Three-Dimensional Imaging the Crystalline Structure of Polypeptoid Nanosheet with Atomic Resolution*

Electron microscopy imaging of three-dimensional structures in soft materials with atomic resolution is challenging because soft materials are unstable under the electron beam, and techniques such as x-ray scattering or diffraction are not able to provide atomic resolution phase information in position space. The experiments were conducted on self-assembled crystalline polypeptoid nanosheets. Low-dose cryogenic electron microscopy micrographs were obtained from frozen hydrated crystalline nanosheets on the novel ultra-flat supporting grid at different tilting angles. A hybrid processing of crystallographic, tomography and single particle methods, developed for cryo-electron microscopy of biological macromolecules, was used to resolve the structure of crystals with atomic resolution in three dimensions. Our approach is robust and enable direct visualization of the arrangement of polypeptoid backbones and side chains in crystalline nanosheets. It also revealed the effect of side chain chemistry on the crystalline structure of polypeptoid nanosheets.