thus altering the ionic response of the biomimetic nanochannel for direct Casp-3 detection. The practical applicability of this biomimetic nanochannel is confirmed by probing intracellular Casp-3 fluctuation upon drug stimulation and quantifying the Casp-3 evolution during induced apoptosis. This work realizes ionic single-cell Casp-3 analysis and provides a different perspective for single-cell protein analysis.

Exploring Cyclic Peptide Nanotube Stability Across Diverse Lipid Bilayers and Unveiling Water Transport Dynamics

Cyclic Peptide Nanotubes (CPNTs) have emerged as compelling candidates for various applications, particularly as nanochannels within lipid bilayers. In this study, the stability of two CPNTs, namely 8 × [(Cys-Gly-Met-Gly)2] and 8 × [(Gly-Leu)4], are comprehensively investigated across different lipid bilayers, including 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), a mixed model membrane (POPE/POPG), and a realistic yeast model membrane. The results demonstrate that both CPNTs maintain their tubular structures in all lipid bilayers, with [(Cys-Gly-Met-Gly)2] showing increased stability over an extended period in these lipid membranes. The insertion of CPNTs shows negligible impact on lipid bilayer properties, including area per lipid, volume per lipid, and bilayer thickness. The study demonstrates that the CPNT preserves its two-line water movement pattern within all the lipid membranes, reaffirming their potential as water channels. The MSD curves further reveal that the dynamics of water molecules inside the nanotube are similar for all the bilayer systems with minor differences that arise due to different lipid environments.

A novel protein purification scheme based on salt inducible self-assembling peptides

BACKGROUND

Protein purification remains a critical need for biosciences and biotechnology. It frequently requires multiple rounds of chromatographic steps that are expensive and time-consuming. Our lab previously reported a cleavable self-aggregating tag (cSAT) scheme for streamlined protein expression and purification. The tag consists of a self-assembling peptide (SAP) and a controllable self-cleaving intein. The SAP drives the target protein into an active aggregate, then by intein-mediated cleavage, the target protein is released. Here we report a novel cSAT scheme in which the self-assembling peptide is replaced with a salt inducible self-assembling peptide. This allows a target protein to be expressed first in the soluble form, and the addition of salt then drives the target protein into the aggregated form, followed by cleavage and release.

RESULTS

In this study, we used MpA (MKQLEDKIEELLSKAAMKQLEDKIEELLSK) as a second class of self-assembling peptide in the cSAT scheme. This scheme utilizes low salt concentration to keep the fusion protein soluble, while eliminating insoluble cellular matters by centrifugation. Salt then triggers MpA-mediated self-aggregation of the fusion, removing soluble background host cell proteins. Finally, intein-mediated cleavage releases the target protein into solution. As a proof-of-concept, we successfully purified four proteins and peptides (human growth hormone, 22.1 kDa; LCB3, 7.7 kDa; SpyCatcherΔN-ELP-SpyCatcherΔN, 26.2 kDa; and xylanase, 45.3 kDa) with yields ranging from 12 to 87 mg/L. This was comparable to the classical His-tag method both in yield and purity (72-97%), but without the His-tag. By using a further two-step column purification process that included ion-exchange chromatography and size-exclusion chromatography, the purity was increased to over 99%.

CONCLUSION

Our results demonstrate that a salt-inducible self-assembling peptide can serve as a controllable aggregating tag, which might be advantageous in applications where soluble expression of the target protein is preferred. This work also demonstrates the potential and advantages of utilizing salt inducible self-assembling peptides for protein separation.

Dynamic Control of Cyclic Peptide Assembly to Form Higher-Order Assemblies

Chirality correction, asymmetry, ring-chain tautomerism and hierarchical assemblies are fundamental phenomena in nature. They are geometrically related and may impact the biological roles of a protein or other supermolecules. It is challenging to study those behaviors within an artificial system due to the complexity of displaying these features. Herein, we design an alternating D,L peptide to recreate and validate the naturally occurring chirality inversion prior to cyclization in water. The resulting asymmetrical cyclic peptide containing a 4-imidazolidinone ring provides an excellent platform to study the ring-chain tautomerism, thermostability and dynamic assembly of the nanostructures. Different from traditional cyclic D,L peptides, the formation of 4-imidazolidinone promotes the formation of intertwined nanostructures. Analysis of the nanostructures confirmed the left-handedness, representing chirality induced self-assembly. This proves that a rationally designed peptide can mimic multiple natural phenomena and could promote the development of functional biomaterials, catalysts, antibiotics, and supermolecules.

