Structure-determining step in the hierarchical assembly of peptoid nanosheets

Rational Design and Multicomponent Synthesis of Lipid-Peptoid Nanocomposites towards a Customized Drug Delivery System Assembly

Nanotechnology has assumed a significant role over the last decade in the development of various technologies applied to health sciences. This becomes even more evident with its application in controlled drug delivery systems. In this context, peptoids are a promising class of compounds for application as nanocarriers in drug delivery systems. These compounds can be obtained efficiently and with highly functionalized structural diversity via the Ugi 4-component reaction (U-4CR). Herein, we report the design of the process control strategy for the future development of lipid-peptoid-based customized drug delivery system assemblies. Over 20 lipid-peptoid nanocomposites were synthesized via the U-4CR in good to excellent yields. These products were successfully submitted to the nanoparticle formation by the emulsification-evaporation process from lipophilic solution and analyzed via Dynamic Light Scattering (DLS). Several molecules generated nanoparticles with a size ≤200 nm, making them good candidates for drug delivery systems, such as in cancer treatment.

Development of a Systematic and Extensible Force Field for Peptoids (STEPs)

Peptoids (N-substituted glycines) are a class of biomimetic polymers that have attracted significant attention due to their accessible synthesis and enzymatic and thermal stability relative to their naturally occurring counterparts (polypeptides). While these polymers provide the promise of more robust functional materials via hierarchical approaches, they present a new challenge for computational structure prediction for material design. The reliability of calculations hinges on the accuracy of interactions represented in the force field used to model peptoids. For proteins, structure prediction based on sequence and de novo design has made dramatic progress in recent years; however, these models are not readily transferable for peptoids. Current efforts to develop and implement peptoid-specific force fields are spread out, leading to replicated efforts and a fragmented collection of parameterized sidechains. Here, we developed a peptoid-specific force field containing 70 different side chains, using GAFF2 as starting point. The new model is validated based on the generation of Ramachandran-like plots from DFT optimization compared against force field reproduced potential energy and free energy surfaces as well as the reproduction of equilibrium cis/trans values for some residues experimentally known to form helical structures. Equilibrium cis/trans distributions (Kct) are estimated for all parameterized residues to identify which residues have an intrinsic propensity for cis or trans states in the monomeric state.

Two-Dimensional Supramolecular Polymerization of DNA Amphiphiles is Driven by Sequence-Dependent DNA-Chromophore Interactions

Two-dimensional (2D) assemblies of water-soluble block copolymers have been limited by a dearth of systematic studies that relate polymer structure to pathway mechanism and supramolecular morphology. Here, we employ sequence-defined triblock DNA amphiphiles for the supramolecular polymerization of free-standing DNA nanosheets in water. Our systematic modulation of amphiphile sequence shows the alkyl chain core forming a cell membrane-like structure and the distal π-stacking chromophore block folding back to interact with the hydrophilic DNA block on the nanosheet surface. This interaction is crucial to sheet formation, marked by a chiral "signature", and sensitive to DNA sequence, where nanosheets form with a mixed sequence, but not with a homogeneous poly(thymine) sequence. This work opens the possibility of forming well-ordered, bilayer-like assemblies using a single DNA amphiphile for applications in cell sensing, nucleic acid therapeutic delivery and enzyme arrays.

Uncovering the Optimal Molecular Characteristics of Hydrophobe-Containing Polypeptoids to Induce Liposome or Cell Membrane Fragmentation

