Self-Assembly of a Designed Alternating Arginine/Phenylalanine Oligopeptide

Reexamining the diverse functions of arginine in biochemistry

Arginine in a free-state and as part of peptides and proteins shows distinct tendency to form clusters. In free-form, it has been found useful in cryoprotection, as a drug excipient for both solid and liquid formulations, as an aggregation suppressor, and an eluent in protein chromatography. In many cases, the mechanisms by which arginine acts in all these applications is either debatable or at least continues to attract interest. It is quite possible that arginine clusters may be involved in many such applications. Furthermore, it is possible that such clusters are likely to behave as intrinsically disordered polypeptides. These considerations may help in understanding the roles of arginine in diverse applications and may even lead to better strategies for using arginine in different situations.

Antibacterial and Cytocompatible pH-Responsive Peptide Hydrogel

A short peptide, FHHF-11, was designed to change stiffness as a function of pH due to changing degree of protonation of histidines. As pH changes in the physiologically relevant range, G' was measured at 0 Pa (pH 6) and 50,000 Pa (pH 8). This peptide-based hydrogel is antimicrobial and cytocompatible with skin cells (fibroblasts). It was demonstrated that the incorporation of unnatural AzAla tryptophan analog residue improves the antimicrobial properties of the hydrogel. The material developed can have a practical application and be a paradigm shift in the approach to wound treatment, and it will improve healing outcomes for millions of patients each year.

Looking into a crystal ball: printing and patterning self-assembled peptide nanostructures

The solution processability of organic semiconductors and conjugated polymers along with the advent of nanomaterials as conducting inks have revolutionized next-generation flexible consumer electronics. Another equally important class of nanomaterials, self-assembled peptides, heralded as next-generation materials for bioelectronics, have a lot of potential in printed technology. In this minireview, we address the self-assembly process in dipeptides, their application in electronics, and recent progress in three-dimensional printing. The prospect of a generalizable path for nanopatterning self-assembled peptides using ice lithography and its challenges are further discussed.

Self-assembly pathways in a triphenylalanine peptide capped with aromatic groups

Peptide derivatives and, most specifically, their self-assembled supramolecular structures are being considered in the design of novel biofunctional materials. Although the self-assembly of triphenylalanine homopeptides has been found to be more versatile than that of homopeptides containing an even number of residues (i.e. diphenylalanine and tetraphenylalanine), only uncapped triphenylalanine (FFF) and a highly aromatic analog blocked at both the N- and C-termini with fluorenyl-containing groups (Fmoc-FFF-OFm), have been deeply studied before. In this work, we have examined the self-assembly of a triphenylalanine derivative bearing 9-fluorenylmethyloxycarbonyl and benzyl ester end-capping groups at the N- and C-termini, respectively (Fmoc-FFF-OBzl). The antiparallel arrangement clearly dominates in β-sheets formed by Fmoc-FFF-OBzl, whereas the parallel and antiparallel dispositions are almost isoenergetic in Fmoc-FFF-OFm β-sheets and the parallel one is slightly favored for FFF. The effects of both the peptide concentration and the medium on the self-assembly process have been examined considering Fmoc-FFF-OBzl solutions in a wide variety of solvent:co-solvent mixtures. In addition, Fmoc-FFF-OBzl supramolecular structures have been compared to those obtained for FFF and Fmoc-FFF-OFm under identical experimental conditions. The strength of π-π stacking interactions involving the end-capping groups plays a crucial role in the nucleation and growth of supramolecular structures, which determines the resulting morphology. Finally, the influence of a non-invasive external stimulus, ultrasounds, on the nucleation and growth of supramolecular structures has been examined. Overall, FFF-based peptides provide a wide range of supramolecular structures that can be of interest in the biotechnological field.

Designer peptides as versatile building blocks for functional materials

Peptides and pseudopeptides show distinct self-assembled nanostructures such as fibers, nanotubes, vesicles, micelles, toroids, helices and rods. The formation of such molecular communities through the collective behavior of molecules is not fully understood at a molecular level. All these self-assembled nanostructured materials have a wide range of applications such as drug delivery, gene delivery, biosensing, bioimaging, catalysis, tissue engineering, nano-electronics and sensing. Self-assembly is one of the most efficient and a simple strategy to generate complex functional materials. Owing to its significance, the last few decades witnessed a remarkable advancement in the field of self-assembling peptides with a plethora of new designer synthetic systems being discovered. These systems range from amphiphilic, cyclic, linear and polymeric peptides. This article presents only selected examples of such self-assembling peptides and pseudopeptides.

