Protein-mimicking molecules called peptoids can take on a never-before-seen secondary structure when they form, researchers report. “We’re interested in making synthetic versions of nature’s structures,” says team leader Stephen Whitelam of Lawrence Berkeley National Laboratory. “We found an interesting nanomaterial that formed through a rule that nature doesn’t use.” Whitelam, Ranjan V. Mannige, and coworkers used molecular dynamics simulations to show that peptoid nanosheets are built from a zigzag structure they call a sigma (Σ) strand (Nature 2015, DOI: 10.1038/nature15363). Protein backbones usually twist in only one direction, which leads to the familiar α helices and β strands. In peptoid backbones, in contrast, adjoining monomers twist in opposite directions, rendering the strand approximately linear. The Σ strands assemble into an extended two-dimensional structure. The proposed structure is consistent with atomic force microscopy and powder X-ray diffraction measurements of the peptoid nanosheets. The researchers predict that with proper choice of side-chain interactions other secondary structure motifs should be possible that can be used to design three-dimensional nanostructures.