Cellular functions of membrane proteins are strongly coupled to their structures and aggregation states in the cellular membrane. Molecular agents that can induce the fragmentation of lipid membranes are highly sought after as they are potentially useful for extracting membrane proteins in their native lipid environment. Toward this goal, we investigated the fragmentation of synthetic liposome using hydrophobe-containing polypeptoids (HCPs), a class of facially amphiphilic pseudo-peptidic polymers. A series of HCPs with varying chain lengths and hydrophobicities have been designed and synthesized. The effects of polymer molecular characteristics on liposome fragmentation are systemically investigated by a combination of light scattering (SLS/DLS) and transmission electron microscopy (cryo-TEM and negative stained TEM) methods. We demonstrate that HCPs with a sufficient chain length (DP_n ≈ 100) and intermediate hydrophobicity (PNDG mol % = 27%) can most effectively induce the fragmentation of liposomes into colloidally stable nanoscale HCP-lipid complexes owing to the high density of local hydrophobic contact between the HCP polymers and lipid membranes. The HCPs can also effectively induce the fragmentation of bacterial lipid-derived liposomes and erythrocyte ghost cells (i.e., empty erythrocytes) to form nanostructures, highlighting the potential of HCPs as novel macromolecular surfactants toward the application of membrane protein extraction.

Tuning the Packing Density of Gold Nanoparticles in Peptoid Nanosheets Prepared at the Oil-Water Interface

Two-dimensional (2D) arrays of gold nanoparticles that can freely float in water are promising materials for solution-based plasmonic applications like sensing. To be effective sensors, it is critical to control the interparticle gap distance and thus the plasmonic properties of the 2D arrays. Here, we demonstrate excellent control over the interparticle gap distance in a family of freely floating gold nanoparticle-embedded peptoid nanosheets. Nanosheets are made via monolayer assembly and collapse at the oil-water interface, allowing for fine control over the solution nanoparticle concentration during assembly. We used surface pressure measurements to monitor the assembly of the peptoid, nanoparticle, and combined system at the oil-water interface to determine a workable range of nanosheet assembly conditions suitable for controlling the interparticle gap distances within the nanosheets. These measurements revealed that the extent of nanoparticle adsorption to the peptoid monolayer can be tuned by varying the bulk nanoparticle concentration, but the ability for the monolayer to collapse into nanosheets is compromised at high nanoparticle concentrations. Peptoid nanosheets were synthesized with varying bulk nanoparticle concentrations and analyzed using light microscopy and UV-visible spectroscopy. Based on the spectral shift of the localized surface plasmon resonance peaks for the nanoparticles in the nanosheets relative to those well dispersed in toluene, we estimate that we can access interparticle gap distances within the nanosheet interior between 2.9 ± 0.5 and 9 ± 2 nm. Our results suggest that the minimum interparticle distance achievable by this method is limited by the nanoparticle ligand length, and so has the potential to be further tuned by varying the ligand chemical structure. The ability to quantitatively control and monitor the assembly conditions by this method provide an opportunity to readily tune the optoelectronic properties of this new class of 2D nanomaterial, making it a promising platform for plasmonic-based sensing applications.

Solution Self-Assembly of Coil-Crystalline Diblock Copolypeptoids Bearing Alkyl Side Chains

Polypeptoids, a class of synthetic peptidomimetic polymers, have attracted increasing attention due to their potential for biotechnological applications, such as drug/gene delivery, sensing and molecular recognition. Recent investigations on the solution self-assembly of amphiphilic block copolypeptoids highlighted their capability to form a variety of nanostructures with tailorable morphologies and functionalities. Here, we review our recent findings on the solutions self-assembly of coil-crystalline diblock copolypeptoids bearing alkyl side chains. We highlight the solution self-assembly pathways of these polypeptoid block copolymers and show how molecular packing and crystallization of these building blocks affect the self-assembly behavior, resulting in one-dimensional (1D), two-dimensional (2D) and multidimensional hierarchical polymeric nanostructures in solution.