Nanostructure Formation and Cell Spheroid Morphogenesis of a Peptide Supramolecular Hydrogel

Peptide-based hydrogels have attracted much attention due to their extraordinary applications in biomedicine and offer an excellent mimic for the 3D microenvironment of the extracellular matrix. These hydrated matrices comprise fibrous networks held together by a delicate balance of intermolecular forces. Here, we investigate the hydrogelation behavior of a designed decapeptide containing a tetraleucine self-assembling backbone and fibronectin-related tripeptides near both ends of the strand. We have observed that this synthetic peptide can produce hydrogel matrices entrapping >99% wt/vol % water. Ultrastructural analyses combining atomic force microscopy, small-angle neutron scattering, and X-ray diffraction revealed that amyloid-like fibrils form cross-linked networks endowed with remarkable thermal stability, the structure of which is not disrupted up to temperatures >80 °C. We also examined the interaction of peptide hydrogels with either NIH3T3 mouse fibroblasts or HeLa cells and discovered that the matrices sustain cell viability and induce morphogenesis into grape-like cell spheroids. The results presented here show that this decapeptide is a remarkable building block to prepare highly stable scaffolds simultaneously endowed with high water retention capacity and the ability to instruct cell growth into tumor-like spheroids even in noncarcinoma lineages.

Amyloids as Building Blocks for Macroscopic Functional Materials: Designs, Applications and Challenges

Amyloids are self-assembled protein aggregates that take cross-β fibrillar morphology. Although some amyloid proteins are best known for their association with Alzheimer’s and Parkinson’s disease, many other amyloids are found across diverse organisms, from bacteria to humans, and they play vital functional roles. The rigidity, chemical stability, high aspect ratio, and sequence programmability of amyloid fibrils have made them attractive candidates for functional materials with applications in environmental sciences, material engineering, and translational medicines. This review focuses on recent advances in fabricating various types of macroscopic functional amyloid materials. We discuss different design strategies for the fabrication of amyloid hydrogels, high-strength materials, composite materials, responsive materials, extracellular matrix mimics, conductive materials, and catalytic materials.

Sound methods for the synthesis of nanoparticles from biological molecules

The development of simple, green, reproducible, and scalable approaches for synthesizing nanoparticles from biomolecules is important to advance nanomaterials towards therapeutic applications. Microreactors generated by high frequency ultrasound provide a one pot-platform to alter the physiochemical properties and stability of various types of biomolecules to ultimately generate multifunctional nanoparticles with controlled size and morphology. Herein, recent advancements in the field of nanoparticles fabrication from amino acids, phenolics, peptides and proteins using both high and low frequency ultrasound are reviewed. In particular, the sound driven self-assembly of biomolecules into nanoparticles by using high frequency ultrasound, as an emerging and innovative approach, is discussed in detail.

Amyloid-like Self-Assembly of a Hydrophobic Cell-Penetrating Peptide and Its Use as a Carrier for Nucleic Acids

Cell-penetrating peptides (CPPs) are a topical subject potentially exploitable for creating nanotherapeutics for the delivery of bioactive loads. These compounds are often classified into three major categories according to their physicochemical characteristics: cationic, amphiphilic, and hydrophobic. Among them, the group of hydrophobic CPPs has received increasing attention in recent years due to toxicity concerns posed by highly cationic CPPs. The hexapeptide PFVYLI (P, proline; F, phenylalanine; V, valine; Y, tyrosine; L, leucine; and I, isoleucine), a fragment derived from the C-terminal portion of α1-antitrypsin, is a prototypal example of hydrophobic CPP. This sequence shows reduced cytotoxicity and a capacity of nuclear localization, and its small size readily hints at its suitability as a building block to construct nanostructured materials. In this study, we examine the self-assembling properties of PFVYLI and investigate its ability to form noncovalent complexes with nucleic acids. By using a combination of biophysical tools including synchrotron small-angle X-ray scattering and atomic force microscopy-based infrared spectroscopy, we discovered that this CPP self-assembles into discrete nanofibrils with remarkable amyloidogenic features. Over the course of days, these fibrils coalesce into rodlike crystals that easily reach the micrometer range. Despite lacking cationic residues in the composition, PFVYLI forms noncovalent complexes with nucleic acids that retain β-sheet pairing found in amyloid aggregates. In vitro vectorization experiments performed with double-stranded DNA fragments indicate that complexes promote the internalization of nucleic acids, revealing that tropism toward cell membranes is preserved upon complexation. On the other hand, transfection assays with splice-correction oligonucleotides (SCOs) for luciferase expression show limited bioactivity across a narrow concentration window, suggesting that the propensity to form amyloidogenic aggregates may trigger endosomal entrapment. We anticipate that the findings presented here open perspectives for using this archetypical hydrophobic CPP in the fabrication of nanostructured scaffolds, which potentially integrate properties of amyloids and translocation capabilities of CPPs.

