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)-β(3) 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 C(53)H(66)N(6)O(9)⋅CH(3)OH are: space group P2(1), a = 9.854(2) Å, b = 10.643(2) Å, c = 25.296(4) Å, β = 100.39(2)°, Z = 2, agreement factor R(1) = 0.065 for 3,706 data observed >4σ(F) and a resolution of 0.90 Å.