Hierarchical assemblies of polypeptoids for rational design of advanced functional nanomaterials

Polypeptoids (poly-N-substituent glycines) are a class of highly tailorable peptidomimetic polymers. Polypeptoids have identical backbones as polypeptides (poly-C-substituent glycines), but sidechains of polypeptoids are appended to backbone nitrogen rather than α-carbon of polypeptides. As a result, peptoid backbone lacks of chirality and hydrogen bond donors. This unique structure gives polypeptoids a combined merit of both high stability as synthetic polymers and biocompatibility as biopolymers. In addition, peptoid sequences can be engineered precisely to assemble specific crystalline patterns such as spheres, fibers, ribbons, tubes, and sheets, which shows promising potentials of polypeptoids for different applications such as antimicrobials, catalysts, drug delivery, and templating inorganic materials. In this review, we summarize recent investigations into hierarchical self-assembly pathways and molecular structures of peptoid crystals that are of interest as templates for fabricating functional materials for potential biomedical, biochemical, and bioengineering applications. This review provides a summary of recent experimental and computational studies of polypeptoid assembly in solution and solid-liquid interfaces, current achievements in the field, and discusses future challenges and opportunities for the rational design of self-assembled polypeptoid nanomaterials.

Hierarchical Nanomaterials Assembled from Peptoids and Other Sequence-Defined Synthetic Polymers

In nature, the self-assembly of sequence-specific biopolymers into hierarchical structures plays an essential role in the construction of functional biomaterials. To develop synthetic materials that can mimic and surpass the function of these natural counterparts, various sequence-defined bio- and biomimetic polymers have been developed and exploited as building blocks for hierarchical self-assembly. This review summarizes the recent advances in the molecular self-assembly of hierarchical nanomaterials based on peptoids (or poly-N-substituted glycines) and other sequence-defined synthetic polymers. Modern techniques to monitor the assembly mechanisms and characterize the physicochemical properties of these self-assembly systems are highlighted. In addition, discussions about their potential applications in biomedical sciences and renewable energy are also included. This review aims to highlight essential features of sequence-defined synthetic polymers (e.g., high stability and protein-like high-information content) and how these unique features enable the construction of robust biomimetic functional materials with high programmability and predictability, with an emphasis on peptoids and their self-assembled nanomaterials.

Early-Stage Aggregation and Crystalline Interactions of Peptoid Nanomembranes

Fully synthetic peptoid membranes are known to mimic important features of biological membranes, with several advantages over other biomimetic membranes. A fundamental understanding of how the individual peptoid amphiphiles assemble in solution to form the bilayer membrane is key to unlocking their versatility for application in a broad range of processes. In this study, in situ X-ray scattering and molecular dynamics simulations are used to understand the early stages of assembly of three different peptoids that exhibit distinctly different crystallization kinetics. The in situ measurements reveal that the peptoids aggregate first into a nascent phase that is less crystalline than the assembled peptoid membrane. Anisotropic aromatic interactions are determined to be the dominant driving force in the early stages of membrane formation. These results provide key insights into how the peptoid assembly may be manipulated during the early stages of assembly and nucleation and growth.

Controlled ring-opening polymerization of N-(3-tert-butoxy-3-oxopropyl) glycine derived N-carboxyanhydrides towards well-defined peptoid-based polyacids

Polypeptoids bearing carboxylic acid groups on the N-substituent are useful building blocks for the construction of peptidomimetic supramolecular assemblies with stimuli-responsive properties.

Lanthanide conjugates as versatile instruments for therapy and diagnostics

Lanthanides have demonstrated outstanding properties in many fields of research including biology and medicinal chemistry. Their unique luminescence and magnetic properties make them the metals of choice for next generation theranostics that efficiently combine the two central pillars of medicine - diagnostics and therapy. Attached to targeting units, lanthanide complexes pave the way for real-time imaging of drug uptake and distribution as well as specific regulation of subcellular processes with few side effects. This enables individualized treatment options for severe diseases characterized by altered cell expression. The highly diverse results achieved as well as insights into the challenges that research in this area has to face in the upcoming years will be summarized in the present review.