Self-Assembly of Angiotensin-Converting Enzyme Inhibitors Captopril and Lisinopril and Their Crystal Structures

The peptide angiotensin-converting enzyme inhibitors captopril and lisinopril are unexpectedly shown to exhibit critical aggregation concentration (CAC) behavior through measurements of surface tension, electrical conductivity, and dye probe fluorescence. These three measurements provide similar values for the CAC, and there is also evidence from circular dichroism spectroscopy for a possible conformational change in the peptides at the same concentration. Cryogenic transmission electron microscopy indicates the formation of micelle-like aggregates above the CAC, which can thus be considered a critical micelle concentration, and the formation of aggregates with a hydrodynamic radius of ∼6-7 nm is also evidenced by dynamic light scattering. We also used synchrotron radiation X-ray diffraction to determine the single-crystal structure of captopril and lisinopril. Our results improve the accuracy of previous data reported in the literature, obtained using conventional X-ray sources. We also studied the structure of aqueous solutions containing captopril or lisinopril at high concentrations. The aggregation may be driven by intermolecular interactions between the proline moiety of captopril molecules or between the phenylalanine moiety of lisinopril molecules.

Amphipathic design dictates self-assembly, cytotoxicity and cell uptake of arginine-rich surfactant-like peptides

Amphiphilicity is the most critical parameter in the self-assembly of surfactant-like peptides (SLPs), regulating the way by which hydrophobic attraction holds peptides together. Its effects go beyond supramolecular assembly and may also trigger different cell responses of bioactive peptide-based nanostructures. Herein, we investigate the self-assembly and cellular effects of nanostructures based on isomeric SLPs composed by arginine (R) and phenylalanine (F). Two amphipathic designs were studied: a diblock construct F4R4 and its bolaamphiphile analog R2F4R2. A strong sequence-dependent polymorphism emerges with appearance of globules and vesicle-like assemblies, or flat nanotapes and cylindrical micelles. The diblock construct possesses good cell penetrating capabilities and effectiveness to kill SK-MEL-28 melanoma tumor cells, in contrast to reduced intracellular uptake and low cytotoxicity exhibited by the bolaamphiphilic form. Our findings demonstrate that amphipathic design is a relevant variable for self-assembling SLPs to modulate different cellular responses and may assist in optimizing the production of nanostructures based on arginine-enriched sequences in cell penetrating and antimicrobial peptides.

Amyloid Formation by Short Peptides in the Presence of Dipalmitoylphosphatidylcholine Membranes

The aggregation of two short peptides, [RF] and [RF]₄ (where R = arginine and F = phenylalanine), at dipalmitoylphosphatidylcholine (DPPC) model membranes was investigated at the air-water interface using the Langmuir technique and vesicles in aqueous solutions. The molar ratio of the peptide and lipid components was varied to provide insights into the peptide-membrane interactions, which might be related to their cytotoxicity. Both peptides exhibited affinity to the DPPC membrane interface and rapidly adopted β-sheet-rich structures upon adsorption onto the surface of the zwitterionic membrane. Results from adsorption isotherm and small-angle X-ray scattering experiments showed changes in the structural and thermodynamic parameters of the membrane with increasing peptide concentration. Using atomic force microscopy, we showed the appearance of pores through the bilayer membranes and peptide aggregation at different interfaces, suggesting that the hydrophobic residues might have an effect on both pore size and layer structure, facilitating the membrane disruption and leading to different cytotoxicity effects.

Sono-Assembly of the [Arg-Phe]4 Octapeptide into Biofunctional Nanoparticles

High-frequency ultrasound treatment is found to be a one-pot green technique to produce peptide-based nanostructures by ultrasound assisted self-assembly of oligopeptides. [Arg-Phe]₄ octapeptides, consisting of alternating arginine (Arg/R) and phenylalanine (Phe/F) sequences, were subjected to 430 kHz ultrasound in aqueous solution in the absence of any external agents, to form [RF]₄ nanoparticles ([RF]₄-NPs), ~220 nm in diameter. A comprehensive analysis of the obtained nanoparticles demonstrated that the aromatic moieties of the oligopeptides can undergo oxidative coupling to form multiple oligomeric species, which then self-assemble into well-defined fluorescent nanoparticles. [RF]₄-NPs were functionalized with polyethylene glycol (PEGylated) to improve their colloidal stability. Unlike the parent peptide, the PEGylated [RF]₄-NPs showed limited cytotoxicity towards MDA-MB-231 cells. Furthermore, the intracellular trafficking of PEGylated [RF]₄-NPs was investigated after incubation with MDA-MB-231 cells to demonstrate their efficient endo-lysosomal escape. This work highlights that the combined use of ultrasonic technologies and peptides enables easy fabrication of nanoparticles, with potential application in drug delivery.

