Conformational Adaptability and Thermostability in α/β-Peptide Fibrils Induced by β-Amino Acid Substitution

The self-assembly of peptides into amyloid fibrils enables the design of functional biomaterials, yet the conformational constraints of α-peptides limit the attainable supramolecular diversity. Here, we introduce β-amino acids, β-phenylalanine (β-Phe), and β-homophenylalanine (β-hPhe) into the reversible fibril-forming core sequence hnRAC1 to generate α/β-peptide variants with distinct architectures and enhanced thermal stability. Cryo-EM reveals that β-modified peptides assemble into polymorphic fibrils with cross-β structures that differ markedly from each other and from native hnRAC1. Comparative structural analysis indicates that backbone extension by β-residues increases subunit conformational heterogeneity, enabling tighter packing and formation of more thermostable fibrils. Examination of intra- and intermolecular contacts shows that enhanced π–π stacking, hydrophobic interactions, hydrogen bonds, and electrostatic interactions likely contribute to fibril stabilization. These results show that minimal backbone modifications can remodel amyloid architecture, offering a generalizable strategy for designing structurally diverse and robust peptide-based biomaterials.