Curvature instability in a chiral amphiphile self-assembly

Real-time Observation of Macroscopic Helical Morphologies under Optical Microscope: A Curious Case of π-π Stacking Driven Molecular Self-assembly of an Organic Gelator Devoid of Hydrogen Bonding

Supramolecular assemblies such as tubules/helix/double helix/helical tape etc. are usually submicron objects preventing direct observation under optical microscope. Chiral-pure form of these assemblies is important for potential applications. Herein, we report a rare phenomenon wherein a DMSO gel of a simple terpyridine derivative [(4-CNPhe)4PyTerp] produced macroscopic helical morphologies (μm length scale) which could be observed under optical microscope, formation of which could be monitored by optical videography, stable enough to withstand acidic vapour, robust enough to display reversible gel↔sol in response to acidic and ammonia vapour and sturdy enough to be maneuvered with a needle. These properties appeared to be unique to the title compound as the other related derivatives failed to display such assembly structures. SXRD and MD simulation studies suggested that weak interactions (π-π stacking) played a crucial role in the self-assembly process.

Building blocks of non-Euclidean ribbons: size-controlled self-assembly via discrete frustrated particles

Geometric frustration offers a pathway to soft matter self-assembly with controllable finite sizes. While the understanding of frustration in soft matter assembly derives almost exclusively from continuum elastic descriptions, a current challenge is to understand the connection between microscopic physical properties of misfitting "building blocks" and emergent assembly behavior at the mesoscale. We present and analyze a particle-based description of what is arguably the best studied example for frustrated soft matter assembly, negative-curvature ribbon assembly, observed in both assemblies of chiral surfactants and shape-frustrated nanoparticles. Based on our particle model, known as saddle wedge monomers, we numerically test the connection between microscopic shape and interactions of the misfitting subunits and the emergent behavior at the supra-particle scale, specifically focussing on the propagation and relaxation of inter-particle strains, the emergent role of extrinsic shape on frustrated ribbons and the equilibrium regime of finite width selection. Beyond the intuitive role of shape misfit, we show that self-limitation is critically dependent on the finite range of cohesive interactions, with larger size finite assemblies requiring increasing short-range interparticle forces. Additionally, we demonstrate that non-linearities arising from discrete particle interactions alter self-limiting behavior due to both strain-softening in shape-flattened assembly and partial yielding of highly strained bonds, which in turn may give rise to states of hierarchical, multidomain assembly. Tracing the regimes of frustration-limited assembly to the specific microscopic features of misfitting particle shapes and interactions provides necessary guidance for translating the theory of size-programmable assembly into design of intentionally-frustrated colloidal particles.

Phenol-soluble modulins PSMα3 and PSMβ2 form nanotubes that are cross-α amyloids

Phenol-soluble modulins (PSMs) are peptide-based virulence factors that play significant roles in the pathogenesis of staphylococcal strains in community-associated and hospital-associated infections. In addition to cytotoxicity, PSMs display the propensity to self-assemble into fibrillar species, which may be mediated through the formation of amphipathic conformations. Here, we analyze the self-assembly behavior of two PSMs, PSMα3 and PSMβ2, which are derived from peptides expressed by methicillin-resistant Staphylococcus aureus (MRSA), a significant human pathogen. In both cases, we observed the formation of a mixture of self-assembled species including twisted filaments, helical ribbons, and nanotubes, which can reversibly interconvert in vitro. Cryo–electron microscopy structural analysis of three PSM nanotubes, two derived from PSMα3 and one from PSMβ2, revealed that the assemblies displayed remarkably similar structures based on lateral association of cross-α amyloid protofilaments. The amphipathic helical conformations of PSMα3 and PSMβ2 enforced a bilayer arrangement within the protofilaments that defined the structures of the respective PSMα3 and PSMβ2 nanotubes. We demonstrate that, similar to amyloids based on cross-β protofilaments, cross-α amyloids derived from these PSMs display polymorphism, not only in terms of the global morphology (e.g., twisted filament, helical ribbon, and nanotube) but also with respect to the number of protofilaments within a given peptide assembly. These results suggest that the folding landscape of PSM derivatives may be more complex than originally anticipated and that the assemblies are able to sample a wide range of supramolecular structural space.