Role of Sheet-Edge Interactions in β-sheet Self-Assembling Peptide Hydrogels

Hydrogels' hydrated fibrillar nature makes them the material of choice for the design and engineering of 3D scaffolds for cell culture, tissue engineering, and drug-delivery applications. One particular class of hydrogels which has been the focus of significant research is self-assembling peptide hydrogels. In the present work, we were interested in exploring how fiber-fiber edge interactions affect the self-assembly and gelation properties of amphipathic peptides. For this purpose, we investigated two β-sheet-forming peptides, FEFKFEFK (F8) and KFEFKFEFKK (KF8K), the latter one having the fiber edges covered by lysine residues. Our results showed that the addition of the two lysine residues did not affect the ability of the peptides to form β-sheet-rich fibers, provided that the overall charge carried by the two peptides was kept constant. However, it did significantly reduce edge-driven hydrophobic fiber-fiber associative interactions, resulting in reduced tendency for KF8K fibers to associate/aggregate laterally and form large fiber bundles and consequently network cross-links. This effect resulted in the formation of hydrogels with lower moduli but faster dynamics. As a result, KF8K fibers could be aligned only under high shear and at high concentration while F8 hydrogel fibers were found to align readily at low shear and low concentration. In addition, F8 hydrogels were found to fragment at high concentration because of the high aggregation state stabilizing the fiber bundles, resulting in fiber breakage rather than disentanglement and alignment.

Self-assembly and intracellular delivery of DNA by a truncated fragment derived from the Trojan peptide Penetratin

Penetratin is a short Trojan peptide that attracts great interest in biomedical research for its capacity to translocate biological membranes. Herein, we study in detail both self-assembly and intracellular delivery of DNA by the heptamer KIWFQNR, a truncated peptide derived from Penetratin. This shortened sequence possesses a unique design with bolaamphiphilic characteristics that preserves the longest noncationic amino acid portion found in Penetratin. These features convey amphipathicity to assist self-assembly and make it a suitable model for exploring the role of hydrophobic residues for peptide interaction and cell uptake. We show that the fragment forms peptiplexes (i.e., peptide-DNA complexes), and aggregates into long nanofibers with clear β-sheet signature. The supramolecular structure of nanofibers is likely composed of DNA cores surrounded by a peptide shell to which the double helix behaves as a template and induces fibrillization. A nucleation and growth mechanism proceeding through liquid-liquid phase separation of coacervates is proposed for describing the self-assembly of peptiplexes. We also demonstrate that peptiplexes deliver double-stranded 200 bp DNA into HeLa cells, indicating its potential for preparing non-viral vectors for oligonucleotides through noncovalent strategies. Since the main structural features of native Penetratin are conserved in this simpler fragment, our findings also highlight the role of uncharged amino acids for structuration, and thus for the ability of Penetratin to cross cell membranes.

Designer Self-Assembling Peptide Hydrogels to Engineer 3D Cell Microenvironments for Cell Constructs Formation and Precise Oncology Remodeling in Ovarian Cancer

Designer self-assembling peptides form the entangled nanofiber networks in hydrogels by ionic-complementary self-assembly. This type of hydrogel has realistic biological and physiochemical properties to serve as biomimetic extracellular matrix (ECM) for biomedical applications. The advantages and benefits are distinct from natural hydrogels and other synthetic or semisynthetic hydrogels. Designer peptides provide diverse alternatives of main building blocks to form various functional nanostructures. The entangled nanofiber networks permit essential compositional complexity and heterogeneity of engineering cell microenvironments in comparison with other hydrogels, which may reconstruct the tumor microenvironments (TMEs) in 3D cell cultures and tissue-specific modeling in vitro. Either ovarian cancer progression or recurrence and relapse are involved in the multifaceted TMEs in addition to mesothelial cells, fibroblasts, endothelial cells, pericytes, immune cells, adipocytes, and the ECM. Based on the progress in common hydrogel products, this work focuses on the diverse designer self-assembling peptide hydrogels for instructive cell constructs in tissue-specific modeling and the precise oncology remodeling for ovarian cancer, which are issued by several research aspects in a 3D context. The advantages and significance of designer peptide hydrogels are discussed, and some common approaches and coming challenges are also addressed in current complex tumor diseases.

