X‑ray and Electron Diffraction Observations of Steric Zipper Interactions in Metal-Induced Peptide Cross‑β Nanostructures

The steric zipper is a common hydrophobic packing structure of peptide side chains that forms between two adjacent β-sheet layers in amyloid and related fibrils. Although previous studies have revealed that peptide fragments derived from native protein sequences exhibit steric zipper structures, their de novo designs have rarely been studied. Herein, steric zipper structures were artificially constructed in the crystalline state by metal-induced folding and assembly of tetrapeptide fragments Boc-3pa-X¹-3pa-X²-OMe (3pa: β-(3-pyridyl)-l-alanine; X¹ and X²: hydrophobic amino acids). Crystallographic studies revealed two types of packing structures, interdigitation and hydrophobic contact, that result in a class 1 steric zipper geometry when the X¹ and X² residues contain alkyl side chains. Furthermore, a class 3 steric zipper geometry was also observed for the first time among any reported steric zippers when using tetrapeptide fragments with (X¹, X²) = (Thr, Thr) and (Phe, Leu). The system could also be extended to a knob–hole-type zipper using a pentapeptide sequence.