Constructing synthetic-protein assemblies from de novo designed 3₁₀ helices

Compared with the iconic α helix, 3 10 helices occur much less frequently in protein structures. The different 3 10 -helical parameters lead to energetically less favourable internal energies, and a reduced tendency to pack into defined higher-order structures. Consequently, in natural proteins, 3 10 helices rarely extend past 6 residues, and do not form regular supersecondary, tertiary, or quaternary interactions. Here, we show that despite their absence in nature, synthetic protein-like assemblies can be built from 3 10 helices. We report the rational design, solution-phase characterisation, and an X-ray crystal structure for water-soluble bundles of 3 10 helices with consolidated hydrophobic cores. The design uses 6-residue repeats informed by analysing natural 3 10 helices, and incorporates aminoisobutyric acid residues. Design iterations reveal a tipping point between α-helical and 3 10 -helical folding, and identify features required for stabilising assemblies in this unexplored region of protein-structure space.