Amyloid Peptide Mixtures: Self-Assembly, Hydrogelation, Nematic Ordering, and Catalysts in Aldol Reactions

Morphological, spectroscopic, and scattering studies of the self-assembly and aggregation of mixtures of [RF]4 and P[RF]4 peptides (where R = arginine; F = phenylalanine; P = proline), in solution and as hydrogels, were performed to obtain information about polymorphism. CD data confirmed a β-sheet secondary structure in aqueous solution, and TEM images revealed nanofibers with diameters of ∼10 nm and micrometer lengths. SAXS curves were fitted using a mass fractal-component and a long cylinder shell form factor for the liquid samples, and only a long cylinder shell form factor for the gels. Increasing the P[RF]4 content in the systems leads to a reduction in cylinder radius and core scattering density, suggesting an increase in packing of the peptide molecules; however, the opposite effect is observed for the gels, where the scattering density is higher in the shell for the systems containing higher P[RF]4 content. These compounds show potential as catalysts in the asymmetric aldol reactions, with cyclohexanone and p-nitrobenzaldehyde in aqueous media. A moderate conversion (36.9%) and a good stereoselectivity (69:31) were observed for the system containing only [RF]4. With increasing P[RF]4 content, a considerable decrease of the conversion was observed, suggesting differences in the self-assembly and packing factor. Rheological measurements were performed to determine the shear moduli for the soft gels.

Selective Antibacterial Activity and Lipid Membrane Interactions of Arginine-Rich Amphiphilic Peptides

The self-assembly behavior and antimicrobial activity of two designed amphiphilic peptides, R3F3 and R4F4, containing short hydrophobic phenylalanine (F) and cationic arginine (R) sequences, are investigated. The conformation of the peptides was examined using circular dichroism and FTIR spectroscopy, which show that they have a disordered secondary structure. Concentration-dependent fluorescence assays show the presence of a critical aggregation concentration (cac) for each peptide. Above the cac, small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) reveal a population of twisted tapes for R3F3 and nanosheets for R4F4. The interaction of the peptides with model bacterial membranes comprising mixtures of the lipids DPPG [1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol] and DPPE [1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine], was studied using SAXS and cryogenic-TEM. Analysis of the SAXS structure factor indicates that R3F3 interacts with lipid bilayers by inducing correlation between bilayers, whereas R4F4 interacts with the bilayers causing an increase in polydispersity of the vesicle wall thickness. Both peptides break vesicles with a 1:3 DPPG:DPPE composition, which is close to the ratio of PG and PE lipids observed in the lipid membrane of Pseudomonas aeruginosa, a pathogen responsible for serious infections and which has developed antimicrobial resistant strains. Both peptides show activity against this bacterium in planktonic form. Peptide R4F4 shows particularly strong bioactivity against this microbe, with a minimum inhibitory concentration (MIC) value in the range of concentrations where the peptide is cytocompatible. It was further shown to have activity against other Pseudomonas species including the common plant pathogen Pseudomonas syringae. Finally, we show that R4F4 inhibits the development of P. aeruginosa biofilms. This was examined in detail and a proposed mechanism involving binding of the signaling molecule c-di-GMP is suggested, based on circular dichroism spectroscopy studies and Congo red assays of extracellular polysaccharides produced by the stressed bacteria. Thus, R4F4 is a promising candidate antimicrobial peptide with activity against Pseudomonas species.

Self-assembled gold nanoparticles and amphiphile peptides: a colorimetric probe for copper(ii) ion detection

We show that arginine/phenylalanine based peptides can be used to control the aggregation of gold nanoparticles in different ways. The arrangement provides a colorimetric approach to detect Cu²⁺ ions in water.

pH-Responsive Biocompatible Supramolecular Peptide Hydrogel

Peptide-based hydrogels are highly promising for various biomedical applications owing to their precise self-assembly, biocompatibility, and sensitivity toward biologically relevant external stimuli. Herein, we report pH-responsive self-assembly and gelation of a highly biocompatible amphiphilic peptide PEP-1. This is an octa-peptide and double mutant of a naturally occurring β-strand peptide fragment of the protein Galectin-1, available in bovine spleen. PEP-1 was synthesized by using the Rink amide resin as the solid support in a homemade apparatus. At pH 7.4, it exhibits spontaneous gelation with very high yield stress of 88.0 Pa and gel-to-sol temperature of 84 °C at C = 2.0 wt %. Microscopy studies revealed entangled fibrillar morphology whereas circular dichroism, Fourier tranform IR, and Thioflavin T assay indicated formation of β-sheet rich secondary structure. The assembled state was found to be stable in neutral pH whereas either decrease or increase in the pH resulted in disassembly owing to the presence of the pH responsive Asp and Lys residues. The gel network showed ability to entrap water-soluble guest molecules such as Calcein which could be selectively released at acidic pH whereas under neutral condition the release was negligible. MTT assay revealed remarkable biocompatibility of the PEP-1 gel as almost 100% cells were alive after 48 h incubation in the presence of PEP-1 (2.0 mg/mL).

