Linear oligopeptides. 188.Crystallographic characterization of the conformation of the 1-aminocyclopentane-1-carboxylic acid residue in simple derivatives

Abstract

The crystal structures of six derivatives of the Cα,α-dialkylated α-aminoacid residue 1-aminocyclopentane-1-carboxylic acid (Acc5) have been determined. These are Z-Acc5-OH, Z-Acc5-OH acetone hemi-solvate, Fmoc-Acc5-OH, mClAc-Acc5-OH, pBrBz-Acc5-OH, and the 5(4H)-oxazolone from pBrBz-Acc5-OH. The values determined for the torsion angles about the [Formula: see text] and Cα—CO (ψ) bonds correspond to folded, potentially α/310-helical conformations for the Acc5 residue of Z-Acc5-OH, Z-Acc5-OH acetone hemi-solvate, mClAc-Acc5-OH, and pBrBz-Acc5-OH. The structure of Fmoc-Acc5-OH, however, is unusual in having a semi-extended conformation for the [Formula: see text], ψ angles, accompanied by a cis arrangement of the amide group of the secondary urethane moiety. The dependence of the N—Cα—C′ bond angle on the ψ torsion angle and the asymmetry at the Cα atom are relevant features of the Acc5 residues. The position of the cyclopentyl ring relative to the peptide chain and its puckering parameters for the six compounds are also discussed.