Stereochemistry of peptides containing 1-aminocyclopentanecarboxylic acid (Acc⁵): solution and solid-state conformations of Boc-Acc⁵-Acc⁵-NHMe

Abstract

The conformational analysis of a protected homodipeptide of 1-aminocyclopentanecarboxylic acid (Acc⁵) has been carried out. ¹H-nmr studies establish a β-turn conformation for Boc-Acc⁵-Acc⁵-NHMe in chloroform and dimethylsulfoxide solutions involving the methylamide NH in an intramolecular hydrogen bond. Supportive evidence for the formation of an intramolecular hydrogen bond is obtained from ir studies. X-ray diffraction studies reveal a type III β-turn conformation in the solid state stabilized by a 4 →1 hydrogen bond between the Boc CO and methylamide NH groups. The Φ, ψ values for both Acc⁵ residues are close to those expected for an ideal 3₁₀-helical conformation ( Φ≃ ± 60°, Ψ∼ ± 30°).