Conformational Preference of β-Aryldehydroalanine. Solid-State Conformation of Tripeptide Possessing a (Z)-β-(1-Pyrenyl)dehydroalanine Residue in the Second Position

• Published in: Bulletin of the Chemical Society of Japan Vol. 74; no. 5; pp. 959 - 966
• Main Authors: Inai, Yoshihito, Oshikawa, Tatsuo, Yamashita, Mitsuji, Hirabayashi, Tadamichi, Kurokawa, Yuji
• Format: Journal Article
• Language: English
• Published: The Chemical Society of Japan 01.05.2001
• ISSN: 0009-2673; 1348-0634
• DOI: 10.1246/bcsj.74.959

In this paper we attempted to reveal how a large β-substituent (pyrenyl group) of dehydroresidue affects the structural and conformational preferences of (Z)-β-substituted α,α-dehydroalanine. An X-ray crystallographic analysis was carried out for Boc–L-Ala–ΔZPyr–L-Val–OMe (Boc = t-butyloxycarbonyl; OMe = methoxy) possessing a novel (Z)-β-(1-pyrenyl)dehydroalanine (ΔZPyr) residue. Single crystals were obtained by slow evaporation from an ethanol solution in the monoclinic space group P21. The bond lengths and bond angles of the ΔZPyr residue were essentially similar to other β-substituted dehydroalanines. The tripeptide took a type II β-turn conformation involving an intramolecular hydrogen bond between CO(Boc) and NH(Val), like small peptides possessing a (Z)-β-(1-naphthyl)dehydroalanine or a (Z)-β-phenyldehydroalanine residue. The similarity between ΔZPyr and other dehydroalanines in structural property and conformational preference should be responsible for the non-planarity of the ΔZPyr residue to give an extremely high energy in the planar orientation of the pyrenyl group relative to the Cα=Cβ-Cγ plane; the non-planarity should weaken the π-conjugation effect of the pyrenyl group on the structural property and conformational preference of the ΔZPyr residue. The pyrenyl rings were packed in a partially overlapped face-to-face orientation with the shortest center-to-center distance of 5.96 Å. The CD spectrum of a KBr disk prepared from the crystal showed strong excitonic interactions between pyrenyl groups, reflecting a regular arrangement of the pyrenyl groups in the crystal structure.