Infinite Pleated β-Sheet Formed by the β-Hairpin Boc-β-Phe-β-Phe-D-Pro-Gly-β-Phe-β-Phe-OMe

A β-hairpin conformation and extended β-pleated sheet assembly have been characterized by single crystal x-ray diffraction for the synthetic peptide t-butoxycarbonyl-β-Phe-β-Phe-D-Pro-Gly-β-Phe-β-Phe-methyl ester [β-Phe: (S)-β3 homophenylalanine]. The centrally located D-Pro-Gly segment nucleates a chain reversal in a type II′ β-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 β-sheet. The β-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, β-sheets formed by α-residues have alternating directions for the hydrogen bonds, thus resulting in an "apolar" sheet. The crystallographic parameters for C53H66N6O9·CH3OH are: space group P21, a = 9.854(2) Å, b = 10.643(2) Å, c = 25.296(4) Å, β = 100.39(2)°, Z = 2, agreement factor R1 = 0.065 for 3,706 data observed >4σ(F) and a resolution of 0.90 Å.