Frustrated self-assembly of non-Euclidean crystals of nanoparticles

Self-organized complex structures in nature, e.g., viral capsids, hierarchical biopolymers, and bacterial flagella, offer efficiency, adaptability, robustness, and multi-functionality. Can we program the self-assembly of three-dimensional (3D) complex structures using simple building blocks, and reach similar or higher level of sophistication in engineered materials? Here we present an analytic theory for the self-assembly of polyhedral nanoparticles (NPs) based on their crystal structures in non-Euclidean space. We show that the unavoidable geometrical frustration of these particle shapes, combined with competing attractive and repulsive interparticle interactions, lead to controllable self-assembly of structures of complex order. Applying this theory to tetrahedral NPs, we find high-yield and enantiopure self-assembly of helicoidal ribbons, exhibiting qualitative agreement with experimental observations. We expect that this theory will offer a general framework for the self-assembly of simple polyhedral building blocks into rich complex morphologies with new material capabilities such as tunable optical activity, essential for multiple emerging technologies.

Supramolecular Interactions and Morphology of Self-Assembling Peptide Amphiphile Nanostructures

The morphology of supramolecular peptide nanostructures is difficult to predict given their complex energy landscapes. We investigated peptide amphiphiles containing β-sheet forming domains that form twisted nanoribbons in water. We explained the morphology based on a balance between the energetically favorable packing of molecules in the center of the nanostructures, the unfavorable packing at the edges, and the deformations due to packing of twisted β-sheets. We find that morphological polydispersity of PA nanostructures is determined by peptide sequences, and the twisting of their internal β-sheets. We also observed a change in the supramolecular chirality of the nanostructures as the peptide sequence was modified, although only amino acids with l-configuration were used. Upon increasing charge repulsion between molecules, we observed a change in morphology to long cylinders and then rodlike fragments and spherical micelles. Understanding the self-assembly mechanisms of peptide amphiphiles into nanostructures should be useful to optimize their well-known functions.

Control of Peptide Amphiphile Supramolecular Nanostructures by Isosteric Replacements

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

Directed Assembly of Multi-Walled Nanotubes and Nanoribbons of Amino Acid Amphiphiles Using a Layer-by-Layer Approach

Monodisperse unilamellar nanotubes (NTs) and nanoribbons (NRs) were transformed to multilamellar NRs and NTs in a well-defined fashion. This was done by using a step-wise approach in which self-assembled cationic amino acid amphiphile (AAA) formed the initial NTs or NRs, and added polyanion produced an intermediate coating. Successive addition of cationic AAA formed a covering AAA layer, and by repeating this layer-by-layer (LBL) procedure, multi-walled nanotubes (mwNTs) and nanoribbons were formed. This process was structurally investigated by combining small-angle neutron scattering (SANS) and cryogenic-transmission electron microscopy (cryo-TEM), confirming the multilamellar structure and the precise layer spacing. In this way the controlled formation of multi-walled suprastructures was demonstrated in a simple and reproducible fashion, which allowed to control the charge on the surface of these 1D aggregates. This pathway to 1D colloidal materials is interesting for applications in life science and creating well-defined building blocks in nanotechnology.

Equilibrium mechanisms of self-limiting assembly

Self-assembly is a ubiquitous process in synthetic and biological systems, broadly defined as the spontaneous organization of multiple subunits (e.g. macromolecules, particles) into ordered multi-unit structures. The vast majority of equilibrium assembly processes give rise to two states: one consisting of dispersed disassociated subunits, and the other, a bulk-condensed state of unlimited size. This review focuses on the more specialized class of self-limiting assembly, which describes equilibrium assembly processes resulting in finite-size structures. These systems pose a generic and basic question, how do thermodynamic processes involving non-covalent interactions between identical subunits "measure" and select the size of assembled structures? In this review, we begin with an introduction to the basic statistical mechanical framework for assembly thermodynamics, and use this to highlight the key physical ingredients that ensure equilibrium assembly will terminate at finite dimensions. Then, we introduce examples of self-limiting assembly systems, and classify them within this framework based on two broad categories: self-closing assemblies and open-boundary assemblies. These include well-known cases in biology and synthetic soft matter - micellization of amphiphiles and shell/tubule formation of tapered subunits - as well as less widely known classes of assemblies, such as short-range attractive/long-range repulsive systems and geometrically-frustrated assemblies. For each of these self-limiting mechanisms, we describe the physical mechanisms that select equilibrium assembly size, as well as potential limitations of finite-size selection. Finally, we discuss alternative mechanisms for finite-size assemblies, and draw contrasts with the size-control that these can achieve relative to self-limitation in equilibrium, single-species assemblies.