Self-assembly of l-phenylalanine amino acid: electrostatic induced hindrance of fibril formation

Nanostructure morphology originating from the self-assembly of molecules has attracted substantial attention due to its role in toxic amyloid fibril formation and immense potential in the design and fabrication of novel biomaterials. This study presents the role of intermolecular electrostatic interaction on the self-assembly process of l-phenylalanine (L-Phe) amino acid. We have employed attenuated total reflection Fourier transform infrared spectroscopy to probe the existence of different ionization states of the amino acid in various pH aqueous solutions. The self-assembly process of L-Phe in the aqueous phase is explored by using circular dichroism absorption and nuclear magnetic resonance spectroscopic tools. The observed spectral features have shown the signature of higher order structures and possible perturbation in the π-π stacking aromatic interactions for the cationic and anionic states of the amino acid. Scanning electron microscopy is used to probe the self-assembled morphology of the L-Phe amino acid dried samples prepared from the same pH aqueous solutions. We find that for the case of zwitterionic states the self-assembly nanostructures are dominated by the presence of fibrillar morphology, however interestingly for cationic and anionic states the morphology is dominated by the presence of flakes. Our finding demonstrates the potential influence of intermolecular electrostatic interaction over the aromatic π-π stacking interaction in hindering the fibril formation.

Amyloidogenic model peptides as catalysts for stereoselective aldol reactions

Different polymorphic forms of peptide assemblies influence the stereoselectivity of aldol reactions in water medium.

Microbial nanowires - Electron transport and the role of synthetic analogues

Electron transfer is central to cellular life, from photosynthesis to respiration. In the case of anaerobic respiration, some microbes have extracellular appendages that can be utilised to transport electrons over great distances. Two model organisms heavily studied in this arena are Shewanella oneidensis and Geobacter sulfurreducens. There is some debate over how, in particular, the Geobacter sulfurreducens nanowires (formed from pilin nanofilaments) are capable of achieving the impressive feats of natural conductivity that they display. In this article, we outline the mechanisms of electron transfer through delocalised electron transport, quantum tunnelling, and hopping as they pertain to biomaterials. These are described along with existing examples of the different types of conductivity observed in natural systems such as DNA and proteins in order to provide context for understanding the complexities involved in studying the electron transport properties of these unique nanowires. We then introduce some synthetic analogues, made using peptides, which may assist in resolving this debate. Microbial nanowires and the synthetic analogues thereof are of particular interest, not just for biogeochemistry, but also for the exciting potential bioelectronic and clinical applications as covered in the final section of the review.

STATEMENT OF SIGNIFICANCE

Some microbes have extracellular appendages that transport electrons over vast distances in order to respire, such as the dissimilatory metal-reducing bacteria Geobacter sulfurreducens. There is significant debate over how G. sulfurreducens nanowires are capable of achieving the impressive feats of natural conductivity that they display: This mechanism is a fundamental scientific challenge, with important environmental and technological implications. Through outlining the techniques and outcomes of investigations into the mechanisms of such protein-based nanofibrils, we provide a platform for the general study of the electronic properties of biomaterials. The implications are broad-reaching, with fundamental investigations into electron transfer processes in natural and biomimetic materials underway. From these studies, applications in the medical, energy, and IT industries can be developed utilising bioelectronics.

Chiral organocatalysts based on lipopeptide micelles for aldol reactions in water

A comprehensive study of the self-assembly in water of a lipopeptide consisting of a sequence of l-proline, l-arginine and l-tryptophan with a hydrocarbon chain has been performed. Fluorescence assays were used to determine the critical aggregation concentration. In situ small-angle X-ray scattering (SAXS) and molecular dynamics simulations showed the presence of spherical micelles with diameters around 6 nm. In agreement with these results, cryo-TEM images showed globular aggregates with diameters ranging from ≈4 nm up to ≈9 nm. Furthermore, the lipopeptide catalytic activity has been tested for the direct aldol reaction between cyclohexanone and p-nitrobenzaldehyde, and we have observed that the self-association of the organocatalyst played a critical role in the enhanced activity. Water affects the selectivity, and poor results are obtained under neat reaction conditions. The location of the catalytic groups at the lipopetide/water solvent interface also endowed unusual selectivity in the catalyzed aldol reactions. Under optimized reaction conditions, high yields (up to >99%), good enantioselectivity (ee up to 85%) and high diastereoselectivity (ds up to 92 : 8) were obtained.

