Polypeptides are polymers of amino acids joined by amide bonds between the carboxyl group of one residue and the amino group of the next ^[1]. The backbone repeats over three atoms per residue, and the dihedral angles of that backbone decide whether the chain adopts an alpha-helix, a beta-sheet or a random coil ^[1]. Sequence, and not composition alone, fixes the conformation, so two polymers with identical amino acid content can fold differently ^[2]. Synthesis proceeds either by stepwise coupling of protected amino acids or by ring-opening polymerisation of N-carboxyanhydrides, and the routes differ in the control they give over sequence and dispersity ^[3]. The amide bond is cleaved by proteases, which makes the backbone degradable under physiological conditions ^[2].