Chain Length Effects on Helix-Hairpin Distribution in Short Peptides with Aib-~DAla and Aib-Aib Segments

摘要

The Aib-~DAla dipeptide segment has a tendency to form both type-I'/III' and type-I/III β-turns. The occurrence of prime turns facilitates the formation of $-hairpin conformations, while type-I/III turns can nucleate helix formation. The octapeptide Boc-Leu-Phe-Val-Aib-~DAla-Leu-Phe-Val-OMe (1) has been previously shown to form a β-hairpin in the crystalline state and in solution. The effects of sequence truncation have been examined using the model peptides Boc-Phe-Val-Aib-Xxx-Leu-Phe-NHMe (2, 6), Boc-Val-Aib-Xxx-Leu-NHMe (3, 7), and Boc-Aib-Xxx-NHMe (4, 8), where Xxx = DAla, Aib. For peptides with central Aib-Aib segments, Boc-Phe-Val-Aib-Aib-Leu-Phe-NHMe (6), Boc-Val-Aib-Aib-Leu-NHMe (7), and Boc-Aib-Aib-NHMe (8) helical conformations have been established by NMR studies in both hydrogen bonding (CD_3OH) and non-hydrogen bonding (CDCl_3) solvents. In contrast, the corresponding hexapeptide Boc-Phe- Val-Aib-~DAla-Leu-Phe- Val-NHMe (2) favors helical conformations in CDCl_3 and β-hairpin conformations in CD_3OH. The β-turn conformations (type-I'/III) stabilized by intramolecular 4→1 hydrogen bonds are observed for the peptide Boc-Aib-~DAla-NHMe (4) and Boc-Aib-Aib-NHMe (8) in crystals. The tetrapeptide Boc-Val-Aib-Aib-Leu-NHMe (7) adopts an incipient 310-helical conformation stabilized by three 4→1 hydrogen bonds. The peptide Boc-Val-Aib-~DAla-Leu-NHMe (3) adopts a novel α-turn conformation, stabilized by three intramolecular hydrogen bonds (two 4→1 and one 5→1). The Aib-~DAla segment adopts a type-11 β-turn conformation. The observation of an NOE between Val (1) NH?HNCH_3 (5) in CD_3OH suggests, that the solid state conformation is maintained in methanol solutions.