Rippled β-Sheet Formation by an Amyloid-β Fragment Indicates Expanded Scope of Sequence Space for Enantiomeric β-Sheet Peptide Coassembly

In 1953, Pauling and Corey predicted that enantiomeric β-sheet peptides would coassemble into so-called "rippled" β-sheets, in which the β-sheets would consist of alternating l- and d-peptides. To date, this phenomenon has been investigated primarily with amphipathic peptide sequences composed of alternating hydrophilic and hydrophobic amino acid residues. Here, we show that enantiomers of a fragment of the amyloid-β (Aβ) peptide that does not follow this sequence pattern, amyloid-β (16-22), readily coassembles into rippled β-sheets. Equimolar mixtures of enantiomeric amyloid-β (16-22) peptides assemble into supramolecular structures that exhibit distinct morphologies from those observed by self-assembly of the single enantiomer pleated β-sheet fibrils. Formation of rippled β-sheets composed of alternating l- and d-amyloid-β (16-22) is confirmed by isotope-edited infrared spectroscopy and solid-state NMR spectroscopy. Sedimentation analysis reveals that rippled β-sheet formation by l- and d-amyloid-β (16-22) is energetically favorable relative to self-assembly into corresponding pleated β-sheets. This work illustrates that coassembly of enantiomeric β-sheet peptides into rippled β-sheets is not limited to peptides with alternating hydrophobic/hydrophilic sequence patterns, but that a broader range of sequence space is available for the design and preparation of rippled β-sheet materials.