Atomic-level engineering and imaging of polypeptoid crystal lattices

Abstract

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.

Keywords: cryo-TEM; nanosheets; peptoid polymers; polymer amphiphiles; supramolecular assembly.

Fig. 1.

Assembly of amphiphilic diblock copolypeptoids into nanosheets via evaporation of a THF/water solution. (A) Chemical structures of diblock copolypeptoids 1 through 10. The Nte block of compound 5 was increased to 6 monomers to increase the water solubility of this peptoid. (B and D) Representative TEM images of nanosheets. (C and E) Representative AFM images of nanosheets. The inset graphs are thickness profiles of nanosheets. (F) Proposed nanosheet structures of amphiphilic diblock copolypeptoids: Polypeptoid chains are packed antiparallel along the c direction and parallel along the a direction. The hydrophobic block (yellow color) is crystalline and the hydrophilic block (blue color) is amorphous.

Fig. 2.

Nanosheet assembled from Nte₄-Npe₆ (1). (A) Chemical structure of Nte₄-Npe₆ (1). (B) Molecular model of sheet 1. The molecules are packed antiparallel along c direction and parallel along a direction. (C) Cryo-TEM image of sheet 1 from b direction (top view) showing parallel V shapes along c direction. (D) Top view of the hydrophobic domain in B from b direction showing parallel V shapes along c direction. The structure is overlapped with cryo-TEM image shown in C.

Fig. 3.

Nanosheet assembled from Nte₄-N4Brpe₆ (4). (A) Chemical structure of Nte₄-N4Brpe₆ (4). (B) Molecular model of sheet 4. The molecules are packed antiparallel along c direction and parallel along a direction. (C) Cryo-TEM image of sheet 4 from b direction (top view) showing antiparallel V shapes along the c direction. The Br atoms show a tip-to-tip packing (red box). (D) Top view of the hydrophobic domain in B from b direction showing antiparallel V shapes along the c direction. The structure is overlapped with cryo-TEM image shown in C.

Fig. 4.

Nanosheet assembled from Nte₄-(N4BrpeNpe)₃ (9). (A) Chemical structure of Nte₄-(N4BrpeNpe)₃ (9). (B) Molecular model of sheet 9. The molecules are packed antiparallel along c direction and parallel along a direction. The green and red planes show the opposite chirality of adjacent backbones. (C) Cryo-TEM image of sheet 9 from b direction (top view) showing parallel V shapes along the c direction. (D) Top view of the hydrophobic domain in B from b direction showing parallel V shapes along the c direction. The structure is overlapped with cryo-TEM image shown in C.

Fig. 5.

DSC measurements for dry sheets 1, 4, 7, and 9.

Fig. 6.

WAXS measurements of dry sheets 1, 4, 7, and 9 at room temperature. (A) WAXS measurements showing the peaks corresponding to the c dimension. The peak at q = 0.4 Å⁻¹ is from the Kapton windows. (B) WAXS measurements showing the peaks corresponding to the a dimension.