Molecular Dynamics Simulation Study of the Protonation State Dependence of Glutamic Acid Transport through a Cyclic Peptide Nanotube

The effect of the protonation state of glutamic acid on its translocation through cyclic peptide nanotubes (CPNs) was assessed by using molecular dynamics (MD) simulations. Anionic (GLU-), neutral zwitterionic (GLU0), and cationic (GLU+) forms of glutamic acid were selected as three different protonation states for an analysis of energetics and diffusivity for acid transport across a cyclic decapeptide nanotube. Based on the solubility-diffusion model, permeability coefficients for the three protonation states of the acid were calculated and compared with experimental results for CPN-mediated glutamate transport through CPNs. Potential of mean force (PMF) calculations reveal that, due to the cation-selective nature of the lumen of CPNs, GLU-, so-called glutamate, shows significantly high free energy barriers, while GLU+ displays deep energy wells and GLU0 has mild free energy barriers and wells inside the CPN. The considerable energy barriers for GLU- inside CPNs are mainly attributed to unfavorable interactions with DMPC bilayers and CPNs and are reduced by favorable interactions with channel water molecules through attractive electrostatic interactions and hydrogen bonding. Unlike the distinct PMF curves, position-dependent diffusion coefficient profiles exhibit comparable frictional behaviors regardless of the charge status of three protonation states due to similar confined environments imposed by the lumen of the CPN. The calculated permeability coefficients for the three protonation states clearly demonstrate that glutamic acid has a strong protonation state dependence for its transport through CPNs, as determined by the energetics rather than the diffusivity of the protonation state. In addition, the permeability coefficients also imply that GLU- is unlikely to pass through a CPN due to the high energy barriers inside the CPN, which is in disagreement with experimental measurements, where a considerable amount of glutamate permeating through the CPN was detected. To resolve the discrepancy between this work and the experimental observations, several possibilities are proposed, including a large concentration gradient of glutamate between the inside and outside of lipid vesicles and bilayers in the experiments, the glutamate activity difference between our MD simulations and experiments, an overestimation of energy barriers due to the artifacts imposed in MD simulations, and/or finally a transformation of the protonation state from GLU- to GLU0 to reduce the energy barriers. Overall, our study demonstrates that the protonation state of glutamic acid has a strong effect on the transport of the acid and suggests a possible protonation state change for glutamate permeating through CPNs.

Enzyme-Powered Tubular Microrobotic Jets as Bioinspired Micropumps for Active Transmembrane Drug Transport

In nature, there exist a variety of transport proteins on cell membranes capable of actively moving cargos across biological membranes, which plays a vital role in the living activities of cells. Emulating such biological pumps in artificial systems may bring in-depth insights on the principles and functions of cell behaviors. However, it poses great challenges due to difficulty in the sophisticated construction of active channels at the cellular scale. Here, we report the development of bionic micropumps for active transmembrane transportation of molecular cargos across living cells that is realized by enzyme-powered microrobotic jets. By immobilizing urease onto the surface of a silica-based microtube, the prepared microjet is capable of catalyzing the decomposition of urea in surrounding environments and generating microfluidic flow through the inside channel for self-propulsion, which is verified by both numerical simulation and experimental results. Therefore, once naturally endocytosed by the cell, the microjet enables the diffusion and, more importantly, active transportation of molecular substances between the extracellular and intracellular ends with the assistance of generated microflow, thus serving as an artificial biomimetic micropump. Furthermore, by constructing enzymatic micropumps on cancer cell membranes, enhanced delivery of anticancer doxorubicin into cells as well as improved killing efficacy are achieved, which demonstrates the effectiveness of the active transmembrane drug transport strategy in cancer treatment. This work not only extends the applications of micro/nanomachines in biomedical fields but also provides a promising platform for future cell biology research at cellular and subcellular scales.