A combined experimental and computational approach reveals how aromatic peptide amphiphiles self-assemble to form ion-conducting nanohelices

Reported here is a combined experimental-computational strategy to determine structure-property-function relationships in persistent nanohelices formed by a set of aromatic peptide amphiphile (APA) tetramers with the general structure K S XEK S , where KS= S-aroylthiooxime modified lysine, X = glutamic acid or citrulline, and E = glutamic acid. In low phosphate buffer concentrations, the APAs self-assembled into flat nanoribbons, but in high phosphate buffer concentrations they formed nanohelices with regular twisting pitches ranging from 9-31 nm. Coarse-grained molecular dynamics simulations mimicking low and high salt concentrations matched experimental observations, and analysis of simulations revealed that increasing strength of hydrophobic interactions under high salt conditions compared with low salt conditions drove intramolecular collapse of the APAs, leading to nanohelix formation. Analysis of the radial distribution functions in the final self-assembled structures led to several insights. For example, comparing distances between water beads and beads representing hydrolysable KS units in the APAs indicated that the KS units in the nanohelices should undergo hydrolysis faster than those in the nanoribbons; experimental results verified this hypothesis. Simulation results also suggested that these nanohelices might display high ionic conductivity due to closer packing of carboxylate beads in the nanohelices than in the nanoribbons. Experimental results showed no conductivity increase over baseline buffer values for unassembled APAs, a slight increase (0.4 × 102 μS/cm) for self-assembled APAs under low salt conditions in their nanoribbon form, and a dramatic increase (8.6 × 102 μS/cm) under high salt conditions in their nanohelix form. Remarkably, under the same salt conditions, these self-assembled nanohelices conducted ions 5-10-fold more efficiently than several charged polymers, including alginate and DNA. These results highlight how experiments and simulations can be combined to provide insight into how molecular design affects self-assembly pathways; additionally, this work highlights how this approach can lead to discovery of unexpected properties of self-assembled nanostructures.

Inception Mechanisms of Tunneling Nanotubes

Tunneling nanotubes (TNTs) are thin membranous tubes that interconnect cells, representing a novel route of cell-to-cell communication and spreading of pathogens. TNTs form between many cell types, yet their inception mechanisms remain elusive. We review in this study general concepts related to the formation and stability of membranous tubular structures with a focus on a deviatoric elasticity model of membrane nanodomains. We review experimental evidence that tubular structures initiate from local membrane bending facilitated by laterally distributed proteins or anisotropic membrane nanodomains. We further discuss the numerical results of several theoretical and simulation models of nanodomain segregation suggesting the mechanisms of TNT inception and stability. We discuss the coupling of nanodomain segregation with the action of protruding cytoskeletal forces, which are mostly provided in eukaryotic cells by the polymerization of f-actin, and review recent inception mechanisms of TNTs in relation to motor proteins.

Shape and fluctuations of positively curved ribbons

We study the shape and shape fluctuations of incompatible, positively curved ribbons, with a flat reference metric and a spherelike reference curvature. Such incompatible geometry is likely to occur in many self-assembled materials and other experimental systems. Ribbons of this geometry exhibit a sharp transition between a rigid ring and an anomalously soft spring as a function of their width. As a result, the temperature dependence of these ribbons' shape is unique, exhibiting a nonmonotonic dependence of the persistence and Kuhn lengths on the temperature and width. We map the possible configuration phase space and show the existence of three phases: At high temperatures it is the ideal chain phase, where the ribbon is well described by classical models (e.g., wormlike chain model). The second phase, for cold and narrow ribbons, is the plane ergodic phase; a ribbon in this phase might be thought of as made out of segments that gyrate within an oblate spheroid with extreme aspect ratio. The third phase, for cold, wide ribbons, is a direct result of the residual stress caused by the incompatibility, called the random structured phase. A ribbon in this phase behaves on large scales as an ideal chain. However, the segments of this chain are not straight; rather they may have different shapes, mainly helices (both left and right handed) of various pitches.

