Structural Diversity in the Self-Assembly of Pseudopeptidic Macrocycles

Biodegradable Nanobowls with Controlled Dents

Nanobowls show promising potential in biomedical applications, such as bioimaging, cargo delivery, and disease theranostics, due to their unique concave structure and interior cavities. However, the lack of biodegradable nanobowls with manipulable size (especially the dent size) still exists as an obstacle for their in-depth exploration and application in biomedical fields. Herein, polypeptide-based nanobowls are successfully obtained by the self-assembly of a graft polypeptide [named TPE-P(GAAzo21-stat-GA29)] via a solvent-switch method. Through the synergistic effect between the hydrogen bonding and π-π stacking interactions, the size of nanobowls and the corresponding dents can be facilely controlled by altering either the initial polypeptide concentration or the cosolvents in self-assembly. Furthermore, such polypeptide-based nanobowls are demonstrated to be biocompatible and biodegradable in vitro, which may promote the development of biomedical nanobowls in the future.

L-Phenylalanine-derived pseudopeptidic bioinspired materials: Zn(II) induced fluorescence enhancement and precise tuning of self-assembled nanostructures

The pseudopeptide, owing to its intriguing, sustainable, and easily accessible multifunctional properties, has attracted significant research interest over the years. C2-symmetric pseudopeptidic chiral bioinspired materials have been developed for their selective sensitivity to Zn(II) ions via a turn-on fluorescence under physiological conditions. Moreover, these are promising soft materials for precisely tuning their self-assembled nanostructures after incubating with Zn(II), opening avenues for exploring similar effects in various peptidomimetics.

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.

Synthetic Receptors Based on Abiotic Cyclo(pseudo)peptides

Work on the use of cyclic peptides or pseudopeptides as synthetic receptors started even before the field of supramolecular chemistry was firmly established. Research initially focused on the development of synthetic ionophores and involved the use of macrocycles with a repeating sequence of subunits along the ring to facilitate the correlation between structure, conformation, and binding properties. Later, nonnatural amino acids as building blocks were also considered. With growing research in this area, cyclopeptides and related macrocycles developed into an important and structurally diverse receptor family. This review provides an overview of these developments, starting from the early years. The presented systems are classified according to characteristic structural elements present along the ring. Wherever possible, structural aspects are correlated with binding properties to illustrate how natural or nonnatural amino acids affect binding properties.

Tailoring of Peptide Vesicles: A Bottom-Up Chemical Approach

Spherical ordering from small molecules is a subject of intense interest to chemists. The inherent capability of amphiphiles to assemble spontaneously is the unique feature of the evolutionary process of life. Self-assembly is prevalent in biology and has attracted the interest of scientists across several disciplines. This is because scientists have realized that nature has extensively used this inherent organizational power contained in the molecules. Judicious use of the self-assembly principle is the cornerstone of nature's exotic assemblies. These exotic assemblies lead to unimaginable functions in biology that might not have been predicted from the monomer building blocks alone. Recently, a number of chemical systems that self-assemble in aqueous or organic solvents to form vesicles were reported. This account provides advances made from our laboratory toward designing and understanding the mechanism of formation of spherical vesicular assembly. A bottom-up approach for the de novo design of vesicles using nonlipidated molecular architecture will be a paradigm shift in vesicular research. Vesicles act as a protocell model for studying the origin and evolution of cellular life. They could also act as excellent model systems for studying the fusion of cells and membrane transport. Self-assembled vesicles have enormous potential for several applications such as drug and biomolecule delivery to cells and in materials science. These aspects along with the dynamic nature of vesicular assembly have attracted researchers to the study of spherical assemblies. The common belief was that the molecules that form vesicles must have one polar head and two hydrophobic tails. All attempts to synthesize vesicles are by mimicking nature's strategy, which mainly involves the self-assembly of lipid amphiphiles through a bilayer-like arrangement. Pseudopeptide-based molecules with the ability to form vesicles have changed this long-standing notion. In addition to chemical and medical applications, these peptide vesicles could act as models for protocells, membrane fusion, and the study of the vesiculation mechanism. This Account highlights the progress made toward a heuristic approach to the de novo design of vesicles using pseudopeptides as building blocks.A large number of diverse classes of pseudopeptides showed vesicular assembly. Various acyclic and cyclic molecules were designed and synthesized that showed spherical vesicular assembly. Cystine-based macrocyclic peptides showed drug encapsulation and release. Polymersomes with unusual topology, self-assembling tripodal ligands, and molecules containing amino acids such as lysine, leucine, cystine, and serine were synthesized. The incorporation of a wide variety of amino acids in the vesicle-forming peptides could enhance their scope and applications. The mechanism of vesiculation was also investigated using these designer molecules.