Self-Assembling Peptide-Based Functional Biomaterials

Peptides can self-assemble into various hierarchical nanostructures through noncovalent interactions and form functional materials exhibiting excellent chemical and physical properties, which have broad applications in bio-/nanotechnology. The self-assembly mechanism, self-assembly morphology of peptide supramolecular architecture and their various applications, have been widely explored which have the merit of biocompatibility, easy preparation, and controllable functionality. Herein, we introduce the latest research progress of self-assembling peptide-based nanomaterials and review their applications in biomedicine and optoelectronics, including tissue engineering, anticancer therapy, biomimetic catalysis, energy harvesting. We believe that this review will inspire the rational design and development of novel peptide-based functional bio-inspired materials in the future.

Polymeric Nanotubes as Drug Delivery Vectors─Comparison of Covalently and Supramolecularly Assembled Constructs

Rod-shaped nanoparticles have been identified as promising drug delivery candidates. In this report, the in vitro cell uptake and in vivo pharmacokinetic/bio-distribution behavior of molecular bottle-brush (BB) and cyclic peptide self-assembled nanotubes were studied in the size range of 36-41 nm in length. It was found that BB possessed the longest plasma circulation time (t_1\2 > 35 h), with the cyclic peptide system displaying an intermediate half-life (14.6 h), although still substantially elevated over a non-assembling linear control (2.7 h). The covalently bound BB underwent substantial distribution into the liver, whereas the cyclic peptide nanotube was able to mostly circumvent organ accumulation, highlighting the advantage of the inherent degradability of the cyclic peptide systems through their reversible aggregation of hydrogen bonding core units.

Molecular Self-Assembly and Supramolecular Chemistry of Cyclic Peptides

This Review focuses on the establishment and development of self-assemblies governed by the supramolecular interactions between cyclic peptides. The Review first describes the type of cyclic peptides able to assemble into tubular structures to form supramolecular cyclic peptide nanotubes. A range of cyclic peptides have been identified to have such properties, including α-peptides, β-peptides, α,γ-peptides, and peptides based on δ- and ε-amino acids. The Review covers the design and functionalization of these cyclic peptides and expands to a recent advance in the design and application of these materials through their conjugation to polymer chains to generate cyclic peptide-polymer conjugates nanostructures. The Review, then, concentrates on the challenges in characterizing these systems and presents an overview of the various analytical and characterization techniques used to date. This overview concludes with a critical survey of the various applications of the nanomaterials obtained from supramolecular cyclic peptide nanotubes, with a focus on biological and medical applications, ranging from ion channels and membrane insertion to antibacterial materials, anticancer drug delivery, gene delivery, and antiviral applications.

Self-Assembling Peptides: From Design to Biomedical Applications

Self-assembling peptides could be considered a novel class of agents able to harvest an array of micro/nanostructures that are highly attractive in the biomedical field. By modifying their amino acid composition, it is possible to mime several biological functions; when assembled in micro/nanostructures, they can be used for a variety of purposes such as tissue regeneration and engineering or drug delivery to improve drug release and/or stability and to reduce side effects. Other significant advantages of self-assembled peptides involve their biocompatibility and their ability to efficiently target molecular recognition sites. Due to their intrinsic characteristics, self-assembled peptide micro/nanostructures are capable to load both hydrophobic and hydrophilic drugs, and they are suitable to achieve a triggered drug delivery at disease sites by inserting in their structure's stimuli-responsive moieties. The focus of this review was to summarize the most recent and significant studies on self-assembled peptides with an emphasis on their application in the biomedical field.

