The alanine model dipeptide Ac-Ala-NH2 exists as a mixture of C eq7 and C5 conformers

Abstract

Microwave spectroscopy has been applied to characterize the conformations adopted in the gas phase by a small peptide derived from alanine, N-acetyl-L-alaninamide (Ac-Ala-NH₂). This compound was vaporized by laser ablation and shown to exist as a mixture of C^eq₇ and C₅ conformers stabilized by a CO⋯HN intramolecular hydrogen bond closing a seven- or a five-membered ring, respectively. The complicated quadrupole hyperfine structure originated from two ¹⁴N nuclei has been completely resolved for both species and the derived nuclear quadrupole coupling constants have been used to determine the Ramachandran angles that describe their molecular shapes.