news of the week APRIL 19, 2010 EDITED BY WILLIAM G. SCHULZ & LAUREN K. WOLF
PEPTOID CRYSTALS: Peptide analogs
self-assemble to form versatile ‘molecular paper’
P
EPTIDE ANALOGS called peptoids have been
developed for biomedical applications and potential drug use since their discovery in the early 1990s. Now, researchers report that they can also be used to create defined nanostructures that could have a range of applications in sensors, electronic components, and other devices (Nat. Mater., DOI: 10.1038/ nmat2742). The work “demonstrates for the first time the utility of a bioinspired polymer to create one of the largest two-dimensional organic crystals known,” says Ronald N. Zuckermann of the Molecular Foundry at Lawrence Berkeley National Laboratory (LBNL). Zuckermann is the lead investigator of the study and was a member of the group at Chiron Corp. (now part of Novartis) that developed peptoids, which are oligomers of N-substituted glycines. In characterizing the crystals, the group achieved the first direct visualization of individual polymer chains by transmission electron microscopy (TEM). The peptoid nanosheet crystals are “novel, tunable, and not biodegradable, which is important for making biodevices,” comments Shuguang Zhang of MIT, a leader in amphiphilic peptide self-assembly. “They’re like ‘molecular paper,’ but about 2,000 times less thick than regular paper. And functionalizing them for molecular recognition applications is straightforward.” “This is a landmark publication that takes an important step toward the design of functional materials by applying biological principles to synthetic macromolecules,” says Virgil Percec of the University of Pennsylvania, an expert in molecular self-assembly. “Applications in areas such as imaging, membrane mimetics, sensors, and separations could emerge from these 2-D crystalline sheets.” Zuckermann and coworkers created the crystals by experimenting with mixtures of various peptoids. They found that two oppositely charged peptoid 36-mers with a specific sequence combine in aqueous solution to form free-floating sheets only 2.7 nm thick. The peptoid crystals resemble peptide β-sheets. “But β-sheets always have a twist that is a direct result of peptides’ inherent chirality,” Zuckermann says. “Because peptoids are completely achiral, they can assem-
ble into extended planar structures with no curvature.” The researchers used several microscopy techniques to characterize the crystals. With TEM, they were able to visualize individual peptoid chains in the sheets—a level of structural discrimination that is unique for the technique. This detail was possible because “the sheets hang together even under high vacuum and under intense electron beam irradiation,” Zuckermann explains. Also, the transmission electron microscope used in the study—TEAM 0.5 at LBNL’s National Center for Electron Microscopy—“is one of the newest and most powerful ever built,” he adds. The researchers demonstrated that they can “display biologically active ligands on the surface of the sheets, and these densely functionalized sheets can bind proteins specifically,” making the crystals “an
Peptoid nanosheets, shown in an optical microscopy image (A), form in high yield; have sharp edges, as shown in an SEM image (B); are composed of extended polymer chains lined up side by side in a bilayer (C); and can be observed as individual polymer chains, shown by aberrationcorrected TEM (D). NAT. MATER.
BIOINSPIRED MATERIAL DEBUTS
ideal platform for chemical and biological sensing,” Zuckermann says. “Also, the extended planar form should be ideal for the fabrication of electronic components and other higher order structures. “For sure, these nanosheets are plasma-membrane mimics, which could have application in separation science and in the selective transport of ions, small molecules, and gases,” Zuckermann continues. For example, photosynthetic membrane proteins tend to be unstable when isolated, but embedding them in peptoid crystals could stabilize them, making it possible to create peptoid-crystal-based biosolar cells, Zhang suggests. Peptoid crystals, he says, “should have general interest for biologists and materials scientists in the nanodevices field.”—STU BORMAN
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