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

A -hairpin conformation and extended -pleated sheet assembly have been characterized by single crystal x-ray diffraction for the synthetic peptide t-butoxycarbonyl— -Phe-Phe-D-Pro-GlyPhe-Phe-methyl ester [ -Phe: (S)3 homophenylalanine]. The centrally located D-Pro-Gly segment nucleates a chain reversal in a type II -turn conformation. Two intramolecular cross-strand hydrogen bonds stabilize the peptide fold. Intermolecular NH OAC hydrogen bonds (two on each side of the hairpin) connect the hairpins into an infinitely extended -sheet. The -residues cause all CAO groups to point in the same direction, resulting in a ‘‘polar’’ sheet by the unidirectional alignment of NH OAC hydrogen bonds. In contrast, -sheets formed by -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, 100.39(2)°, Z 2, agreement factor R1 0.065 for 3,706 data observed >4 (F) and a resolution of 0.90 Å.