Morphology transition in helical tubules of a supramolecular gel driven by metal ions

Our aim to access a particular chemical functionality on helical tubules has been achieved by the rational molecular design and synthesis of glucono-appended cardanol derivatives. For the first time, we report a chiral molecular packing with α-helical tubules, and chiral symmetry-breaking upon exposure to Cu²⁺ that generated the final ordered structure via an in situ morphological transition without undergoing any phase change.

Nucleation of twisted and tubular states in chiral ribbons

Bilayers of chiral molecules can self-assemble into twisted and tubular structures, as was recently shown with chiral molecular constituents such as ssDNA-amphiphiles. I show that the dynamics of the transition between these topologies is driven by a nucleation mechanism that bears a striking formal similarity to that encountered in first-order wetting and dewetting transitions. Exploiting this analogy enables the critical nuclei of the transition to be calculated.

Molecular engineering of chiral colloidal liquid crystals using DNA origami

Establishing precise control over the shape and the interactions of the microscopic building blocks is essential for design of macroscopic soft materials with novel structural, optical and mechanical properties. Here, we demonstrate robust assembly of DNA origami filaments into cholesteric liquid crystals, one-dimensional supramolecular twisted ribbons and two-dimensional colloidal membranes. The exquisite control afforded by the DNA origami technology establishes a quantitative relationship between the microscopic filament structure and the macroscopic cholesteric pitch. Furthermore, it also enables robust assembly of one-dimensional twisted ribbons, which behave as effective supramolecular polymers whose structure and elastic properties can be precisely tuned by controlling the geometry of the elemental building blocks. Our results demonstrate the potential synergy between DNA origami technology and colloidal science, in which the former allows for rapid and robust synthesis of complex particles, and the latter can be used to assemble such particles into bulk materials.

pH-Driven Self-Assembly of Acidic Microbial Glycolipids

Microbial glycolipids are a class of well-known compounds, but their self-assembly behavior is still not well understood. While the free carboxylic acid end group makes some of them interesting stimuli-responsive compounds, the sugar hydrophilic group and the nature of the fatty acid chain make the understanding of their self-assembly behavior in water not easy and highly unpredictable. Using cryo-transmission electron microscopy (cryo-TEM) and both pH-dependent in situ and ex situ small angle X-ray scattering (SAXS), we demonstrate that the aqueous self-assembly at room temperature (RT) of a family of β-d-glucose microbial glycolipids bearing a saturated and monounsaturated C18 fatty acid chain cannot be explained on the simple basis of the well-known packing parameter. Using the "pH-jump" process, we find that the molecules bearing a monosaturated fatty acid forms vesicles below pH 6.2, as expected, but the derivative with a saturated fatty acid forms infinite bilayer sheets below pH 7.8, instead of vesicles. We show that this behavior can be explained on the different bilayer membrane elasticity as a function of temperature. Membranes are either flexible or stiff for experiments performed at a temperature respectively above or below the typical melting point, TM, of the lipidic part of each compound. Finally, we also show that the disaccharide-containing acidic cellobioselipid forms a majority of chiral fibers, instead of the expected micelles.

Elasticity and Fluctuations of Frustrated Nanoribbons

We derive a reduced quasi-one-dimensional theory of geometrically frustrated elastic ribbons. Expressed in terms of geometric properties alone, it applies to ribbons over a wide range of scales, allowing the study of their elastic equilibrium, as well as thermal fluctuations. We use the theory to account for the twisted-to-helical transition of ribbons with spontaneous negative curvature and the effect of fluctuations on the corresponding critical exponents. The persistence length of such ribbons changes nonmonotonically with the ribbon's width, dropping to zero at the transition. This and other statistical properties qualitatively differ from those of nonfrustrated fluctuating filaments.

