摘要:
The incorporation of the P-amino acid residues into specific positions in the strands and U-turn segments of peptide hairpins is being systematically explored. The presence of an additional torsion variable about the C(alpha)-C(beta) bond (theta) enhances the conformational repertoire in beta-residues. The conformational analysis of three designed peptide hairpins composed of alpha/beta-hybrid segments is described: Boc-Leu-Val-Val-(D)Pro-betaPhe-Leu-Val-Val-OMe (1), Boc-Leu-Vil-betaVal-(D)Pro-Gly-betaLeu-Val-Val-OMe (2). and Boc-Lcu-Val-betaPhe-Val-(D)Pro-Gly-Leu-betaPhe-Val-Val-OMe (3). 500-MHz H-1-NMR Analysis supports a preponderance of beta-hairpin conformation in solution for all three peptides, with critical cross-strand NOEs providing evidence for the proposed structures. The crystal structure of peptide 2 reveals a beta-hairpin conformation with two beta-residues occupying facing, non-H-bonded positions in antiparallel beta-strands. Notably, betaVal(3) adopts a gauche conformation about the C(alpha)-C(beta) bond (theta = + 65degrees) without disturbing cross-strand H-bonding. The crystal structure of 2, together with previously published crystal structures of peptides 3 and Boc-Phe-betaPhe-(D)Pro-Gly-betaPhe-betaPhe-OMe, provide an opportunity to visualize the packing of peptide sheets with local 'polar segments' formed as a consequence of reversal peptide-bond orientation. The available structural evidence for hairpins suggests that beta-residues can be accommodated into nucleating turn segments and into both the H-bonding and non-H-bonding positions on the strands.