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

摘要

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-b-Phe-b-Phe-methyl ester [b-Phe: (S)-b3 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 CAOgroups to point in the same direction, resulting in a ‘‘polar’’ sheet by the unidirectional alignment of NH…O=C hydrogen bonds. In contrast, beta-sheets formed by beta-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, beta = 100.39(2)°, Z = 2, agreement factor R1 _ 0.065 for 3,706 data observed >4_(F) and a resolution of 0.90 Å.