Conformations of peptides containing 1-aminocyclohexanecarboxylic acid (Acc⁶). Crystal structures of two model peptides

Abstract

The crystal structures of two peptides containing 1-aminocyclohexanecarboxylic acid (Acc⁶) are described. Boc-Aib-Acc⁶-NHMe · H₂O adopts a β-turn conformation in the solid state, stabilized by an intramolecular 4 → 1 hydrogen bond between the Boc CO and methylamide NH groups. The backbone conformational angles (φ_Aib = - 50.3°, ψ_Aib = - 45.8°; φ_Acc6 = - 68.4°, ψ_Acc6 = - 15°) lie in between the values expected for ideal Type I or III β-turns. In Boc-Aib-Acc⁶-OMe, the Aib residue adopts a partially extended conformation (φ_Aib = - 62.2°, ψ_Aib = 143°) while the Acc⁶ residue maintains a helical conformation (φ_Acc6 = 48°, ψ_Acc6= 42.6°). ¹H n.m.r. studies in CDCl₃ and (CD₃)₂SO suggest that Boc-Aib-Acc⁶-NHMe maintains the β-turn conformation in solution.