Self-assembly of amino acids toward functional biomaterials

Biomolecules, such as proteins and peptides, can be self-assembled. They are widely distributed, easy to obtain, and biocompatible. However, the self-assembly of proteins and peptides has disadvantages, such as difficulty in obtaining high quantities of materials, high cost, polydispersity, and purification limitations. The difficulties in using proteins and peptides as functional materials make it more complicate to arrange assembled nanostructures at both microscopic and macroscopic scales. Amino acids, as the smallest constituent of proteins and the smallest constituent in the bottom-up approach, are the smallest building blocks that can be self-assembled. The self-assembly of single amino acids has the advantages of low synthesis cost, simple modeling, excellent biocompatibility and biodegradability in vivo. In addition, amino acids can be assembled with other components to meet multiple scientific needs. However, using these simple building blocks to design attractive materials remains a challenge due to the simplicity of the amino acids. Most of the review articles about self-assembly focus on large molecules, such as peptides and proteins. The preparation of complicated materials by self-assembly of amino acids has not yet been evaluated. Therefore, it is of great significance to systematically summarize the literature of amino acid self-assembly. This article reviews the recent advances in amino acid self-assembly regarding amino acid self-assembly, functional amino acid self-assembly, amino acid coordination self-assembly, and amino acid regulatory functional molecule self-assembly.

3D Hydrazone-Functionalized Covalent Organic Frameworks as pH-Triggered Rotary Switches

The property expansion of 3D functionalized covalent organic frameworks (COFs) is important for developing their potential applications. Herein, the first case of 3D hydrazone-decorated COFs as pH-triggered molecular switches is reported, and their application in the stimuli-responsive drug delivery system is explored. These functionalized COFs with hydrazone groups on the channel walls are obtained via a multi-component bottom-up synthesis strategy. They exhibit a reversible E/Z isomerization at various pH values, confirmed by UV-vis absorption spectroscopy and proton conduction. Remarkably, after loading cytarabine (Ara-C) as a model drug molecule, these pH-responsive COFs show an excellent and intelligent sustained-release effect with an almost fourfold increase in the Ara-C release at pH = 4.8 than at pH = 7.4, which will effectively improve drug-targeting. Thus, these results open a way toward designing 3D stimuli-responsive functionalized COF materials and promote their potential application as drug carriers in the field of disease treatment.

Simulation of the In Vivo Fate of Polymeric Nanoparticles Traced by Environment-Responsive Near-Infrared Dye: A Physiologically Based Pharmacokinetic Modelling Approach

The application of physiologically based pharmacokinetic models to nanoparticles is still very restricted and challenging, owing to the complicated in vivo transport mechanisms involving nanoparticles, including phagocytosis, enhanced permeability and retention effects, cellular recognition, and internalisation, enzymatic degradation, lymphatic transport, and changes in physical properties. In our study, five nanoparticle formulations were synthesised using polycaprolactone as a framework material and methoxy poly (ethylene glycol)-poly(ε-caprolactone) as a long-circulating decorating material, as well as types of environmentally responsive near-infrared aza-boron-dipyrromethene dyes. According to quantification data and direct visualisation involving specific organs, a phagocytosis physiologically based pharmacokinetic model was developed to describe the dynamics of nanoparticles within and between organs in mice, considering cellular mechanisms involving phagocytosis and enhanced permeability and retention effects. Our results offer a better understanding of the in vivo fate of polymeric nanoparticles.

Peptide-Based Nanoassemblies in Gene Therapy and Diagnosis: Paving the Way for Clinical Application

Nanotechnology approaches play an important role in developing novel and efficient carriers for biomedical applications. Peptides are particularly appealing to generate such nanocarriers because they can be rationally designed to serve as building blocks for self-assembling nanoscale structures with great potential as therapeutic or diagnostic delivery vehicles. In this review, we describe peptide-based nanoassemblies and highlight features that make them particularly attractive for the delivery of nucleic acids to host cells or improve the specificity and sensitivity of probes in diagnostic imaging. We outline the current state in the design of peptides and peptide-conjugates and the paradigms of their self-assembly into well-defined nanostructures, as well as the co-assembly of nucleic acids to form less structured nanoparticles. Various recent examples of engineered peptides and peptide-conjugates promoting self-assembly and providing the structures with wanted functionalities are presented. The advantages of peptides are not only their biocompatibility and biodegradability, but the possibility of sheer limitless combinations and modifications of amino acid residues to induce the assembly of modular, multiplexed delivery systems. Moreover, functions that nature encoded in peptides, such as their ability to target molecular recognition sites, can be emulated repeatedly in nanoassemblies. Finally, we present recent examples where self-assembled peptide-based assemblies with "smart" activity are used in vivo. Gene delivery and diagnostic imaging in mouse tumor models exemplify the great potential of peptide nanoassemblies for future clinical applications.