Methods to Characterize the Nanostructure and Molecular Organization of Amphiphilic Peptide Assemblies

Methods to characterize the nanostructure and molecular organization of aggregates of peptides such as amyloid or amphiphilic peptide assemblies are reviewed. We discuss techniques to characterize conformation and secondary structure including circular and linear dichroism and FTIR and Raman spectroscopies, as well as fluorescence methods to detect aggregation. NMR spectroscopy methods, especially solid-state NMR measurements to probe beta-sheet packing motifs, are also briefly outlined. Also discussed are scattering methods including X-ray diffraction and small-angle scattering techniques including SAXS (small-angle X-ray scattering) and SANS (small-angle neutron scattering) and dynamic light scattering. Imaging methods are direct methods to uncover features of peptide nanostructures, and we provide a summary of electron microscopy and atomic force microscopy techniques. Selected examples are highlighted showing data obtained using these techniques, which provide a powerful suite of methods to probe ordering from the molecular scale to the aggregate superstructure.

Amphiphilic Peptide-Based Supramolecular, Noncytotoxic, Stimuli-Responsive Hydrogels with Antibacterial Activity

A series of peptides with a long fatty acyl chain covalently attached to the C-terminal part and a free amine (-NH₂) group at the N-terminus have been designed so that these molecules can be assembled in aqueous medium by using various noncovalent interactions. Five different peptide amphiphiles with a general chemical formula [H₂N-(CH₂)_nCONH-Phe-CONHC₁₂ (n = 1-5, C₁₂ = dodecylamine)] have been synthesized, characterized, and examined for self-assembly and hydrogelation. All of these molecules [P1 (n = 1), P2 (n = 2), P3 (n = 3), P4 (n = 4), P5 (n = 5)] form thermoresponsive hydrogels in water (pH 6.6) with a nanofibrillar network structure. Interestingly, the hydrogels obtained from compounds P4 and P5 exhibit potential antimicrobial activity against Gram-positive bacteria (Staphylococcus aureus, Bacillus subtilis) and Gram-negative bacteria (Escherichia coli). Dose-dependent cell-viability studies using MTT assay (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) by taking human lung carcinoma (A549) cells vividly demonstrates the noncytotoxic nature of these gelator molecules in vitro. Hemolytic studies show nonsignificant or little hemolysis of human erythrocyte cells at the minimum inhibitory concentration (MIC) of these tested bacteria. Interestingly, it has been found that these antibacterial noncytotoxic hydrogels exhibit proteolytic resistance toward the enzymes proteinase K and chymotrypsin. Moreover, the gel strength and gel recovery time have been successfully modulated by varying the alkyl chain length of the N-terminally located amino acid residues. Similarly, the thermal stability of these hydrogels has been nicely tuned by altering the alkyl chain length of the N-terminally located amino acid residues. In the era of antibiotic-resistant strains of bacteria, the discovery of this new class of peptide-based antibacterial, proteolytically stable, injectable, and noncytotoxic soft materials holds future promise for the development of new antibiotics.

Partially Acetylated or Benzoylated Arabinose Derivatives as Structurally Simple Organogelators: Effect of the Ester Protecting Group on Gel Properties

Sugar-based low-molecular-weight gelators (LMWGs) have been used for various applications for a long time. Herein, structurally simple, ester-protected arabinosides are reported as low-molecular-weight organogelators (LMOGs) that are able to gel aromatic solvents, as well as petrol and diesel. Studies on the mechanical strength of the gels, through detailed rheological experiments, indicate that gels from the 1,2-dibenzoylated arabinose gelator possess better mechanical properties than those from the 1,2-diacetylated gelator. These results are interpreted in terms of the tendency of the former to form fibers with comparatively lower diameter than those of the latter, based on detailed field-emission SEM and AFM studies. Investigations of the interactions responsible for the self-assembly of gelators through IR spectroscopy and wide-angle X-ray scattering reveal that the primary interactions responsible are hydrogen bonds between the hydroxyl groups and ester C=O, which is absent in the solid state of the gelators. In addition, π interactions present in the 1,2-dibenzoylated derivative result in a more regular arrangement, which, in turn, leads to better mechanical properties of the gels compared with those of the 1,2-diacetylated gelator.

Nano sand filter with functionalized nanoparticles embedded in anodic aluminum oxide templates

Since the ancient Egyptians had used sand as filter media for water purification, its principle has been inherited through generations and it is still being used now in industries. The sand filter consists of sand literally, and the voids within the sand bed are the pores for filtration. Here we present a filtration principle using nanoparticles, so that the voids between the nanoparticles can be considered as effective pores in nanoscale dimension. Anodic aluminum oxide (AAO) membrane has been used as the working template, and the nanoparticles have been injected and embedded within the pores of the AAO template. Nanoparticles with multiple sizes have been used in order to obtain smaller voids. Moreover, the nanoparticles have been functionalized, or electrically charged, with arginine/phenylalanine (RF) peptide group. In this way, filtration performance for charged particles or molecules, such as methylene blue, has been enhanced. Consequently, this study is expected to provide a new principle for fabrication of nano voids, or nano pores, and for filtration in nanoscale dimension.

