The Singular Gas-Phase Structure of 1-Aminocyclopropanecarboxylic Acid (Ac3c)

The natural nonproteinogenic (alpha-amino acid 1-aminocy-clopropanecarboxylic acid (Ac(3)c) has been vaporized by laser ablation and studied in the gas phase by molecular-beam Fourier transform microwave spectroscopy. Comparison of the experimental rotational and N-14 nuclear quadrupole coupling constants with the values predicted ab initio for these parameters has allowed the unambiguous identification of three Ac(3)c conformers differing in the hydrogen bonding pattern. Two of them resemble those characterized before for the coded aliphatic alpha-amino acids. Remarkably, a third conformer predicted to be energetically accessible for all of these amino acids but never observed (the so-called missing conformer) has been found for Ac(3)c, close in energy to the global minimum. This is the first time that such a conformer, stabilized by an N-H center dot center dot center dot O(H) hydrogen bond, is detected in the rotational spectrum of a gaseous alpha-amino acid with a nonpolar side chain. The conjugative interaction established between the cyclopropane ring and the adjacent carbonyl group seems to be responsible for the unique conformational properties exhibited by Ac(3)c.