The alanine model dipeptide Ac-Ala-NH2 exists as a mixture of Ceq7 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-NH(2)). This compound was vaporized by laser ablation and shown to exist as a mixture of C(eq)(7) and C(5) 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 (14)N 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.