Two-dimensional, “sheet-like” nanostructures are commonly employed in biological systems such as cell membranes, and their unique properties have inspired interest in materials such as graphene. Now, Berkeley Lab scientists have made the largest two-dimensional polymer crystal self-assembled in water to date. This entirely new material mirrors the structural complexity of biological systems with the durable architecture needed for membranes or integration into functional devices.
These self-assembling sheets are made of peptoids, engineered polymers that can flex and fold like proteins while maintaining the robustness of manmade materials. Each sheet is just two molecules thick yet hundreds of square micrometers in area—akin to ‘molecular paper’ large enough to be visible to the naked eye. What’s more, unlike a typical polymer, each building block in a peptoid nanosheet is encoded with structural ‘marching orders’—suggesting its properties can be precisely tailored to an application. For example, these nanosheets could be used to control the flow of molecules, or serve as a platform for chemical and biological detection.
“Our findings bridge the gap between natural biopolymers and their synthetic counterparts, which is a fundamental problem in nanoscience,” said Ronald Zuckermann, Director of the Biological Nanostructures Facility at the Molecular Foundry. “We can now translate fundamental sequence information from proteins to a non-natural polymer, which results in a robust synthetic nanomaterial with an atomically-defined structure.”
The building blocks for peptoid polymers are cheap, readily available and generate a high yield of product, providing a huge advantage over other synthesis techniques. Zuckermann, instrumental in developing the Foundry’s one-of-a-kind robotic synthesis capabilities, worked with his team of coauthors to form libraries of peptoid materials. After screening many candidates, the team landed upon the unique combination of polymer building blocks that spontaneously formed peptoid nanosheets in water.
This research is reported in a paper titled, “Free floating ultra-thin two-dimensional crystals from sequence-specific peptoid polymers,” appearing in the journal Nature Materials
The design and synthesis of protein-like polymers is a fundamental challenge in materials science. A biomimetic approach is to explore the impact of monomer sequence on non-natural polymer structure and function. We present the aqueous self-assembly of two peptoid polymers into extremely thin two-dimensional (2D) crystalline sheets directed by periodic amphiphilicity, electrostatic recognition and aromatic interactions. Peptoids are sequence-specific, oligo-N-substituted glycine polymers designed to mimic the structure and functionality of proteins. Mixing a 1:1 ratio of two oppositely charged peptoid 36mers of a specific sequence in aqueous solution results in the formation of giant, free-floating sheets with only 2.7 nm thickness. Direct visualization of aligned individual peptoid chains in the sheet structure was achieved using aberration-corrected transmission electron microscopy. Specific binding of a protein to ligand-functionalized sheets was also demonstrated. The synthetic flexibility and biocompatibility of peptoids provide a flexible and robust platform for integrating functionality into defined 2D nanostructures.