Evaluation of β‐Amino Acid Replacements in Protein Loops: Effects on Conformational Stability and Structure

β‐Amino acids have a backbone that is expanded by one carbon atom relative to α‐amino acids, and β residues have been investigated as subunits in protein‐like molecules that adopt discrete and predictable conformations. Two classes of β residue have been widely explored in the context of generating α‐helix‐like conformations: β 3 ‐amino acids, which are homologous to α‐amino acids and bear a side chain on the backbone carbon adjacent to nitrogen, and residues constrained by a five‐membered ring, such the one derived from trans ‐2‐aminocyclopentanecarboxylic acid (ACPC). Substitution of α residues with their β 3  homologues within an α‐helix‐forming sequence generally causes a decrease in conformational stability. Use of a ring‐constrained β residue, however, can offset the destabilizing effect of α→β substitution. Here we extend the study of α→β substitutions, involving both β 3 and ACPC residues, to short loops within a small tertiary motif. We start from previously reported variants of the Pin1 WW domain that contain a two‐, three‐, or four‐residue β‐hairpin loop, and we evaluate α→β replacements at each loop position for each variant. By referral to the ϕ , ψ angles of the native structure, one can choose a stereochemically appropriate ACPC residue. Use of such logically chosen ACPC residues enhances conformational stability in several cases. Crystal structures of three β‐containing Pin1 WW domain variants show that a native‐like tertiary structure is maintained in each case.