Science 340, 595–599 (2013)

Credit: © 2013 AAAS

As illustrated by fullerenes (and footballs), a sphere cannot be tiled with regular hexagons only; pentagons are always needed. Derek Woolfson and colleagues now show that coiled-coil peptides can self-assemble into sheets made of hexagons, and that these close up to make spheres. The team designed and synthesized two types of tripod-like module — each made of three distinct coils arranged roughly in parallel — with matching (that is, attractive) ends. When the aggregates were mixed in solution, they formed hexagonal (graphite-like) sheets, which closed in on themselves to form spheres each about 80 nm in diameter. Using atomistic simulations, the researchers show that the formation of closed cages is driven by the minimization of network edges (that is, of unmatched peptide coils), and speculate that the necessary imperfections needed for the network to close up are possible because of the flexibility of the modules, which may allow for mismatched pairings. This modular approach for peptide self-assembly of porous cages may find advantageous uses in drug delivery and synthetic biology.