Influence of Peptoid Sequence on the Mechanisms and Kinetics of 2D Assembly.

Two-dimensional (2D) materials have attracted intense interest due to their potential for applications in fields ranging from chemical sensing to catalysis, energy storage, and biomedicine. Recently, peptoids, a class of biomimetic sequence-defined polymers, have been found to self-assemble into 2D crystalline sheets that exhibit unusual properties, such as high chemical stability and the ability to self-repair. The structure of a peptoid is close to that of a peptide except that the side chains are appended to the amide nitrogen rather than the α carbon. In this study, we investigated the effect of peptoid sequence on the mechanism and kinetics of 2D assembly on mica surfaces using in situ AFM and time-resolved X-ray scattering. We explored three distinct peptoid sequences that are amphiphilic in nature with hydrophobic and hydrophilic blocks and are known to self-assemble into 2D sheets. The results show that their assembly on mica starts with deposition of aggregates that spread to establish 2D islands, which then grow by attachment of peptoids, either monomers or unresolvable small oligomers, following well-known laws of crystal step advancement. Extraction of the solubility and kinetic coefficient from the dependence of the growth rate on peptoid concentration reveals striking differences between the sequences. The sequence with the slowest growth rate in bulk and with the highest solubility shows almost no detachment; i.e., once a growth unit attaches to the island edge, there is almost no probability of detaching. Furthermore, a peptoid sequence with a hydrophobic tail conjugated to the final carboxyl residue in the hydrophilic block has enhanced hydrophobic interactions and exhibits rapid assembly both in the bulk and on mica. These assembly outcomes suggest that, while the π-π interactions between adjacent hydrophobic blocks play a major role in peptoid assembly, sequence details, particularly the location of charged groups, as well as interaction with the underlying substrate can significantly alter the thermodynamic stability and assembly kinetics.

Biomimetic Mineral Synthesis by Nanopatterned Supramolecular-Block Copolymer Templates.

Supramolecular structures of matrix proteins in mineralizing tissues are known to direct the crystallization of inorganic materials. Here we demonstrate how such structures can be synthetically directed into predetermined patterns for which functionality is maintained. The study employs block copolymer lamellar patterns with alternating hydrophilic and hydrophobic regions to direct the assembly of amelogenin-derived peptide nanoribbons that template calcium phosphate nucleation by creating a low-energy interface. Results show that the patterned nanoribbons retain their ß-sheet structure and function and direct the formation of filamentous and plate-shaped calcium phosphate with high fidelity, where the phase, amorphous or crystalline, depends on the choice of mineral precursor and the fidelity depends on peptide sequence. The common ability of supramolecular systems to assemble on surfaces with appropriate chemistry combined with the tendency of many templates to mineralize multiple inorganic materials implies this approach defines a general platform for bottom-up-patterning of hybrid organic-inorganic materials.

Oriented Crystallization of Hydroxyapatite in Self-Assembled Peptide Fibrils as a Bonelike Material.

Controlling oriented crystallization is key to producing bonelike composite materials with a well-organized structure. However, producing this type of composite material using synthetic biopolymers as scaffolds is challenging. Inspired by the molecular structure of collagen-I, a collagenlike peptide─(Pro-Hyp-Gly)10 (POG10)─was designed to produce self-assembled fibrils that resemble the structure of collagen-I fibrils. In addition, the oriented mineralization of HAP crystals is formed in the fibrils that reproduces a bonelike material similar to collagen-I fibril mineralization. Unlike collagen-I fibrils, POG10 fibrils do not contain gap spaces. The molecular simulation results indicate that in addition to space confinement, the molecular field generated by POG10 can also confine the orientation of HAP, enriching our understanding of physical confinement and shedding light on the design of synthetic biopolymer scaffolds for bonelike material fabrication.

Hierarchical Materials from High Information Content Macromolecular Building Blocks: Construction, Dynamic Interventions, and Prediction.

Hierarchical materials that exhibit order over multiple length scales are ubiquitous in nature. Because hierarchy gives rise to unique properties and functions, many have sought inspiration from nature when designing and fabricating hierarchical matter. More and more, however, nature's own high-information content building blocks, proteins, peptides, and peptidomimetics, are being coopted to build hierarchy because the information that determines structure, function, and interfacial interactions can be readily encoded in these versatile macromolecules. Here, we take stock of recent progress in the rational design and characterization of hierarchical materials produced from high-information content blocks with a focus on stimuli-responsive and "smart" architectures. We also review advances in the use of computational simulations and data-driven predictions to shed light on how the side chain chemistry and conformational flexibility of macromolecular blocks drive the emergence of order and the acquisition of hierarchy and also on how ionic, solvent, and surface effects influence the outcomes of assembly. Continued progress in the above areas will ultimately usher in an era where an understanding of designed interactions, surface effects, and solution conditions can be harnessed to achieve predictive materials synthesis across scale and drive emergent phenomena in the self-assembly and reconfiguration of high-information content building blocks.

Radiolysis and Radiation-Driven Dynamics of Boehmite Dissolution Observed by In Situ Liquid-Phase TEM.

Over the last several decades, there have been several studies examining the radiation stability of boehmite and other aluminum oxyhydroxides, yet less is known about the impact of radiation on boehmite dissolution. Here, we investigate radiation effects on the dissolution behavior of boehmite by employing liquid-phase transmission electron microscopy (LPTEM) and varying the electron flux on the samples consisting of either single nanoplatelets or aggregated stacks. We show that boehmite nanoplatelets projected along the [010] direction exhibit uniform dissolution with a strong dependence on the electron dose rate. For nanoplatelets that have undergone oriented aggregation, we show that the dissolution occurs preferentially at the particles at the ends of the stacks that are more accessible to bulk solution than at the others inside the aggregate. In addition, at higher dose rates, electrostatic repulsion and knock-on damage from the electron beam causes delamination of the stacks and dissolution at the interfaces between particles in the aggregate, indicating that there is a threshold dose rate for electron-beam enhancement of dissolution of boehmite aggregates.