Temperature-Dependent Reversible Morphological Transformations in N-Oleoyl β-d-Galactopyranosylamine

Amphiphilic molecules self-assemble into supramolecular structures of various sizes and morphologies depending on their molecular packing and external factors. Transformations between various self-assembled morphologies are a matter of great fundamental interest. Recently, we reported the discovery of a novel class of single-chain galactopyranosylamide amphiphiles that self-assemble to form vesicles in water. Here, we describe how the vesicles composed of the amphiphile N-oleoyl β-d-galactopyranosylamine (GOA) undergo a morphological transition to fibers consisting of mainly flat sheet-like structures. Moreover, we show that this transformation is reversible in a temperature-dependent manner. We used several optical microscopy and electron microscopy techniques, circular dichroism spectroscopy, small-angle X-ray scattering, and differential scanning calorimetry, to fully investigate and characterize the morphological transformations of GOA and provide a structural basis for such phenomena. These studies provide significant molecular insight into the structural polymorphism of sugar-based amphiphiles and foresee future applications in rational design of self-assembled materials.

Pharmaceutical Formulation and Characterization of Dipeptide Nanotubes for Drug Delivery Applications

Peptide nanotubes are promising materials for a variety of biomedical applications with ultrashort (≤7 amino acids) forms providing particular promise for clinical translation. The manufacture of peptide nanotubes has, however, been associated with toxic organic solvents restricting clinical use. The purpose of this work is to formulate dipeptide nanotubes using mild techniques easily translated to industrial upscale and to characterize their physiochemical and biological properties. Phenylalanine-phenylalanine variants can be successfully formulated using distilled water as demonstrated here. Formulations are homogenous in shape (tubular), with apparent size (50-260 nm) and a zeta potential of up to +30 mV. L-(H₂ N-FF-COOH), and D-enantiomers (H₂ N-ff-COOH) demonstrate no toxicity against glioblastoma cells and are explored for ability to deliver a model hydrophilic molecule, sodium fluorescein, at pH 5.5 (tumor) and 7.4 (physiological). Peptide nanotubes loaded with >85% sodium fluorescein, demonstrate burst release characteristics, fitting the Weibull model of drug release. This research provides important data contributing to the pharmaceutical formulation of peptide nanotubes as drug delivery platforms for hydrophilic drugs.

Parallel Versus Antiparallel β-Sheet Structure in Cyclic Peptide Hybrids Containing γ- or δ-Cyclic Amino Acids

Cyclic peptides with disc-shaped structures have emerged as potent building blocks for the preparation of new biomaterials in fields ranging from biological to material science. In this work, we analyze in depth the self-assembling properties of a new type of cyclic peptides based on the alternation of α-residues and cyclic δ-amino acids (α,δ-CPs). To examine the preferred stacking properties adopted by cyclic peptides bearing this type of amino acids, we carried out a synergistic in vitro/in silico approximation by using simple dimeric models and then extended to nanotubes. Although these new cyclic peptides (α,δ-CPs) can interact either in a parallel or antiparallel fashion, our results confirm that although the parallel β-sheet is more stable, it can be switched to the antiparallel stacking by choosing residues that can establish favorable cross-strand interactions. Moreover, the subsequent comparison by using the same methodology but applied to α,γ-CPs models, up to the moment assumed as antiparallel-like d,l-α-CPs, led to unforeseen conclusions that put into question preliminary conjectures about these systems. Surprisingly, they tend to adopt a parallel β-sheet directed by the skeleton interactions. These results imply a change of paradigm with respect to cyclic peptide designs that should be considered for dimers and nanotubes.

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.

