1.The alanine model dipeptide Ac-Ala-NH2 exists as a mixture of C7(eq) and C5 conformers.
Cabezas C1, Varela M, Cortijo V, Jiménez AI, Peña I, Daly AM, López JC, Cativiela C, Alonso JL. Phys Chem Chem Phys. 2013 Feb 21;15(7):2580-5. doi: 10.1039/c2cp43924g. Epub 2013 Jan 14.
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.