Nanoparticle-Mediated Assembly of Peptoid Nanosheets Functionalized with Solid-Binding Proteins: Designing Heterostructures for Hierarchy.

The fabrication of ordered architectures that intimately integrate polymer, protein, and inorganic components remains difficult. Two promising building blocks to tackle this challenge are peptoids, peptide mimics capable of self-assembly into well-defined structures, and solid-binding peptides, which offer a biological path to controlled inorganic assembly. Here, we report on the synthesis of 3.3-nm-thick thiol-reactive peptoid nanosheets from equimolar mixtures of unmodified and maleimide-derivatized versions of the Nbpe6Nce6 oligomer, optimize the location of engineered cysteine residues in silica-binding derivatives of superfolder green fluorescent protein for maleimide conjugation, and react the two components to form protein-peptoid hybrids exhibiting partial or uniform protein coverage on both of their sides. Using 10 nm silica nanoparticles, we trigger the stacking of these 2D structures into a multilayered material composed of alternating peptoid, protein, and organic layers. This simple and modular approach to hierarchical hybrid synthesis should prove useful in bioimaging and photocatalysis applications.

Visualization of Aluminum Ions at the Mica Water Interface Links Hydrolysis State-to-Surface Potential and Particle Adhesion.

When hydrolyzable cations such as aluminum interact with solid-water interfaces, macroscopic interfacial properties (e.g., surface charge and potential) and interfacial phenomena (e.g., particle adhesion) become tightly linked with the microscopic details of ion adsorption and speciation. We use in situ atomic force microscopy to directly image individual aluminum ions at a mica-water interface and show how adsorbate populations change with pH and aluminum activity. Complementary streaming potential measurements then allow us to build a triple layer model (TLM) that links surface potentials to adsorbate populations, via equilibrium binding constants. Our model predicts that hydrolyzed species dominate the mica-water interface, even when unhydrolyzed species dominate the solution. Ab initio molecular dynamics (AIMD) simulations confirm that aluminum hydrolysis is strongly promoted at the interface. The TLM indicates that hydrolyzed adsorbates are responsible for surface-potential inversions, and we find strong correlations between hydrolyzed adsorbates and particle-adhesion forces, suggesting that these species mediate adhesion by chemical bridging.

Growth of Au and ZnS nanostructures via engineered peptide and M13 bacteriophage templates.

We demonstrate directed nucleation of Au and ZnS patterns on templates comprised of functional peptides and an M13 bacteriophage. We discuss the control over nucleation in terms of the interplay between enhanced ion binding and reduced interfacial energy resulting from the presence of the templates.

Surface aggregation of urinary proteins and aspartic Acid-rich peptides on the faces of calcium oxalate monohydrate investigated by in situ force microscopy.

The growth of calcium oxalate monohydrate in the presence of Tamm-Horsfall protein (THP), osteopontin, and the 27-residue synthetic peptides (DDDS)(6)DDD and (DDDG)(6)DDD (D = aspartic acid, S = serine, and G = glycine) was investigated via in situ atomic force microscopy. The results show that these four growth modulators create extensive deposits on the crystal faces. Depending on the modulator and crystal face, these deposits can occur as discrete aggregates, filamentary structures, or uniform coatings. These proteinaceous films can lead to either the inhibition of or an increase in the step speeds (with respect to the impurity-free system), depending on a range of factors that include peptide or protein concentration, supersaturation, and ionic strength. While THP and the linear peptides act, respectively, to exclusively increase and inhibit growth on the (101) face, both exhibit dual functionality on the (010) face, inhibiting growth at low supersaturation or high modulator concentration and accelerating growth at high supersaturation or low modulator concentration. Based on analyses of growth morphologies and dependencies of step speeds on supersaturation and protein or peptide concentration, we propose a picture of growth modulation that accounts for the observations in terms of the strength of binding to the surfaces and steps and the interplay of electrostatic and solvent-induced forces at the crystal surface.

Phosphorylation of osteopontin is required for inhibition of calcium oxalate crystallization.

Under near-physiological pH, temperature, and ionic strength, a kinetics constant composition (CC) method was used to examine the roles of phosphorylation of a 14 amino acid segment (DDVDDTDDSHQSDE) corresponding to potential crystal binding domains within the osteopontin (OPN) sequence. The phosphorylated 14-mer OPN peptide segment significantly inhibits both the nucleation and growth of calcium oxalate monohydrate (COM), inhibiting nucleation by markedly increasing induction times and delaying subsequent growth by at least 50% at concentrations less than 44 nM. Molecular modeling predicts that the doubly phosphorylated peptide binds much more strongly to both (-101) and (010) faces of COM. The estimated binding energies are, in part, consistent with the CC experimental observations. Circular dichroism spectroscopy indicates that phosphorylation does not result in conformational changes in the secondary peptide structure, suggesting that the local binding of negatively charged phosphate side chains to crystal faces controls growth inhibition. These in vitro results reveal that the interactions between phosphorylated peptide and COM crystal faces are predominantly electrostatic, further supporting the importance of macromolecules rich in anionic side chains in the inhibition of kidney stone formation. In addition, the phosphorylation-deficient form of this segment fails to inhibit COM crystal growth up to concentrations of 1450 nM. However, at sufficiently high concentrations, this nonphosphorylated segment promotes COM nucleation. Dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS) results confirm that aggregation of the nonphosphorylated peptide segment takes place in solution above 900 nM when the aggregated peptide particles may exceed a well-defined minimum size to be effective crystallization promoters.