Programming pH-triggered self-assembly transitions via isomerization of peptide sequence

Influence of chirality and sequence in lysine-rich lipopeptide biosurfactants and micellar model colloid systems

Lipopeptides can self-assemble into diverse nanostructures which can be programmed to incorporate peptide sequences to achieve a remarkable range of bioactivities. Here, the influence of peptide sequence and chirality on micelle structure and interactions is investigated in a series of lipopeptides bearing two lysine or D-lysine residues and tyrosine or tryptophan residues, attached to a hexadecyl lipid chain. All molecules self-assemble into micelles above a critical micelle concentration (CMC). Small-angle x-ray scattering (SAXS) is used to probe micelle shape and structure from the form factor and to probe inter-micellar interactions via analysis of structure factor. The CMC is obtained consistently from surface tension and electrical conductivity measurements. We introduce a method to obtain the zeta potential from the SAXS structure factor which is in good agreement with directly measured values. Atomistic molecular dynamics simulations provide insights into molecular packing and conformation within the lipopeptide micelles which constitute model self-assembling colloidal systems and biomaterials.

Orientational phase transition in monolayers of multipolar straight rigid rods: The case of 2-thiophene molecule adsorption on the Au(111) surface

Monte Carlo simulations and finite-size scaling theory have been carried out to study the critical behavior and universality for the isotropic-nematic (IN) phase transition in a system of straight rigid pentamers adsorbed on a triangular lattice with polarized nonhomogeneous intermolecular interactions. The model was inspired by the deposition of 2-thiophene molecules over the Au(111) surface, which was previously characterized by experimental techniques and density functional theory. A nematic phase, observed experimentally by the formation of a self-assembled monolayer of parallel molecules, is separated from the isotropic state by a continuous transition occurring at a finite density. The precise determination of the critical exponents indicates that the transition belongs to the three-state Potts universality class. The finite-size scaling analysis includes the study of mutability and diversity. These two quantities are derived from information theory and they have not previously been considered as part of the conventional treatment of critical phenomena for the determination of critical exponents. The results obtained here contribute to the understanding of formation processes of self-assembled monolayers, phase transitions, and critical phenomena from novel compression algorithms for studying mutual information in sequences of data.

Modeling Interactions within and between Peptide Amphiphile Supramolecular Filaments

Many peptides are able to self-assemble into one-dimensional (1D) nanostructures, such as cylindrical fibers or ribbons of variable widths, but the relationship between the morphology of 1D objects and their molecular structure is not well understood. Here, we use coarse-grained molecular dynamics (CG-MD) simulations to study the nanostructures formed by self-assembly of different peptide amphiphiles (PAs). The results show that ribbons are hierarchical superstructures formed by laterally assembled cylindrical fibers. Simulations starting from bilayer structures demonstrate the formation of filaments, whereas other simulations starting from filaments indicate varying degrees of interaction among them depending on chemical structure. These interactions are verified by observations using atomic force microscopy of the various systems. The interfilament interactions are predicted to be strongest in supramolecular assemblies that display hydrophilic groups on their surfaces, while those with hydrophobic ones are predicted to interact more weakly as confirmed by viscosity measurements. The simulations also suggest that peptide amphiphiles with hydrophobic termini bend to reduce their interfacial energy with water, which may explain why these systems do not collapse into superstructures of bundled filaments. The simulations suggest that future experiments will need to address mechanistic questions about the self-assembly of these systems into hierarchical structures, namely, the preformation of interactive filaments vs equilibration of large assemblies into superstructures.

Enzyme-instructed hybrid nanogel/nanofiber oligopeptide hydrogel for localized protein delivery

Enzyme-catalysis self-assembled oligopeptide hydrogel holds great interest in drug delivery, which has merits of biocompatibility, biodegradability and mild gelation conditions. However, its application for protein delivery is greatly limited by inevitable degradation of enzyme on the encapsulated proteins leading to loss of protein activity. Moreover, for the intracellularly acted proteins, cell membrane as a primary barrier hinders the transmembrane delivery of proteins. The internalized proteins also suffer from acidic and enzymatic degradation in endosomes and lysosomes. We herein develop a protease-manipulated hybrid nanogel/nanofiber hydrogel for localized delivery of intracellularly acted proteins. The embedded polymeric nanogels (CytoC/aNGs) preserve activity of cytochrome c (CytoC) that is an intracellular activator for cell apoptosis as a model protein against proteolysis, and do not affect the gelation properties of the protease-catalysis assembled hydrogels. The injectable hydrogel (CytoC/aNGs/Gel) serves as a reservoir to enhance intratumoral retention and realize sustainable release of CytoC/aNGs. The released CytoC/aNGs increase cellular uptake of CytoC and enhance its intracellular delivery to its target site, cytoplasm, resulting in favorable apoptosis-inducing and cytotoxic effects. We show that a single local administration of CytoC/aNGs/Gel efficiently inhibit the tumor growth in the breast tumor mouse model.

