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

The crystal structures of six derivatives of the C α,α -dialkylated α-aminoacid residue 1-aminocyclopentane-1-carboxylic acid (Acc 5 ) have been determined. These are Z-Acc 5 -OH, Z-Acc 5 -OH acetone hemi-solvate, Fmoc-Acc 5 -OH, mClAc-Acc 5 -OH, pBrBz-Acc 5 -OH, and the 5(4H)-oxazolone from pBrBz-Acc 5 -OH. The values determined for the torsion angles about the [Formula: see text] and C α —CO (ψ) bonds correspond to folded, potentially α/3 10 -helical conformations for the Acc 5 residue of Z-Acc 5 -OH, Z-Acc 5 -OH acetone hemi-solvate, mClAc-Acc 5 -OH, and pBrBz-Acc 5 -OH. The structure of Fmoc-Acc 5 -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—C α —C′ bond angle on the ψ torsion angle and the asymmetry at the C α atom are relevant features of the Acc 5 residues. The position of the cyclopentyl ring relative to the peptide chain and its puckering parameters for the six compounds are also discussed.