Improving and fine-tuning the properties of peptide-based hydrogels via incorporation of peptide nucleic acids

Peptide self-assemblies have attracted intense research interest over the last few decades thanks to their implications in key biological processes (e.g., amyloid formation) and their use in biotechnological and (bio)material fields. In particular, peptide-based hydrogels have been highly considered as high potential supramolecular materials in the biomedical domain and open new horizons in terms of applications. To further understand their self-assembly mechanisms and to optimize their properties, several strategies have been proposed with the modification of the constituting amino acid chains via, per se, the introduction of d-amino acids, halogenated amino acids, pseudopeptide bonds, or other chemical moieties. In this context, we report herein on the incorporation of DNA-nucleobases into their peptide nucleic acid (PNA) forms to develop a new series of hybrid nucleopeptides. Thus, depending on the nature of the nucleobase (i.e., thymine, cytosine, adenine or guanine), the physicochemical and mechanical properties of the resulting hydrogels can be significantly improved and fine-tuned with, for instance, drastic enhancements of both the gel stiffness (up to 70-fold) and the gel resistance to external stress (up to 40-fold), and the generation of both thermo-reversible and uncommon red-edge excitation shift (REES) properties. To decipher the actual role of each PNA moiety in the self-assembly processes, the induced modifications from the molecular to the macroscopic scales are studied thanks to the multiscale approach based on a large panel of analytical techniques (i.e., rheology, NMR relaxometry, TEM, thioflavin T assays, FTIR, CD, fluorescence, NMR chemical shift index). Thus, such a strategy provides new opportunities to adapt and fit hydrogel properties to the intended ones and pushes back the limits of supramolecular materials.

Nanobowls with controlled openings and interior holes driven by the synergy of hydrogen bonding and π-π interaction

Asymmetric nanoparticles such as nanobowls have promising potential in many fields due to their interior asymmetric cavities and specific concave structure. However, the fabrication of nanobowls and control over their openings and interior holes are still challenging. Herein we demonstrate a versatile strategy for preparing nanobowls with precisely controlled openings and interior holes based on the synergy of hydrogen bonding and π-π interaction of homopolymers. We designed and synthesized a series of amphiphilic homopolymers with an amino alcohol moiety and azobenzene pendant (poly(2-hydroxy-3-((4-(phenyldiazenyl)phenyl)amino)propyl methacrylate) (PHAzoMA)). The homopolymers can self-assemble into nanobowls due to the heterogeneous shrinkage of the preformed spheres. Upon increasing the molecular weight of the homopolymers from 10.1 to 76.9 kg mol-1, the sizes of the openings of nanobowls can be precisely controlled from 242 to 423 nm with a linear relationship as a result of the enhancement of the hydrogen bonding and π-π interaction between homopolymer chains. Overall, we have prepared finely controlled nanobowls by the synergy of non-covalent interactions such as hydrogen bonding and π-π interaction of polymers, which opens a new avenue for the preparation of asymmetric nanoparticles.

The role of the side chain in the conformational and self-assembly patterns of C2-symmetric Val and Phe pseudopeptidic derivatives

Side chain as the main conformational and self-assembly structural factor for C₂-pseudopeptides.

Solvent Mixing To Induce Molecular Motor Aggregation into Bowl-Shaped Particles: Underlying Mechanism, Particle Nature, and Application To Control Motor Behavior

Control over dynamic functions in larger assemblies is key to many molecular systems, ranging from responsive materials to molecular machines. Here we report a molecular motor that forms bowl-shaped particles in water and how confinement of the molecular motor affects rotary motion. Studying the aggregation process in a broader context, we provide evidence that, in the case of bowl-shaped particles, the structures are not the product of self-assembly, but a direct result of the mixing a good solvent and a (partial) non-solvent and highly independent of the molecular design. Under the influence of the non-solvent, droplets are formed, of which the exterior is hardened due to the increase in the glass transition temperature by the external medium, while the interior of the droplets remains plasticized by the solvent, resulting in the formation of stable bowl-shaped particles with a fluid interior, a glass-like exterior, and a very specific shape: dense spheres with a hole in their side. Applying this to a bulky first-generation molecular motor allowed us to change its isomerization behavior. Furthermore, the motor shows in situ photo-switchable aggregation-induced emission. Strong confinement prohibits the thermal helix inversion step while altering the energy barriers that determine the rotary motion, such that it introduces a reverse trans- cis isomerization upon heating. These studies show a remarkable control of forward and backward rotary motion by simply changing solvent ratios and extent of confinement.

