A flat, extended structural shape that some peptides and proteins fold into — associated with both stability and, in some cases, disease-causing aggregation.Formed when backbone hydrogen bonds develop between adjacent peptide strands running parallel or antiparallel. Amyloid-forming peptides adopt beta-sheet conformations upon aggregation. Cyclic peptides are often designed to restrict conformational flexibility and prevent unwanted beta-sheet formation — one reason cyclization improves stability and receptor selectivity.