Chiral Nanoarchitectonics: Towards the Design, Self-Assembly, and Function of Nanoscale Chiral Twists and Helices

Helical structures such as double helical DNA and the α-helical proteins found in biological systems are among the most beautiful natural structures. Chiral nanoarchitectonics, which is used here to describe the hierarchical formation and fabrication of chiral nanoarchitectures that can be observed by atomic force microscopy (AFM), scanning tunneling microscopy (STM), scanning electron microscopy (SEM), or transmission electron microscopy (TEM), is one of the most effective ways to mimic those natural chiral nanostructures. This article focuses on the formation, structure, and function of the most common chiral nanoarchitectures: nanoscale chiral twists and helices. The types of molecules that can be designed and how they can form hierarchical chiral nanoarchitectures are explored. In addition, new and unique functions such as amplified chiral sensing, chiral separation, biological effects, and circularly polarized luminescence associated with the chiral nanoarchitectures are discussed.

Closed membrane shapes with attached BAR domains subject to external force of actin filaments

Membrane deformations induced by attached BAR superfamily domains could trigger or facilitate the growth of plasma membrane protrusions. The BAR domain family consists of BAR, F-BAR and I-BAR domains, each enforcing a different local curvature when attached to the membrane surface. Our theoretical study mainly focuses on the role of I-BAR in the membrane tubular deformations generated or stabilised by actin filaments. The influence of the area density of membrane attached BAR domains and their intrinsic curvature on the closed membrane shapes (vesicles) was investigated numerically. We derived an analytical approximative expression for the critical relative area density of BARs at which the membrane tubular protrusions on vesicles are most prominent. We have shown that the BARs with a higher intrinsic curvature induce thinner and longer cylindrical protrusions. The average orientation of the membrane attached BARs is altered when the vesicle shape is subjected to external force of growing actin rod-like structure inside a vesicle. The average orientation angle of membrane attached BARs may indicate whether the actin filaments are just stabilising the protrusion or generating it by stretching the vesicle.

DNA nanotubes and helical nanotapes via self-assembly of ssDNA-amphiphiles

DNA nanotubes were created using molecular self-assembly of single-stranded DNA (ssDNA)-amphiphiles composed of a hydrophobic dialkyl tail and polycarbon spacer and a hydrophilic ssDNA headgroup. The nanotube structures were formed by bilayers of amphiphiles, with the hydrophobic components forming an inner layer that was shielded from the aqueous solvent by an outer layer of ssDNA. The nanotubes appeared to form via an assembly process that included transitions from twisted nanotapes to helical nanotapes to nanotubes. Amphiphiles that contained different ssDNA headgroups were created to explore the effect of the length and secondary structure of the ssDNA headgroup on the self-assembly behavior of the amphiphiles in the presence and absence of the polycarbon spacer. It was found that nanotubes could be formed using a variety of headgroup lengths and sequences. The ability to create nanotubes via ssDNA-amphiphile self-assembly offers an alternative to the other purely DNA-based approaches like DNA origami and DNA tile assembly for constructing these structures and may be useful for applications in drug delivery, biosensing, and electronics.

Sol–gel transition accelerated by the co-assembly of two components in supramolecular hydrogels

N-Palmitoyl-Gly-His (PalGH) and glycerol monopalmitate (GMP) in water co-assembled into fibrils with twisted ribbon structures and formed a homogeneous network, resulting in gel formation. The mixture exhibits sol–gel transition while a gel made from only PalGH does not.

Amino acid sequence in constitutionally isomeric tetrapeptide amphiphiles dictates architecture of one-dimensional nanostructures

The switching of two adjacent amino acids can lead to differences in how proteins fold thus affecting their function. This effect has not been extensively explored in synthetic peptides in the context of supramolecular self-assembly. Toward this end, we report here the use of isomeric peptide amphiphiles as molecular building blocks to create one-dimensional (1D) nanostructures. We show that four peptide amphiphile isomers, with identical composition but a different sequence of their four amino acids, can form drastically different types of 1D nanostructures under the same conditions. We found that molecules with a peptide sequence of alternating hydrophobic and hydrophilic amino acids such as VEVE and EVEV self-assemble into flat nanostructures that can be either helical or twisted. On the other hand, nonalternating isomers such as VVEE and EEVV result in the formation of cylindrical nanofibers. Furthermore, we also found that when the glutamic acid is adjacent to the alkyl tail the supramolecular assemblies appear to be internally flexible compared to those with valine as the first amino acid. These results clearly demonstrate the significance of peptide side chain interactions in determining the architectures of supramolecular assemblies.

