Peptide hybrids containing α- and β-amino acids: Structure of a decapeptide β-hairpin with two facing β-phenylalanine residues

A β-hairpin conformation has been characterized in crystals of the decapeptide t-butoxycarbonyl-Leu-Val-βPhe-Val-(D)Pro-Gly-Leu-βPhe-Val-Val-methyl ester [βPhe; (S)-β(3) homophenylalanine] by x-ray diffraction. The polypeptide chain reversal is nucleated by the centrally positioned (D)Pro-Gly segment, which adopts a type-I′ β-turn conformation. Four intramolecular cross-strand hydrogen bonds stabilize the peptide fold. The βPhe(3) and βPhe(8) residues occupy facing positions on the hairpin, with the side chains projecting on opposite faces of the β-sheet. At the site of insertion of β-residues, the polarity of the peptide units along each strand reverses, as compared with the α-peptide segments. In this analog, a small segment of a polar sheet is observed, where adjacent CO and NH groups line up in opposite directions in each strand. In the crystal, an extended β-sheet is formed by hydrogen bonding between strands of antiparallel pairs of β-hairpins. The crystallographic parameters for C(65)H(102)N(10)O(13)⋅ 3H(2)O are: space group P2(1)2(1)2(1); a = 19.059(8) Å, b = 19.470(2) Å, c = 21.077(2) Å; Z = 4; agreement factor R(1) = 9.12% for 3,984 data observed >4σ(F) and a resolution of 0.90 Å.