Lipid-anchor display on peptoid nanosheets via co-assembly for multivalent pathogen recognition

Biological systems have evolved sophisticated molecular assemblies capable of exquisite molecular recognition across length scales ranging from angstroms to microns. For instance, the self-organization of glycolipids and glycoproteins on cell membranes allows for molecular recognition of a diversity of ligands ranging from small molecules and proteins to viruses and whole cells. A distinguishing feature of these 2D surfaces is they achieve exceptional binding selectivity and avidity by exploiting multivalent binding interactions. Here we develop a 2D ligand display platform based on peptoid nanosheets that mimics the structure and function of the cell membrane. A variety of small-molecule lipid-conjugates were co-assembled with the peptoid chains to create a diversity of functionalized nanosheet bilayers with varying display densities. The functional heads of the lipids were shown to be surface-exposed, and the carbon tails immobilized into the hydrophobic interior. We demonstrate that saccharide-functionalized nanosheets (e.g., made from globotriaosylsphingosine or 1,2-dipalmitoyl-sn-glycero-3-phospho((ethyl-1',2',3'-triazole)triethyleneglycolmannose)) can have very diverse binding properties, exhibiting specific binding to multivalent proteins as well as to intact bacterial cells. Analysis of sugar display densities revealed that Shiga toxin 1 subunit B (a pentameric protein) and FimH-expressing Escherichia coli (E. coli) bind through the cooperative binding behavior of multiple carbohydrates. The ability to readily incorporate and display a wide variety of lipidated cargo on the surface of peptoid nanosheets makes this a convenient route to soluble, cell-surface mimetic materials. These materials hold great promise for drug screening, biosensing, bioremediation, and as a means to combat pathogens by direct physical binding through a well-defined, multivalent 2D material.

Uniform, Large-Area, Highly Ordered Peptoid Monolayer and Bilayer Films for Sensing Applications

The production of atomically defined, uniform, large-area 2D materials remains as a challenge in materials chemistry. Many methods to produce 2D nanomaterials suffer from limited lateral film dimensions, lack of film uniformity, or limited chemical diversity. These issues have hindered the application of these materials to sensing applications, which require large-area uniform films to achieve reliable and consistent signals. Furthermore, the development of a 2D material system that is biocompatible and readily chemically tunable has been a fundamental challenge. Here, we report a simple, robust method for the production of large-area, uniform, and highly tunable monolayer and bilayer films, from sequence-defined peptoid polymers, and their application as highly selective molecular recognition elements in sensor production. Monolayers and bilayer films were produced on the centimeter scale using Langmuir-Blodgett methods and exhibited a high degree of uniformity and ordering as evidenced by atomic force microscopy, electron diffraction, and grazing incidence X-ray scattering. We further demonstrated the utility of these films in sensing applications by employing the biolayer interferometry technique to detect the specific binding of the pathogen derived proteins, shiga toxin and anthrax protective antigen, to peptoid-coated sensors.

The design and biomedical applications of self-assembled two-dimensional organic biomaterials

Self-assembling 2D organic biomaterials exhibit versatile abilities for structural and functional tailoring, as well as high potential for biomedical applications.

Self-assembly of bioactive peptides, peptide conjugates, and peptide mimetic materials

Molecular self-assembly is a multi-disciplinary field of research, with potential chemical and biological applications. One of the main driving forces of self-assembly is molecular amphiphilicity, which can drive formation of complex and stable nanostructures. Self-assembling peptide and peptide conjugates have attracted great attention due to their biocompatibility, biodegradability and biofunctionality. Understanding assembly enables the better design of peptide amphiphiles which may form useful and functional nanostructures. This review covers self-assembly of amphiphilic peptides and peptide mimetic materials, as well as their potential applications.