pH-triggered formation of nanoribbons from yeast-derived glycolipid biosurfactants

In the present paper, we show that the saturated form of acidic sophorolipids, a family of industrially scaled bolaform microbial glycolipids, unexpectedly forms chiral nanofibers only at pH below 7.5. In particular, we illustrate that this phenomenon derives from a subtle cooperative effect of molecular chirality, hydrogen bonding, van der Waals forces and steric hindrance. The pH-responsive behaviour was shown by Dynamic Light Scattering (DLS), pH-titration and Field Emission Scanning Electron Microscopy (FE-SEM) while the nanoscale chirality was evidenced by Circular Dichroism (CD) and cryo Transmission Electron Microscopy (cryo-TEM). The packing of sophorolipids within the ribbons was studied using Small Angle Neutron Scattering (SANS), Wide Angle X-ray Scattering (WAXS) and 2D (1)H-(1)H through-space correlations via Nuclear Magnetic Resonance under very fast (67 kHz) Magic Angle Spinning (MAS-NMR).

Progress in the direct structural characterization of fibrous amphiphilic supramolecular assemblies in solution by transmission electron microscopic techniques

The self-assembly of amphiphilic molecules into fibrous structures has been the subject of numerous studies over past decades due to various current and promising technical applications. Although very different in their head group chemistry many natural as well as synthetic amphiphilic compounds derived from carbohydrates, carbocyanine dyes, or amino acids tend to form fibrous structures by molecular self-assembly in water predominantly twisted ribbons or tubes. Often a transition between these assembly structures is observed, which is a phenomenon already theoretically approached by Wolfgang Helfrich and still focus point in current research. With the development of suitable sample preparation and electron optical imaging techniques, cryogenic transmission electron microscopy (cryo-TEM) in combination with three-dimensional (3D) reconstruction techniques has become a particular popular direct characterization technique for supramolecular assemblies in general. Here we review the recent progress in deriving precise structural information from cryo-TEM data of particularly fibrous structures preferably in three dimensions.

Shape selection in chiral ribbons: from seed pods to supramolecular assemblies

We provide a geometric-mechanical model for calculating equilibrium configurations of chemical systems that self-assemble into chiral ribbon structures. The model is based on incompatible elasticity and uses dimensionless parameters to determine the equilibrium configurations. As such, it provides universal curves for the shape and energy of self-assembled ribbons. We provide quantitative predictions for the twisted-to-helical transition, which was observed experimentally in many systems, and demonstrate it with synthetic ribbons made of responsive gels. In addition, we predict the bi-stability of wide ribbons and also show how geometrical frustration can cause arrest of ribbon widening. Finally, we show that the model's predictions provide explanations for experimental observations in different chemical systems.

Growth actuated bending and twisting of single crystals

Crystals of a variety of substances including elements, minerals, simple salts, organic molecular crystals, and high polymers forgo long-range translational order by twisting and bending as they grow. These deviations have been observed in crystals ranging in size from nanometers to centimeters. How and why so many materials choose dramatic non-crystallographic distortions is analyzed, with an emphasis on crystal chemistries that give rise to stresses operating either on surfaces of crystallites or within the bulk.

Supramolecular Polymers Formed by ABC Miktoarm Star Peptides

We report here the design and synthesis of an ABC miktoarm star peptide connecting through a lysine junction a short peptide sequence and two hydrophobic but immiscible blocks (a hydrocarbon and a fluorocarbon). The designed molecule can self-assemble into one-dimensional nanostructures with a great diversity of kinetically evolving morphologies in aqueous solution, while molecules that contain only one of the two hydrophobic blocks form structurally similar filaments. We believe the rich assembly behavior and morphological evolution are a direct reflection of the molecular frustration present within the filament core as a result of the in-compatibility of the fluorocarbon and hydrocarbon segments. Our finding opens new opportunities for creating complex supramolecular polymers through the architecture design of small molecular building units.