Peptide Tectonics: Encoded Structural Complementarity Dictates Programmable Self-Assembly

Programmable self-assembly of peptides into well-defined nanostructures represents one promising approach for bioinspired and biomimetic synthesis of artificial complex systems and functional materials. Despite the progress made over the past two decades in the development of strategies for precise manipulation of the self-assembly of peptides, there is a remarkable gap between current peptide assemblies and biological systems in terms of structural complexity and functions. Here, the concept of peptide tectonics for the creation of well-defined nanostructures predominately driven by the complementary association at the interacting interfaces of tectons is introduced. Peptide tectons are defined as peptide building blocks exhibiting structural complementarity at the interacting interfaces of commensurate domains and undergoing programmable self-assembly into defined supramolecular structures promoted by complementary interactions. Peptide tectons are categorized based on their conformational entropy and the underlying mechanism for the programmable self-assembly of peptide tectons is highlighted focusing on the approaches for incorporating the structural complementarity within tectons. Peptide tectonics not only provides an alternative perspective to understand the self-assembly of peptides, but also allows for precise manipulation of peptide interactions, thus leading to artificial systems with advanced complexity and functions and paves the way toward peptide-related functional materials resembling natural systems.

Advances in intracellular delivery through supramolecular self-assembly of oligonucleotides and peptides

Cells utilize natural supramolecular assemblies to maintain homeostasis and biological functions. Naturally inspired modular assembly of biomaterials are now being exploited for understanding or manipulating cell biology for treatment, diagnosis, and detection of diseases. Supramolecular biomaterials, in particular peptides and oligonucleotides, can be precisely tuned to have diverse structural, mechanical, physicochemical and biological properties. These merits of oligonucleotides and peptides as building blocks have given rise to the evolution of numerous nucleic acid- and peptide-based self-assembling nanomaterials for various medical applications, including drug delivery, tissue engineering, regenerative medicine, and immunotherapy. In this review, we provide an extensive overview of the intracellular delivery approaches using supramolecular self-assembly of DNA, RNA, and peptides. Furthermore, we discuss the current challenges related to subcellular delivery and provide future perspectives of the application of supramolecular biomaterials for intracellular delivery in theranostics.

Temperature-responsive self-assembled nanostructures from lysine-based surfactants with high chain length asymmetry: from tubules and helical ribbons to micelles and vesicles

Stimuli-sensitive self-assembled nanostructures are of great relevance for the templating of nanomaterials and the design of efficient systems for the controlled delivery of molecules. Amino acid-based surfactants often display such fascinating self-assembly due to a combination of molecular features such as critical packing parameter, chirality and H-bonding interactions. Herein, we focus on a family of newly synthesized double-chained alkylcarboxylates derived from l-lysine, and designated by 8Lysn, mLys8, with n, m = 12, 14 and 16, and 12Lys16 and 16Lys12, where the numbers represent the number of C atoms in each hydrocarbon chain. The effects of the chain length asymmetry and structural isomerism of the surfactants on their interfacial properties, thermal behavior and self-assembly in water were investigated by a comprehensive toolbox, including surface tension, DSC, imaging (light microscopy, SEM, TEM and AFM) and SAXS. All the surfactants below their Krafft temperature self-organize into tubular structures of various morphologies (flat structures, twisted and coiled ribbons and hollow tubes), forming hydrogels at low surfactant concentration. Upon the solubilization phase transition, micelles or vesicles are formed depending on the surfactant structure, and the tubule-micelle or tubule-vesicle transition is thermoreversible. A molecular-level rationalization of the observed self-assembly and phase transition features is put forth.

Enhancing Membrane-Disruptive Activity via Hydrophobic Phenylalanine and Lysine Tethered to Poly(aspartic acid)

Amphiphilic polymers with pH-responsive abilities have been widely used as carriers for intracellular delivery of bioactive substances, while their membrane-disruptive activity exerted on cells is a critical characteristic that determines delivery efficiency. Herein, we present a novel method to prepare amphiphilic and pH-responsive polymers by chemically tethering l-phenylalanine methyl ester and followed by N_ε-carbobenzyloxy-l-lysine benzyl ester to the side carboxylic acid groups of poly(aspartic acid). The obtained phenylalanine- and lysine-grafted polymer (PAsp- g-Phe)- g-Lys demonstrated enhanced membrane-disruptive activity at pH 7.4 in comparison with that of PAsp- g-Phe. Moreover, the pH-responsive behavior of the grafted polymers caused by the significantly intensified hydrophobicity could be modulated by the tethered amount of hydrophobic amino acids with phenyl groups. The prepared amphiphilic (PAsp- g-Phe)- g-Lys could facilitate entry of calcein into NIH/3T3 and HeLa cells at physiological pH values, possibly due to local chemical destabilization of cell membranes by the interaction between the polymer and membrane bilayers. Therefore, we have provided a feasible approach to prepare pH-responsive polymers with enhanced membrane-disruptive activity, and the phenylalanine- and lysine-grafted polymers could be a potential candidate for intracellular delivery of bioactive molecules in biomedical applications.