Vesicles: self-assembly beyond biological lipids

This review presents an overview of synthetic systems that self-assemble to form vesicles.

Pseudopeptidic compounds for the generation of dynamic combinatorial libraries of chemically diverse macrocycles in aqueous media

The combination of pseudopeptidic dithiol building blocks leads to the generation of highly diverse dynamic libraries of macrocycles in aqueous media.

Spontaneous vesiculation: a mechanistic insight from the study of hybrid peptide molecules

We report a series of hybrid peptide molecules that display spontaneous vesiculation. A closer look at their vesicle formation revealed a toroidal intermediate en route to the final vesicular form.

From simplicity to complex systems with bioinspired pseudopeptides

This feature article highlights some of the recent advances in creating complexity from simple pseudopeptidic molecules. The bioinspired approaches discussed here allowed an increase in the structural, chemical and interactional complexity (see figure).

Morphology engineering: dramatic roles of serine and threonine in supramolecular assembly

Macrocycles containing serine self-assembled into fibres, while threonine induced vesicular self-assembly. Macrocycles with serine can be driven to form vesicular assembly by incorporating a non-planar spacer.

Highly stable oil-in-water emulsions with a gemini amphiphilic pseudopeptide

A gemini amphiphilic pseudopeptide promotes the spontaneous formation of an oil-in-water emulsion with a high thermal, mechanical and acid-medium stability.

Coordination behaviour of new open chain and macrocyclic peptidomimetic compounds with copper(ii)

Two valine-derived bis(amino amides) ligands have been prepared and fully characterized.

Bioinspired chemistry based on minimalistic pseudopeptides

For years researchers have tried to understand the molecular behavior of complex biomolecules through the development of small molecules that can partially mimic their function. Now researchers are implementing the reverse approach: using the structural and mechanistic knowledge obtained from those complex systems to design small molecules with defined properties and for specific applications. One successful strategy for constructing bioinspired, minimalistic molecules is to combine natural building blocks that provide functional elements with abiotic fragments that serve as structural scaffolds. Therefore pseudopeptidic compounds, most of them based on C2 symmetric structures, represent a unique opportunity to explore and evaluate this approach. Some of these molecules are as simple as two amino acids connected by a diamino spacer. The results in this Account show how bioinspired minimalistic pseudopeptides can form ordered structures, participate in the recognition and transcription of information events in molecular devices, and catalyze reactions. This strategy allows researchers to design and prepare a variety of open-chain and macrocyclic compounds leading to systems that can self-aggregate to form hierarchically ordered micro- and nanostructures. In addition, small changes in the molecule or external stimuli can regulate the self-aggregation pattern. In the same way, researchers can also tune the molecular movements of simple pseudopeptides through environmental factors, providing a means to control new molecular devices. In addition, some of the prepared model compounds have shown interesting properties in molecular recognition and even as sensors for several targets of interest. Finally we have observed remarkable catalytic activities from these types of molecules, although those results are still far from the efficiency shown by natural peptides. This family of pseudopeptidic compounds offers the opportunity for the more elaborate design of relatively simple abiotic but bioinspired systems that display specific properties. In addition, the results can provide additional information that will increase the molecular understanding of the basic principles that underlie the extraordinary behavior of natural systems.

Self-assembly of lipidated pseudopeptidic triazolophanes to vesicles

We have transformed the amino acid serine to 32-membered lipidated cyclophanes employing CuAAc reaction. These serine-based lipidated triazolophanes assemble to sturdy and robust vesicles.

Nucleation and growth of elastin-like peptide fibril multilayers: an in situ atomic force microscopy study

Controlling how molecules assemble into complex supramolecular architectures requires careful consideration of the subtle inter- and intra-molecular interactions that control their association. This is particularly crucial in the context of assembly at interfaces, where both surface chemistry and structure can play a role in directing structure formation. We report here the results of a study into the self-assembly of the elastin-like peptide EP I on structurally modified highly ordered pyrolytic graphite, including the role of spatial confinement on fibril nucleation and the growth of oriented fibril multilayers. In situ atomic force microscopy performed in fluid and at elevated temperature provided direct evidence of frustrated fibril nuclei and oriented growth of independent fibril domains. These results portend the application of this in situ strategy for studies of the nucleation and growth mechanisms of other fibril- and amyloid-forming proteins.