Polymorphic Self-Organization of Lauroyl Peptide in Response to pH and Concentration

Amphipathic peptides are attractive building blocks for the preparation of self-assembling, bio-inspired, and stimuli responsive nanomaterials with pharmaceutical interest. The bioavailability of these materials can be improved with the insertion of d amino acid residues to avoid fast proteolysis in vivo. With this knowledge, a new lauroyl peptide consisting of a sequence of glycine, glycine, d-serine, and d-lysine was designed. In spite of its simple sequence, this lipopeptide self-assembles into spherical micelles at acid pH, when the peptide moiety adopts disordered conformations. Self-aggregates reshape toward fibers at basic pH, following the conformational transition of the peptide region from random coil to β-sheet. Finally, hydrogels are achieved at basic pH and higher concentrations. The transition from random coil to β-sheet conformation of the peptide headgroup obtained by increasing pH was monitored by circular dichroism and vibrational spectroscopy. A structural analysis, performed by combining dynamic light scattering, small-angle X-ray scattering, transmission electron microscopy, and molecular dynamic simulations, demonstrated that the transition allows the self-assemblies to remodel from spherical micelles to rodlike shapes, to long fibers with rectangular cross-section and a head-tail-tail-head structure. The viscoelastic behavior of the hydrogels formed at the highest pH was investigated by rheology measurements.

Chirality-driven molecular packing structure difference and potential application for 3D printing of a series of bola-type Ala–Phe dipeptides

For bola-type dipeptides based on Ala–Phe building block, the chirality of Phe residue at C-terminal determined the handedness of self-assemblies and stacking chirality of carbonyl groups.

The influence of pH, protein concentration and calcium ratio on the formation and structure of nanotubes from partially hydrolyzed bovine α-lactalbumin

Formation of nanotubes from partially hydrolysed α-lactalbumin (α-La) was investigated at five pH values, two concentrations of α-La and two calcium levels. Nanotubes were formed under almost all combinations of the investigated factors, and for the first time the formation of nanotubes at low pH (4.0) and low protein concentration (10 g l-1) was observed. Only one sample (10 g l-1, calcium ratio 2.4, and pH 7.5) formed mainly fibrils instead of nanotubes. By altering the three investigated factors, fibrils and/or aggregates were sometimes formed together with nanotubes resulting in transparent, semi-transparent, or non-transparent gels, or sediments. However, structural modelling based on small-angle X-ray scattering data indicated that the formed nanotubes were only to a minor degree affected by the investigated factors. The majority of the nanotubes were found to have an outer diameter of around 19 nm, an inner diameter of 6.6 nm and a wall thickness of 6.0 nm, except for three samples at low α-La concentrations and high calcium levels which exhibited slightly smaller dimensions. These three factors affected the hydrolysis as well as the self-assembly rate, resulting in the observed differences. However, these factors did not influence the architecture of the self-assembled nanotubes, and the lateral spacing of the individual parallel β-sheet motifs was found to be 1.05 ± 0. 03 nm for all nanotubes. This study provides novel fundamental knowledge of the formation and structure of α-La nanotubes under different conditions, which will facilitate future application of these nanotubes in food and pharmaceutical areas.

Peptide-based building blocks as structural elements for supramolecular Gd-containing MRI contrast agents

Magnetic resonance imaging (MRI) is one of the most important clinic diagnostic tool used to obtain high-quality body images. The administration of low-molecular-weight Gd complex-based MRI contrast agents (CAs) permits to increase the ¹ H relaxation rate of nearby water molecules, thus modulating signal intensity and contrast enhancement. Even if highly accurate, MRI modality suffers from its low sensitivity. Moreover, low-molecular-weight CAs rapidly equilibrate between the intravascular and extravascular spaces after their administration. In order to improve their sensitivity and limit the extravasation phenomenon, several macromolecular and supramolecular multimeric gadolinium complexes (dendrimers, polymers, carbon nanostructures, micelles, and liposomes) have been designed until now. Because of their biocompatibility, low immunogenicity, low cost, and easy synthetic modification, peptides are attractive building blocks for the fabbrication of novel materials for biomedical applications. We report on the state of the art of supramolecular CAs obtained by self-assembly of three different classes of building blocks containing a peptide sequence, a gadolinium complex, and, if necessary, a third functional portion achieving the organization process.