New self-assembling peptide nanotubes of large diameter using δ-amino acids

Here we show that 4-aminocyclohexanecarboxylic acid is a rigid stretcher building block for the preparation of cyclic peptides that self-assemble to form peptide nanotubes with large diameter and hydrophobic pores. The hydrophobic properties of the resulting nanotubes provided by the two methylene groups per δ-residue allow the encapsulation of C60 moieties forming a new type of bionanopeapod structure.

Applications of cyclic peptide nanotubes (cPNTs)

Self-assembled cyclic peptide nanotubes (cPNTs) have recently drawn particular attention as one of the most intriguing nanostructures in the field of nanotechnology. Given their unique features including high surface area, increased drug loading, environmental stability, enhanced permeation, and modifiable drug release, these hollow tubular structures can be constructed with cyclic di-, tri-, tetra-, hexa-, octa-, and decapeptides with various amino acid sequences, enantiomers, and functionalized side chains and can be applied for antiviral and antibacterial drugs, drug delivery and gene delivery vectors, organic electronic devices, and ionic or molecular channels. Recent publications have presented promising results regarding the use of cPNTs as drugs or biomedical devices. However, there is an urgent need for the further in vivo nanotoxicity and safety testing of these nanotubes to evaluate their suitability in different fields.

Designed construction of tween 60@2β-CD self-assembly vesicles as drug delivery carrier for cancer chemotherapy

We report a simple strategy to prepare Tween 60@2β-CD self-assembly vesicles in aqueous solution as a new drug delivery carrier for cancer chemotherapy. The spherical shape of vesicles was confirmed by transmission electron microscopy (TEM) and mean particle sizes were about 33.7 nm, as measured by dynamic light scattering, micro-IR results indicated that the self-assembly vesicles was driven by hydrogen bonding. Hydrophilic doxorubicin (DOX) was successfully loaded into the self-assembly vesicles with drug loading content of 7.85% and loading efficiency of 42%. In addition, an in vitro cytotoxicity study and cellular uptake assays demonstrated that the DOX-loaded Tween 60@2β-CD vesicles markedly enhanced the cellular uptake and cytotoxicity of DOX toward the Hela cells. Furthermore, when used to evaluate the in vivo therapeutic efficacy in mice bearing the breast cell line (4T1), DOX-loaded vesicles exhibited superior inhibition of tumor growth compared with the DOX solutions.

Supramolecular functional assemblies: dynamic membrane transporters and peptide nanotubular composites

The fabrication of functional molecular devices constitutes one of the most important current challenges for chemical sciences. The complex processes accomplished by living systems continuously demand the assistance of non-covalent interactions between molecular building blocks. Additionally, these building blocks (proteins, membranes, nucleotides) are also constituted by self-assembled structures. Therefore, supramolecular chemistry is the discipline required to understand the properties of the minimal self-assembled building blocks of living systems and to develop new functional smart materials. In the first part of this feature article, we highlight selected examples of the preparation of supramolecular membrane transporters with special emphasis on the application of dynamic covalent bonds. In the second section of the paper we review recent breakthroughs in the preparation of peptide nanotube hybrids with functional applications. The development of these devices constitutes an exciting process from where we can learn how to understand and manipulate supramolecular functional assemblies.

Designing phenylalanine-based hybrid biological materials: controlling morphology via molecular composition

Harnessing the self-assembly of peptide sequences has demonstrated great promise in the domain of creating high precision shape-tunable biomaterials. The unique properties of peptides allow for a building block approach to material design. In this study, self-assembly of mixed systems encompassing two peptide sequences with identical hydrophobic regions and distinct polar segments is investigated. The two peptide sequences are diphenylalanine and phenylalanine-asparagine-phenylalanine. The study examines the impact of molecular composition (namely, the total peptide concentration and the relative tripeptide concentration) on the morphology of the self-assembled hybrid biological material. We report a rich polymorphism in the assemblies of these peptides and explain the relationship between the peptide sequence, concentration and the morphology of the supramolecular assembly.