Structure-Rheology Relationship in Nanosheet-Forming Peptoid Monolayers

Peptoid nanosheets are novel protein-mimetic materials that form from the supramolecular assembly of sequence-defined peptoid polymers. The component polymer chains organize themselves via a unique mechanism at the air-water interface, in which the collapse of a compressed peptoid monolayer results in free-floating, bilayer nanosheets. To impart functionality into these bilayer materials, structural engineering of the nanosheet-forming peptoid strand is necessary. We previously synthesized a series of peptoid analogues with modifications to the hydrophobic core in order to probe the nanosheet tolerance to different packing interactions. Although many substitutions were well-tolerated, routine surface pressure measurements and monolayer collapse isotherms were insufficient to explain which molecular processes contributed to the ability or inability of these peptoid analogues to form nanosheets. Here, we show that surface dilational rheology measurements of assembled peptoid monolayers at the air-water interface provide great insight into their nanosheet-forming ability. We find that a key property required for nanosheet formation is the ability to assemble into a solidlike monolayer in which the residence time of the peptoid within the monolayer is very long and does not exchange rapidly with the subphase. These collapse-competent monolayers typically have a characteristic time of diffusion-exchange values, τ_D, of >5000 s. Thus, rheological measurements provide an efficient method for assessing the nanosheet-forming ability of peptoid analogues. Results from these studies can be used to guide the rational design of peptoids for assembly into functional nanosheets.

Colloidal Assembly of Hierarchically Structured Porous Supraparticles from Flower-Shaped Protein-Inorganic Hybrid Nanoparticles

Mimicry of biomineralization is an attractive strategy to fabricate nanostructured hybrid materials. While biomineralization involves processes that organize hybrid clusters into complex structures with hierarchy, arrangement of artificial components in biomimetic approaches has been challenging. Here, we demonstrate self-assembly of hierarchically structured porous supraparticles from protein-inorganic hybrid flower-shaped (FS) nanoparticle building blocks. In our strategy, the FS nanoparticles self-assemble via high valency interactions in combination with interfacial adsorption and compression. The flower-like shape directed robust assembly of the FS nanoparticles into chain-like clusters in solution, which were further assembled into spherical supraparticles during rotation of FS nanoparticle solution. Continuously expanding and contracting the air-water interface during rotation catalyzed assembly of FS nanoparticle clusters, indicating that adsorption and compression of the building blocks at the interface were critical. The resulting supraparticles contain hierarchical pores which are translated from the structural characteristics of individual FS nanoparticle building blocks. The protein-inorganic supraparticles are protein-compatible, have large surface area, and provide specific affinity recognition for robust protein immobilization. A variety of functional proteins could be immobilized to the porous supraparticles, making it a general platform that could provide benefits for many applications.

Highly stable and self-repairing membrane-mimetic 2D nanomaterials assembled from lipid-like peptoids

An ability to develop sequence-defined synthetic polymers that both mimic lipid amphiphilicity for self-assembly of highly stable membrane-mimetic 2D nanomaterials and exhibit protein-like functionality would revolutionize the development of biomimetic membranes. Here we report the assembly of lipid-like peptoids into highly stable, crystalline, free-standing and self-repairing membrane-mimetic 2D nanomaterials through a facile crystallization process. Both experimental and molecular dynamics simulation results show that peptoids assemble into membranes through an anisotropic formation process. We further demonstrated the use of peptoid membranes as a robust platform to incorporate and pattern functional objects through large side-chain diversity and/or co-crystallization approaches. Similar to lipid membranes, peptoid membranes exhibit changes in thickness upon exposure to external stimuli; they can coat surfaces in single layers and self-repair. We anticipate that this new class of membrane-mimetic 2D nanomaterials will provide a robust matrix for development of biomimetic membranes tailored to specific applications.

Biomimetic membranes can be used for various applications such as sensors and separations. Here, Chen et al. report the assembly of lipid-like peptoids into stable and self-repairing 2D membrane nanomaterials that change in thickness when under external stimuli.