Self-Assembled Nanostructures Regulate H2S Release from Constitutionally Isomeric Peptides

We report here on three constitutionally isomeric peptides, each of which contains two glutamic acid residues and two lysine residues functionalized with S-aroylthiooximes (SATOs), termed peptide-H2S donor conjugates (PHDCs). SATOs decompose in the presence of cysteine to generate hydrogen sulfide (H2S), a biological signaling gas with therapeutic potential. The PHDCs self-assemble in aqueous solution into different morphologies, two into nanoribbons of different dimensions and one into a rigid nanocoil. The rate of H2S release from the PHDCs depends on the morphology, with the nanocoil-forming PHDC exhibiting a complex release profile driven by morphological changes promoted by SATO decomposition. The nanocoil-forming PHDC mitigated the cardiotoxicity of doxorubicin more effectively than its nanoribbon-forming constitutional isomers as well as common H2S donors. This strategy opens up new avenues to develop H2S-releasing biomaterials and highlights the interplay between structure and function from the molecular level to the nanoscale.

Self-assembling prodrugs

Covalent modification of therapeutic compounds is a clinically proven strategy to devise prodrugs with enhanced treatment efficacies. This prodrug strategy relies on the modified drugs that possess advantageous pharmacokinetic properties and administration routes over their parent drug. Self-assembling prodrugs represent an emerging class of therapeutic agents capable of spontaneously associating into well-defined supramolecular nanostructures in aqueous solutions. The self-assembly of prodrugs expands the functional space of conventional prodrug design, affording a possible pathway to more effective therapies as the assembled nanostructure possesses distinct physicochemical properties and interaction potentials that can be tailored to specific administration routes and disease treatment. In this review, we will discuss the various types of self-assembling prodrugs in development, providing an overview of the methods used to control their structure and function and, ultimately, our perspective on their current and future potential.

Balancing the intermolecular forces in peptide amphiphiles for controlling self-assembly transitions

While the influence of alkyl chain length and headgroup size on self-assembly behaviour has been well-established for simple surfactants, the rational control over the pH- and concentration-dependent self-assembly behaviour in stimuli responsive peptides remains an elusive goal. Here, we show that different amphiphilic peptides can have similar self-assembly phase diagrams, providing the relative strengths of the attractive and repulsive forces are balanced. Using palmitoyl-YYAAEEEEK(DO3A:Gd)-NH2 and palmitoyl-YAAEEEEK(DO3A:Gd)-NH2 as controls, we show that reducing hydrophobic attractive forces through fewer methylene groups in the alkyl chain will lead to a similar self-assembly phase diagram as increasing the electrostatic repulsive forces via the addition of a glutamic acid residue. These changes allow creation of self-assembled MRI vehicles with slightly different micelle and nanofiber diameters but with minimal changes in the spin-lattice T1 relaxivity. These findings reveal a powerful strategy to design self-assembled vehicles with different sizes but with similar self-assembly profiles.

Enzymatic activation of cell-penetrating peptides in self-assembled nanostructures triggers fibre-to-micelle morphological transition

We report here a proof-of-concept design of a multi-domain cell-penetrating peptide amphiphile (CPPA) which can self-assemble into fibrous nanostructures and transform into spherical micelles upon enzymatic degradation by matrix metalloproteinase-2 (MMP-2) up-regulated in the tumour environment. Concomitant with this morphological transition, the cell-penetrating peptide (CPP), which was previously buried inside the CPPA fibers, could be presented on the surface of the CPPA micelles, enhancing their cell-penetrating ability. These multifunctional and enzyme-responsive CPP nanostructures hold potential as nanocarriers for tumour-targeted intracellular delivery of therapeutic and diagnostic agents.

Conformation and self-assembly changes of isomeric peptide amphiphiles influenced by switching tyrosine in the sequences

By simply switching the location of Tyr in isomeric peptide amphiphiles of C₁₂-(GA)₃GY, the varied conformations between the β-sheet and disordered one of these peptide amphiphiles and the alternating morphologies between nanofibers and nanospheres of their assemblies are revealed.

Molecular packing and the handedness of the self-assemblies of C17H35CO-Ala-Phe sodium salts

Molecular packing structure dominates the handedness of the self-assemblies of a series of lipodipeptide sodium salts.

Self-assembled peptide nanostructures for functional materials

Nature is an important inspirational source for scientists, and presents complex and elegant examples of adaptive and intelligent systems created by self-assembly. Significant effort has been devoted to understanding these sophisticated systems. The self-assembly process enables us to create supramolecular nanostructures with high order and complexity, and peptide-based self-assembling building blocks can serve as suitable platforms to construct nanostructures showing diverse features and applications. In this review, peptide-based supramolecular assemblies will be discussed in terms of their synthesis, design, characterization and application. Peptide nanostructures are categorized based on their chemical and physical properties and will be examined by rationalizing the influence of peptide design on the resulting morphology and the methods employed to characterize these high order complex systems. Moreover, the application of self-assembled peptide nanomaterials as functional materials in information technologies and environmental sciences will be reviewed by providing examples from recently published high-impact studies.