Cyclic Peptide-Polymer Nanotubes as Efficient and Highly Potent Drug Delivery Systems for Organometallic Anticancer Complexes

Functional drug carrier systems have potential for increasing solubility and potency of drugs while reducing side effects. Complex polymeric materials, particularly anisotropic structures, are especially attractive due to their long circulation times. Here, we have conjugated cyclic peptides to the biocompatible polymer poly(2-hydroxypropyl methacrylamide) (pHPMA). The resulting conjugates were functionalized with organoiridium anticancer complexes. Small angle neutron scattering and static light scattering confirmed their self-assembly and elongated cylindrical shape. Drug-loaded nanotubes exhibited more potent antiproliferative activity toward human cancer cells than either free drug or the drug-loaded polymers, while the nanotubes themselves were nontoxic. Cellular accumulation studies revealed that the increased potency of the conjugate appears to be related to a more efficient mode of action rather than a higher cellular accumulation of iridium.

One-dimensional poly(L-lysine)-block-poly(L-threonine) assemblies exhibit potent anticancer activity by enhancing membranolysis

Herein, we report the oncolytic activity of cationic, one-dimensional (1D) fibril assemblies formed from coil-sheet poly(L-lysine)-block-poly(L-threonine) (PLL-b-PLT) block copolypeptides for cancer therapy. The 1D fibril assemblies can efficiently interact with negatively charged cellular and mitochondrial membranes via electrostatic interactions, leading to necrosis via membrane lysis and apoptosis via the mitochondria-lytic effect. The concept is analogous to that of 1D drug carriers that exhibit enhanced cell penetration. In comparison to free PLL chains, PLL-b-PLT fibril assemblies exhibit selective cytotoxicity toward cancer cells, low hemolysis activity, enhanced membranolytic activity, and a different apoptosis pathway, which may be due to differences in the peptide-membrane interactions. Antitumor studies using a metastatic LL2 lung carcinoma model indicate that the fibril assemblies significantly inhibited tumor growth, improved survival in tumor-bearing mice and suppressed lung metastasis without obvious body weight loss. An additive efficacy was also observed for treatment with both PLL-b-PLT and cisplatin. These results support the feasibility of using 1D fibril assemblies as potential apoptotic anticancer therapeutics.

STATEMENT OF SIGNIFICANCE

We report that cationic, one-dimensional (1D) fibril assemblies formed by coil-sheet poly(L-lysine)-block-poly(L-threonine) (PLL-b-PLT) block copolypeptides exhibited potent anticancer activity by enhancing membranolysis. The 1D fibril assemblies can efficiently interact with negatively charged cellular and mitochondrial membranes via electrostatic interactions, leading to necrosis via membrane lysis and apoptosis via mitochondria-lytic effect. Moreover, the fibril assemblies exhibited low hemolytic activity and selective cytotoxicity toward cancer cell, which is advantageous as compared to PLL and most antimicrobial/anticancerous peptides. This study provides a new concept of using cationic, 1D fibril assemblies for cancer therapy.

Self-assembling peptide-based building blocks in medical applications

Peptides and peptide-conjugates, comprising natural and synthetic building blocks, are an increasingly popular class of biomaterials. Self-assembled nanostructures based on peptides and peptide-conjugates offer advantages such as precise selectivity and multifunctionality that can address challenges and limitations in the clinic. In this review article, we discuss recent developments in the design and self-assembly of various nanomaterials based on peptides and peptide-conjugates for medical applications, and categorize them into two themes based on the driving forces of molecular self-assembly. First, we present the self-assembled nanostructures driven by the supramolecular interactions between the peptides, with or without the presence of conjugates. The studies where nanoassembly is driven by the interactions between the conjugates of peptide-conjugates are then presented. Particular emphasis is given to in vivo studies focusing on therapeutics, diagnostics, immune modulation and regenerative medicine. Finally, challenges and future perspectives are presented.

Recent advances in controlling the internal and external properties of self-assembling cyclic peptide nanotubes and dimers

Tuning the internal and external properties of self-assembling cyclic peptide nanotubes.