Design, Synthesis, Assembly, and Engineering of Peptoid Nanosheets

Two-dimensional (2D) atomically defined organic nanomaterials are an important material class with broad applications. However, few general synthetic methods exist to produce such materials in high yields and to precisely functionalize them. One strategy to form ordered 2D organic nanomaterials is through the supramolecular assembly of sequence-defined synthetic polymers. Peptoids, one such class of polymer, are designable bioinspired heteropolymers whose main-chain length and monomer sequence can be precisely controlled. We have recently discovered that individual peptoid polymers with a simple sequence of alternating hydrophobic and ionic monomers can self-assemble into highly ordered, free-floating nanosheets. A detailed understanding of their molecular structure and supramolecular assembly dynamics provides a robust platform for the discovery of new classes of nanosheets with tunable properties and novel applications. In this Account, we discuss the discovery, characterization, assembly, molecular modeling, and functionalization of peptoid nanosheets. The fundamental properties of peptoid nanosheets, their mechanism of formation, and their application as robust scaffolds for molecular recognition and as templates for the growth of inorganic minerals have been probed by an arsenal of experimental characterization techniques (e.g., scanning probe, electron, and optical microscopy, X-ray diffraction, surface-selective vibrational spectroscopy, and surface tensiometry) and computational techniques (coarse-grained and atomistic modeling). Peptoid nanosheets are supramolecular assemblies of 16-42-mer chains that form molecular bilayers. They span tens of microns in lateral dimensions and freely float in water. Their component chains are highly ordered, with chains nearly fully extended and packed parallel to one another as a result of hydrophobic and electrostatic interactions. Nanosheets form via a novel interface-catalyzed monolayer collapse mechanism. Peptoid chains first assemble into a monolayer at either an air-water or oil-water interface, on which peptoid chains extend, order, and pack into a brick-like pattern. Upon mechanical compression of the interface, the monolayer buckles into stable bilayer structures. Recent work has focused on the design of nanosheets with tunable properties and functionality. They are readily engineerable, as functional monomers can be readily incorporated onto the nanosheet surface or into the interior. For example, functional hydrophilic "loops" have been displayed on the surfaces of nanosheets. These loops can interact with specific protein targets, serving as a potentially general platform for molecular recognition. Nanosheets can also bind metal ions and serve as 2D templates for mineral growth. Through our understanding of the formation mechanism, along with predicted features ascertained from molecular modeling, we aim to further design and synthesize nanosheets as robust protein mimetics with the potential for unprecedented functionality and stability.

Improved chemical and mechanical stability of peptoid nanosheets by photo-crosslinking the hydrophobic core

Peptoid nanosheets can be broadly functionalized for a variety of applications. However, they are susceptible to degradation when exposed to chemical or mechanical stress. To improve their strength, photolabile monomers were introduced in order to crosslink the nanosheet interior. Photo-crosslinking produced a more robust material that can survive sonication, lyophilization, and other biochemical manipulations.

Two-dimensional polymers: concepts and perspectives

The emerging class of 2D polymers is explored from physicochemical, synthetic, and analytical viewpoints. Prospects for their properties are provided.

Two-Dimensional Peptide and Protein Assemblies

Two-dimensional nanoscale assemblies (nanosheets) represent a promising structural platform to arrange molecular and supramolecular substrates with precision for integration into devices. This nanoarchitectonic approach has gained significant traction over the last decade, as a general concept to guide the fabrication of functional nanoscale devices. Sequence-specific biomolecules, e.g., peptides and proteins, may be considered excellent substrates for the fabrication of two-dimensional nanoarchitectonics. Molecular level instructions can be encoded within the sequence of monomers, which allows for control over supramolecular structure if suitable design principles could be elaborated. Due to the complexity of interactions between protomers, the development of principles aimed toward rational design of peptide and protein nanosheets is at a nascent stage. This review discusses the known two-dimensional peptide and protein assemblies to further our understanding of how to control the arrangement of molecules in two-dimensions.

