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

摘要

In this paper we attempted to reveal how a large #beta#-substituent (pyrenyl group) of dehydroresidue affects the structural and conformational preferences of (Z)-#beta#-substituted #alpha#,#alpha#-dehydroalanine.An X-ray crystallographic analysis was carried out for Boc-L-Ala #DELTA#~ZPyr-L-Val-OMe (Boc= t-butyloxycarbonyl; OMe= methoxy) possessing a novel (Z)-#beta#-(1-pyrenyl)ehydroalanine (#DELTA#~ZPyr) residue.Single crystal were obtained by slow evaporation from an ethanol solution in the monoclinic space group P2_1.The bond lengths and bond angles of the #DELTA#~ZPyr residue were essentially similar to other #beta#-substituted dehydroalanines.The tripeptide took a type II #beta#-turn conformation involving anintramolecular hydrogen bond bteween CO(Boc) and NH(Val), like small peptides possessign a (Z)-#beta#-(1-naphthyl)dehydroalanine or a (Z)-#beta#-phenyldehydroalanine residue.The similarity between #DELTA#~ZPyr and other dehydroalanines in structural property and conformational preference should be responsible for the non-planarity of the #DELTA#~ZPyr residue to given an extremely hihg energy in the planar orientation of the pyrenyl gtoup relative to teh C~(#alpha#)=C_~(#beta#)-C~(#gamma#) plane; the non-planarity should weaken the #pi#-conjugation effect of the pyrenyl group on the structural property and conformational preferenc of the #DELTA#~ZPyr residue.The pyrenyl rings were packed in a partially overlapped face-to-face orientation with the shortest center-to center distance of 5.96 A.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.