The Singular Gas-Phase Structure of 1-Aminocyclopropanecarboxylic Acid (Ac_3c)

摘要

The natural nonproteinogenic a-amino acid 1-aminocy- clopropanecarboxylic acid (Ac_3c) 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 nuclear quadrupole coupling constants with the values predicted ab initio for these parameters has allowed the unambiguous identification of three Ac_3c conformers differing in the hydrogen bonding pattern. Two of them resemble those characterized before for the coded aliphatic a-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_3c, close in energy to the global minimum. This is the first time that such a conformer, stabilized by an N—H • • • O(H) hydrogen bond, is detected in the rotational spectrum of a gaseous a-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_3c.