Crystallographic characterization of the conformation of the 1-aminocyclohexane-1-carboxylic acid residue in simple derivatives and peptides

Abstract

The crystal structures of 1-aminocyclohexane-1-carboxylic acid (H-Acc⁶-OH) and six derivatives (including dipeptides) have been determined. The derivatives are Boc-Acc⁶-OH, Boc-(Acc⁶)₂-OH, Boc-L-Met-Acc⁶-OMe, ClCH₂CO-Acc⁶-OH, p-BrC₆H₄CO-Acc⁶-OH oxazolone, and the symmetrical anhydride from Z-Acc⁶-OH, [(Z-Acc⁶)₂O]. The cyclohexane rings in all the structures adopt an almost perfect chair conformation. The amino group occupies the axial position in six structures; the free amino acid is the only example where the carbonyl group occupies an axial position. The values determined for the torsion angles about the N-C^α (Φ) and C^α-CO (ψ) bonds correspond to folded, potentially helical conformations for the Acc⁶ residue.