Atomic-scale imaging of self-assembled polypeptoid crystals with varying molecular side chains*

Crystal engineering requires a knowledge of the weak noncovalent interactions that produce a three-dimensional crystal structure. In spite of significant progress, it is not yet possible to design molecules that would yield a targeted crystal structure due to uncertainties in the relative importance of these interactions. Polypeptoids are sequence-defined polymers that have a similar structure to peptides, except the side chain is appended to the nitrogen rather than the alpha carbon in the backbone. When amphiphilic diblock copolypeptoids are dissolved in water, they self-assemble into well-ordered planar crystals. The resulting crystals are sensitive to radiation damage when imaged in a transmission electron microscope. In order to achieve atomic-scale images of these crystals, techniques from the structural biology community are adopted to produce images without damaging the crystals. In this work, a series of polypeptoids with a phenyl, amino, or methyl group in the side chain is created and the crystal structure is studied using atomic-scale TEM images.  The atomic-scale information obtained from TEM images enriches our knowledge of the competing factors that determine polypeptoid crystal structure and is a step towards enabling crystal engineering of these materials.