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

A beta-hairpin conformation and extended beta-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[double bond]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[double bond]O groups to point in the same direction, resulting in a "polar" sheet by the unidirectional alignment of NH...O[double bond]C hydrogen bonds. In contrast, beta-sheets formed by alpha-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) x CH(3)OH are: space group P2(1), a = 9.854(2) A, b = 10.643(2) A, c = 25.296(4) A, beta = 100.39(2) degrees, Z = 2, agreement factor R(1) = 0.065 for 3,706 data observed >4 sigma(F) and a resolution of 0.90 A.