Rational design of supramolecular nanomaterials fundamentally depends upon an atomic-level understanding of their structure and how it responds to chemical modifications. Here we studied a series of crystalline diblock copolypeptoids by a combination of sequence-controlled synthesis, cryogenic transmission electron microscopy, and molecular dynamics simulation. This family of amphiphilic polypeptoids formed free-floating 2-dimensional monolayer nanosheets, in which individual polymer chains and their relative orientations could be directly observed. Furthermore, bromine atom side-chain substituents in nanosheets were directly visualized by cryogenic transmission electron microscopy, revealing atomic details in position space inaccessible by conventional scattering techniques. While the polypeptoid backbone conformation was conserved across the set of molecules, the nanosheets exhibited different lattice packing geometries dependent on the aromatic side chain para substitutions. Peptoids are inherently achiral, yet we showed that sequences containing an asymmetric aromatic substitution pattern pack with alternating rows adopting opposite backbone chiralities. These atomic-level insights into peptoid nanosheet crystal structure provide guidance for the future design of bioinspired nanomaterials with more precisely controlled structures and properties.

Polypeptoids, a class of peptidomimetic polymers, have emerged at the forefront of macromolecular and supramolecular science and engineering as the technological relevance of these polymers continues to be demonstrated. The chemical and structural diversity of polypeptoids have enabled access to and adjustment of a variety of physicochemical and biological properties (eg, solubility, charge characteristics, chain conformation, HLB, thermal processability, degradability, cytotoxicity and immunogenicity). These attributes have made this synthetic polymer platform a potential candidate for various biomedical and biotechnological applications. This review will provide an overview of recent development in synthetic methods to access polypeptoid polymers with well-defined structures and highlight some of the fundamental physicochemical and biological properties of polypeptoids that are pertinent to the future development of functional materials based on polypeptoids. | I N TR ODU C TI ONPolypeptoids composed of N-substituted polyglycine backbones are structural mimics of polypeptides ( Figure 1). Because of N-substitution, polypeptoids lack stereogenic centers and hydrogen bonding interactions along the main chains, in sharp contrast to polypeptides.As a result, the global conformations of polypeptoids are strongly dependent on the N-substituent structures, giving rise to random coils or well-defined secondary structures [eg, polyproline I (PPI) helix [1][2][3][4][5][6] and R-sheets] [7][8][9][10][11][12] that are reminiscent of those of polypeptides. The polypeptoid backbone containing tertiary amide linkages is highly polar and hydrophilic. The physicochemical properties of polypeptoids can be tailored by the N-substituent structures, enabling control over the hydrophilicity and lipophilicity balance (HLB), charge characteristics, [13,14] backbone conformation, [1][2][3][4][5][6][7][8][9][10][11][12] solubility, [15][16][17][18][19][20] thermal and crystallization properties of the polypeptoids. [21][22][23][24] Without extensive hydrogen bonding, polypeptoids are thermally processable similar to conventional thermoplastics, [20][21][22][23][24] whereas polypeptides undergo thermal degradation before they can be melt-processed due to the extensive hydrogen bonding interactions. While polypeptoids exhibited enhanced proteolytic stability relative to peptides, [25,26] they can be oxidatively degraded under conditions that mimic tissue inflammation, [27] suggesting their potential in vivo uses as biodegradable materials.Recent advances in the controlled polymerization methods have enabled access to a suite of structurally well-defined polypeptoids with various N-substituent structures and molecular architectures, setting the stage for the future development of polypeptoid materials for various targeted applications. Several review articles on the synthesis, properties, and application of polypeptoids for biomedical or nonbiomedical uses have been published in recent years. [28][29][30][31][32] As a result, this...

Polypeptoid homopolymers and block copolymers undergo thermal transitions in the solid state that can be detected by differential scanning calorimetry (DSC), but so far there is neither consensus on the underpinnings of the observed thermal transitions, nor consensus on the expected number of transitions. We synthesized a series of polypeptoid diblock copolymers containing hydrophobic alkyl sidechains and hydrophilic ethyleneoxide sidechains, systematically varying side-chain length (S), backbone mainchain length (N), block copolymer composition (n/m), and N-terminal group, 1 and studied their thermal transitions by a combination of X-ray scattering and DSC. The thermal transitions are largely unaffected by S, N, and n/m, but strongly affected by the N-terminal group. Block copolymers with an acetylated N-terminus exhibit two thermal transitions. The low temperature thermal transition is due to a transition from a crystalline phase to a sanidic liquid crystalline mesophase. The molecules adopt planar, board-like conformations and are arranged in a rectangular crystal lattice with extended backbones that run parallel to each other. The side-chains extend on either side and are located within the plane of the backbone. The liquid crystalline phase is characterized by conformational disorder in dimensions normal to the molecular plane. The high temperature thermal transition is due to melting of the liquid crystalline phase to give an isotropic phase. Block copolymers with a free N-terminus (non-acetylated) exhibit only one thermal transition, and similar out-of-plane conformational disorder. This disorder appears to be due to a difference in the pendant side chain display angle of the terminal nitrogen atom.

DNA nanotechnology has established approaches for designing programmable and precisely controlled nanoscale architectures through specific Watson−Crick base-pairing, molecular plasticity, and intermolecular connectivity. In particular, superior control over DNA origami structures could be beneficial for biomedical applications, including biosensing, in vivo imaging, and drug and gene delivery. However, protecting DNA origami structures in complex biological fluids while preserving their structural characteristics remains a major challenge for enabling these applications. Here, we developed a class of structurally well-defined peptoids to protect DNA origamis in ionic and bioactive conditions and systematically explored the effects of peptoid architecture and sequence dependency on DNA origami stability. The applicability of this approach for drug delivery, bioimaging, and cell targeting was also demonstrated. A series of peptoids (PE1–9) with two types of architectures, termed as “brush” and “block,” were built from positively charged monomers and neutral oligo-ethyleneoxy monomers, where certain designs were found to greatly enhance the stability of DNA origami. Through experimental and molecular dynamics studies, we demonstrated the role of sequence-dependent electrostatic interactions of peptoids with the DNA backbone. We showed that octahedral DNA origamis coated with peptoid (PE2) can be used as carriers for anticancer drug and protein, where the peptoid modulated the rate of drug release and prolonged protein stability against proteolytic hydrolysis. Finally, we synthesized two alkyne-modified peptoids (PE8 and PE9), conjugated with fluorophore and antibody, to make stable DNA origamis with imaging and cell-targeting capabilities. Our results demonstrate an approach toward functional and physiologically stable DNA origami for biomedical applications.