Abstract
Our entire living world is constructed from just seven major protein secondary structures: α-helices, β-sheets, 310-helices, π-helices, turns, bends, and polyPro-helices. All other polypeptide is classified as unstructured coil. Because only a tiny fraction of theoretically possible protein sequences have ever been sampled by evolutionary processes, it is conceivable that other secondary structures remain undiscovered. Using physical and mathematical modelling, we identified an unprecedented structure with spiral geometry. Upon using this to search uncharted polypeptide sequence space in silico, we discovered sequences that appeared to support the spiral structure. Theta-spirals (ϑ-spirals) were confirmed in vitro using NMR spectroscopy, becoming the eighth protein secondary structure, and revealing a new class of biological building block: noncanonical protein secondary structures.