Formation of rippled β-sheets from mixed chirality linear and cyclic peptides-new structural motifs based on the...

The rippled β-sheet is a structural motif formed by certain racemic peptides that is distinct from the commonly known pleated β-sheet. Although the structure was predicted in 1953, unambiguous crystallographic observation of a rippled β-sheet was not reported until 2022. The structural foundation of the rippled β-sheet field continues to expand, stimulating new research questions, both fundamental and applied. Recent studies found that racemic peptides of varied length and amino acid composition assemble into rippled β-sheets. Intriguingly, certain rippled sheets were found to encapsulate small molecules in ways that could become useful in drug delivery, or to trap harmful substances. These and many other potential applications hinge on the development of a comprehensive structural foundation based on both experiment and theory. In this paper we introduce the concept of the single-component rippled-sheet, composed of joined segments of L and D chirality. The scope of rippled sheet-forming motifs is expanded to include two unexplored classes of rippled sheets: single-component cyclic and linear peptide chimeras. We report on the design, synthesis, and crystal structural characterization of eight self-assembling peptide systems. All five linear systems, in which amino acid sequence, charge and chirality were varied, formed rippled β-sheets with distinct two- and three-dimensional lattices. Of the three cyclic peptides, however, only one system formed a rippled β-sheet, while the other two formed pleated β-sheets. Molecular modeling is used to better understand chiral selection in cyclic systems.