Tuning nanostructure dimensions with supramolecular twisting

Peptide amphiphiles are molecules containing a peptide segment covalently bonded to a hydrophobic tail and are known to self-assemble in water into supramolecular nanostructures with shape diversity ranging from spheres to cylinders, twisted ribbons, belts, and tubes. Understanding the self-assembly mechanisms to control dimensions and shapes of the nanostructures remains a grand challenge. We report here on a systematic study of peptide amphiphiles containing valine-glutamic acid dimeric repeats known to promote self-assembly into belt-like flat assemblies. We find that the lateral growth of the assemblies can be controlled in the range of 100 nm down to 10 nm as the number of dimeric repeats is increased from two to six. Using circular dichroism, the degree of β-sheet twisting within the supramolecular assemblies was found to be directly proportional to the number of dimeric repeats in the PA molecule. Interestingly, as twisting increased, a threshold is reached where cylinders rather than flat assemblies become the dominant morphology. We also show that in the belt regime, the width of the nanostructures can be decreased by raising the pH to increase charge density and therefore electrostatic repulsion among glutamic acid residues. The control of size and shape of these nanostructures should affect their functions in biological signaling and drug delivery.

pH sensitive tubules of a bile acid derivative: a tubule opening by release of wall leaves

Tubules formed by self-assembly of organic molecules have vast potential for nanotechnology applications, and the introduction of sensitivity to stimuli into self-assembly tubules represents a particularly attractive feature. Here we report the preparation and characterization of a molecule obtained by chemical modification of a natural bile acid, a biological surfactant, that self-assembles in pH sensitive tubules in aqueous solutions. The tubules, which are rigid, single-walled and with a diameter of 60 nm, form at pH 8-9 and open up when the pH is increased. The transition is reversible, it occurs in the pH range of 9-10 with an opening mechanism that is remarkably different from those so far proposed in the literature. It involves a release of wall layers similar to leaves, and is determined by a drastic pH-triggered change in the molecular arrangement, which in turn induces a radical modification of the wall curvature. The description of the morphological transformation is provided by means of cryogenic transmission electron microscopy and represents, to our knowledge, the first detailed visualization of pH stimulated tubule opening. UV and circular dichroism spectroscopies are used to investigate the evolution at the molecular level.

Effect of chain chirality on the self-assembly of sickle hemoglobin

We present simulation results on the self-assembly behavior of sickle hemoglobin (HbS). A coarse-grained HbS model, which contains hydrophilic and hydrophobic particles explicitly, is constructed to match the structural properties and physical description of HbS. The hydrophobic interactions are shown to be necessary with chirality being the main driver for the formation of HbS fibers. In the absence of chain chirality, only small self-assembled aggregates are observed whereas self-assembled elongated steplike bundle microstructures appear when we include chain chirality. We also investigate the effect of confinement on self-assembly, and find that elongated fibers-similar to open-space ones-can be obtained in hard confinement domains but cannot be formed within compliant red blood cell (RBC) domains under the same assumptions. We show, however, that by placing explicitly HbS fibers inside the RBCs and subjecting them to linear elongation and bending, we obtain different types of sickle-shaped RBCs as observed in sickle cell anemia.

Advances in cryogenic transmission electron microscopy for the characterization of dynamic self-assembling nanostructures

Elucidating the structural information of nanoscale materials in their solvent-exposed state is crucial, as a result, cryogenic transmission electron microscopy (cryo-TEM) has become an increasingly popular technique in the materials science, chemistry, and biology communities. Cryo-TEM provides a method to directly visualize the specimen structure in a solution-state through a thin film of vitrified solvent. This technique complements X-ray, neutron, and light scattering methods that probe the statistical average of all species present; furthermore, cryo-TEM can be used to observe changes in structure over time. In the area of self-assembly, this tool has been particularly powerful for the characterization of natural and synthetic small molecule assemblies, as well as hybrid organic-inorganic composites. In this review, we discuss recent advances in cryogenic TEM in the context of self-assembling systems with emphasis on characterization of transitions observed in response to external stimuli.