Insights into Peptoid Helix Folding Cooperativity from an Improved Backbone Potential

Peptoids (N-substituted oligoglycines) are biomimetic polymers that can fold into a variety of unique structural scaffolds. Peptoid helices, which result from the incorporation of bulky chiral side chains, are a key peptoid structural motif whose formation has not yet been accurately modeled in molecular simulations. Here, we report that a simple modification of the backbone φ-angle potential in GAFF is able to produce well-folded cis-amide helices of (S)-N-(1-phenylethyl)glycine (Nspe), consistent with experiment. We validate our results against both QM calculations and NMR experiments. For this latter task, we make quantitative comparisons to sparse NOE data using the Bayesian Inference of Conformational Populations (BICePs) algorithm, a method we have recently developed for this purpose. We then performed extensive REMD simulations of Nspe oligomers as a function of chain length and temperature to probe the molecular forces driving cooperative helix formation. Analysis of simulation data by Lifson-Roig helix-coil theory show that the modified potential predicts much more cooperative folding for Nspe helices. Unlike peptides, per-residue entropy changes for helix nucleation and extension are mostly positive, suggesting that steric bulk provides the main driving force for folding. We expect these results to inform future work aimed at predicting and designing peptoid peptidomimetics and tertiary assemblies of peptoid helices.

The Organic Flatland-Recent Advances in Synthetic 2D Organic Layers

Ultrathin, 2D organic layers of sub-ten nanometer thicknesses and high aspect ratios have received a great deal of attention for their graphene-like topological features and emerging properties. Rational synthetic strategies have led to the realization of periodic 2D layers with unprecedented structural precision. Herein, recent progress on the synthesis of 2D organic layers, including methods based on both non-covalent and covalent interactions, is summarized, and potential applications are highlighted. Such 2D organic nanostructures have a brilliant future as prospective multifunctional materials, showing great promise as platforms for engineering novel optoelectronic, interfacial, and bioactive properties.

Sequence Programmable Peptoid Polymers for Diverse Materials Applications

Polymer sequence programmability is required for the diverse structures and complex properties that are achieved by native biological polymers, but efforts towards controlling the sequence of synthetic polymers are, by comparison, still in their infancy. Traditional polymers provide robust and chemically diverse materials, but synthetic control over their monomer sequences is limited. The modular and step-wise synthesis of peptoid polymers, on the other hand, allows for precise control over the monomer sequences, affording opportunities for these chains to fold into well-defined nanostructures. Hundreds of different side chains have been incorporated into peptoid polymers using efficient reaction chemistry, allowing for a seemingly infinite variety of possible synthetically accessible polymer sequences. Combinatorial discovery techniques have allowed the identification of functional polymers within large libraries of peptoids, and newly developed theoretical modeling tools specifically adapted for peptoids enable the future design of polymers with desired functions. Work towards controlling the three-dimensional structure of peptoids, from the conformation of the amide bond to the formation of protein-like tertiary structure, has and will continue to enable the construction of tunable and innovative nanomaterials that bridge the gap between natural and synthetic polymers.

Peptoid nanosheets exhibit a new secondary-structure motif

A promising route to the synthesis of protein-mimetic materials that are capable of complex functions, such as molecular recognition and catalysis, is provided by sequence-defined peptoid polymers--structural relatives of biologically occurring polypeptides. Peptoids, which are relatively non-toxic and resistant to degradation, can fold into defined structures through a combination of sequence-dependent interactions. However, the range of possible structures that are accessible to peptoids and other biological mimetics is unknown, and our ability to design protein-like architectures from these polymer classes is limited. Here we use molecular-dynamics simulations, together with scattering and microscopy data, to determine the atomic-resolution structure of the recently discovered peptoid nanosheet, an ordered supramolecular assembly that extends macroscopically in only two dimensions. Our simulations show that nanosheets are structurally and dynamically heterogeneous, can be formed only from peptoids of certain lengths, and are potentially porous to water and ions. Moreover, their formation is enabled by the peptoids' adoption of a secondary structure that is not seen in the natural world. This structure, a zigzag pattern that we call a Σ('sigma')-strand, results from the ability of adjacent backbone monomers to adopt opposed rotational states, thereby allowing the backbone to remain linear and untwisted. Linear backbones tiled in a brick-like way form an extended two-dimensional nanostructure, the Σ-sheet. The binary rotational-state motif of the Σ-strand is not seen in regular protein structures, which are usually built from one type of rotational state. We also show that the concept of building regular structures from multiple rotational states can be generalized beyond the peptoid nanosheet system.