Infinite pleated β-sheet formed by the β-hairpin Boc-β-Phe-β-Phe-D-Pro-Gly-β-Phe-β-Phe-OMe

A beta-hairpin conformation and extended p-pleated sheet assembly have been characterized by single crystal x-ray diffraction for the synthetic peptide t-butoxycarbonyl-beta-Phe-beta-Phe-D-Pro-Gly-beta-Phe-beta-Phe-methyl ester [beta-Phe: (S)-beta(3) homophenylalanine]. The centrally located D-Pro-Gly segment nucleates a chain reversal in a type II' beta-turn conformation. Two intramolecular cross-strand hydrogen bonds stabilize the peptide fold. Intermolecular NH...O=C hydrogen bonds (two on each side of the hairpin) connect the hairpins into an infinitely extended beta-sheet. The beta-residues cause all C=O groups to point in the same direction, resulting in a "polar" sheet by the unidirectional alignment of NH...O=C hydrogen bonds. In contrast, P-sheets formed by a-residues have alternating directions for the hydrogen bonds, thus resulting in an "apolar" sheet. The crystallographic parameters for C53H66N6O9.CH3OH are: space group P2(1), a = 9.854(2) Angstrom, b = 10.643(2) Angstrom, c = 25.296(4) Angstrom, beta = 100.39(2)degrees, Z = 2, agreement factor R-1 = 0.065 for 3,706 data observed >4sigma(F) and a resolution of 0.90 Angstrom.