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Abstract

Polyglutamine (polyQ) expansion beyond a pathological threshold is associated with a number of neurodegenerative diseases. The polyQ tract of several proteins, such as androgen receptor (AR)¹, huntingtin² and CBP³, can adopt an α-helix structure, in which the helical propensity is influenced by the flanking regions and the polyQ length. We have recently reported that the α-helix formed by the polyQ tract of AR is stabilized by unusual bifurcated hydrogen bonds where the side and main chains of glutamine residues simultaneously donate a hydrogen to the backbone carbonyl of residue i-4.¹ Using a combination of solution NMR and molecular dynamics we have studied in detail how the sequence context influences the helical content of the polyQ tract of AR and expanded the analysis to the tract of the TBP protein. Finally, we have exploited our observations to present rules to design peptides that fold into short single α-helices by concatenating glutamine side chain to main chain hydrogen bonds. The resulting peptides are uncharged, contain only natural amino acids, and their sequences can be optimised to interact with specific targets. An important feature of these peptides is their versatility: several residues can act as efficient H-bond acceptors and the peptides can also incorporate a pH-sensitive switch or salt bridges to further stabilize helicity. As a proof of concept, we have designed two peptides to bind the globular target RAP74-CTD. Remarkably, we show several examples of natural occurring sequences combining such strategies that may represent a new class of uncharged single α-helices (SAH) that may have remained undetected.