Structural behaviour and gene delivery in complexes formed between DNA and arginine-containing peptide amphiphiles

We describe in depth the structure of complexes formed between DNA and two classes of arginine-containing peptide amphiphiles, namely, the lipopeptide PRW-C₁₆ (P = proline, R = arginine, W = tryptophan, C₁₆ = C16 : 0 alkyl chain) and the bolaamphiphile RFL₄FR (R = arginine, F = phenylalanine, L = leucine). A combination of X-ray and neutron scattering provided unprecedented insights into the local structure of these complexes. Lipopeptide-based complexes self-assembled into layered structures with large-scale fractal features, hosting DNA in the interstices. Bola-amphiphile scaffolds were characterized by planar structures with DNA strands presumably sandwiched in-between peptide nanotapes. Importantly, complexation did not affect the structural integrity of DNA in either of the two complexes. The bolaamphiphile conjugates displayed high levels of molecular ordering in contrast to the liquid-crystalline features observed in lipopeptide assemblies. Peptide-DNA complexes were assessed for their potential as a means to deliver the reporter vector pEGFP-N1 into SW480 human colon carcinoma cells. Successfully transfected cells expressed green fluorescent protein. The potentiating effect of PRW-C₁₆ on the cellular uptake of ectopic DNA was found to be much greater than that observed with RFL₄FR. In contrast to the bolaamphiphile-based conjugate, the liquid-crystalline nature of the lipopeptide complex is likely to play a key role in DNA release and transfection efficiency since these weakly bound structures require lower energy expenditure during disassembly and load release.

Peptide based hydrogels for cancer drug release: modulation of stiffness, drug release and proteolytic stability of hydrogels by incorporating d-amino acid residue(s)

Synthetic tripeptide based noncytotoxic hydrogelators have been discovered for releasing an anticancer drug at physiological pH and temparature. Interestingly, gel stiffness, drug release capacity and proteolytic stability of these hydrogels have been successfully modulated by incorporating d-amino acid residues, indicating their potential use for drug delivery in the future.

Metal induced self-assembly of designed V-shape protein into 2D wavy supramolecular nanostructure

In order to understand and imitate the more complex bio-processes and fascinating functions in nature, protein self-assembly has been studied and has attracted more and more interest in recent years. Artificial self-assemblies of proteins have been constructed through many strategies. However, the design of complicated protein self-assemblies utilizing the special profile of building blocks remains a challenge. We herein report linear and 2D nanostructures constructed from a V shape SMAC protein and induced by metal coordination. Zigzag nanowires and wavy 2D nanostructures have been demonstrated by AFM and TEM. The zigzag nanowires can translate to a 2D nanostructure with an excess of metal ions, which reveals the step by step assembly process. Fluorescence and UV/Vis spectra have also been obtained to further study the mechanism and process of self-assembly. Upon the protein nanostructure, fluorescence resonance energy transfer (FRET) could also be detected using fluorescein modified proteins as building blocks. This article provides an approach for designing and controlling self-assembled protein nanostructures with a distinctive topological morphology.

Investigating hydrogel formation using in situ variable-temperature scanning probe microscopy

The assembly and disassembly of fibres formed by a low molecular weight hydrogelator are imaged at high resolution.

The assembly and disassembly of supramolecular gel fibres are observed in situ using variable temperature scanning probe microscopy. The results show that fibre formation can be monitored at high resolution at a surface, and the final fibre morphologies are broadly consistent with those found by ex situ analysis of the bulk gel. The impact of a gelation inhibitor upon the fibre morphology is successfully investigated, providing direct evidence for the mechanism of inhibition as a function of additive concentration.

Programming Bioactive Architectures with Cyclic Peptide Amphiphiles

We present a versatile approach for the synthesis of cyclic peptide amphiphiles of the hormone somatostatin (SST) with tunable lipophilic tails to program bioactive nanoarchitectures. A novel bis-alkylation reagent is synthesized that facilitates the functionalization of SST with a thiol anchor. Different hydrophobic moieties are introduced inspired by a biomimetic palmitoylation approach which opens access to cyclic peptide amphiphiles that display rich self-organization and cell membrane interactions.

Self-assembly pathway of peptide nanotubes formed by a glutamatic acid-based bolaamphiphile

The self-assembly of peptide nanotubes formed by an l-glutamic acid-based bolaamphiphile is shown to proceed via a remarkable mechanism where the peptide conformation changes from β-sheet to unordered.