Chiral self-assembly of designed amphiphiles: optimization for nanotube formation

Four amphiphiles with L-aspartic acid headgroups (Asp) and a diphenyldiazenyl group (Azo) contained within the hydrophobic tails were designed and synthesized for self-assembly into helically based nanotubes. The amphiphiles of the form R'-{4-[(4-alkylphenyl)diazenyl]phenoxy}alkanoyl-L-aspartic acid (where R' is 10 or 11) varied only in alkyl chain lengths either side of the azo group, having 4, 7, or 10 carbon distal chains and 10 or 11 carbon proximal chains (R-Azo-R'-Asp, where R denotes the number of carbons in the distal chain and R' denotes the number of carbons in the proximal chain). Despite the molecular similarities, distinct differences were identified in the chiral order of the structures self-assembled from hot methanolic aqueous solutions using microscopy and spectroscopic analyses. This was reflected in dominant thermodynamic aggregate morphologies that ranged from amorphous material for 10-Azo-10-Asp, through twisted ribbons (196 ± 49 nm pitch) for 7-Azo-11-Asp, to the desired helically based nanotubes for 4- and 7-Azo-10-Asp (81 ± 11 and 76 ± 6 nm diameters, respectively). Another key variable in the self-assembly of the amphiphiles was the use of a second method to precipitate aggregates from solution at room temperature. This method enabled the isolation of thermodynamically unstable and key transitional structures. Helical ribbons were precursor structures to the nanotubes formed from 4- and 7-Azo-10-Asp as well as the wide, flattened nanotube structures (587 ± 85 nm width) found for 4-Azo-10-Asp. Overall, the results highlighted the interplay of influence of the headgroup and the hydrophobic tail on self-assembly, providing a basis for future rational design of self-assembling amphiphiles.

Phase networks of cross-β peptide assemblies

Recent evidence suggests that simple peptides can access diverse amphiphilic phases, and that these structures underlie the robust and widely distributed assemblies implicated in nearly 40 protein misfolding diseases. Here we exploit a minimal nucleating core of the Aβ peptide of Alzheimer's disease to map its morphologically accessible phases that include stable intermolecular molten particles, fibers, twisted and helical ribbons, and nanotubes. Analyses with both fluorescence lifetime imaging microscopy (FLIM) and transmission electron microscopy provide evidence for liquid-liquid phase separations, similar to the coexisting dilute and dense protein-rich liquid phases so critical for the liquid-solid transition in protein crystallization. We show that the observed particles are critical for transitions to the more ordered cross-β peptide phases, which are prevalent in all amyloid assemblies, and identify specific conditions that arrest assembly at the phase boundaries. We have identified a size dependence of the particles in order to transition to the para-crystalline phase and a width of the cross-β assemblies that defines the transition between twisted fibers and helically coiled ribbons. These experimental results reveal an interconnected network of increasing molecularly ordered cross-β transitions, greatly extending the initial computational models for cross-β assemblies.

Sub-persistence-length complex scaling behavior in lysozyme amyloid fibrils

We combine atomic force microscopy single-molecule analysis with polymer physics concepts to study molecular conformations of lysozyme amyloid fibrils. We resolve a wavy structure of the fibrils in which the scaling behavior varies at multiple length scales. Bond and pair correlation functions, end-to-end distribution, and wormlike chain model identify three characteristic length scales. At short length scales (≈150 nm), there is a first bending transition of the fibrils corresponding to a bending length L(b). At larger length scales (>2L(b)), fibrils become pseudoperiodic and start to undulate. Finally, at length scales larger than the persistence length (~ μm), the fibrils become flexible and follow a 2D self-avoiding random walk. We interpret these results in terms of the twisting of the fibrils and the impact this has on the area moment of inertia and the propensity of the fibril to bend.

Adaptive supramolecular nanomaterials based on strong noncovalent interactions

Noncovalent systems are adaptive and allow facile processing and recycling. Can they be at the same time robust? How can one rationally design such systems? Can they compete with high-performance covalent materials? The recent literature reveals that noncovalent systems can be robust yet adaptive, self-healing, and recyclable, featuring complex nanoscale structures and unique functions. We review such systems, focusing on the rational design of strong noncovalent interactions, kinetically controlled pathway-dependent processes, complexity, and function. The overview of the recent examples points at the emergent field of noncovalent nanomaterials that can represent a versatile, multifunctional, and environmentally friendly alternative to conventional covalent systems.