The Shape of the Simplest Non‐proteinogenic Amino Acid α‐Aminoisobutyric Acid (Aib).

The simplest non‐proteinogenic amino acid α‐aminoisobutyric acid (Aib), an analogue of glycine and alanine, has been vaporized by laser ablation and probed by high‐resolution Fourier transform microwave spectroscopic techniques. Comparison of the experimental rotational and 14N nuclear quadrupole constants with that predicted ab initio has allowed the identification of three conformers of Aib exhibiting three types of hydrogen‐bond interactions I (NH⋅⋅⋅O=C, cis‐COOH), II (OH⋅⋅⋅N, trans‐COOH), and III (N−H⋅⋅⋅O−H, cis‐COOH) within the amino acid backbone. The observation of conformer III, not detected previously for related proteinogenic amino acids with a nonpolar side chain in a supersonic expansion, indicates that the presence of the methyl groups should restrict the conformational relaxation from conformer Aib‐III to Aib‐I. For conformer Aib‐II, the rotational spectra of the 13C isotopomers reveal a tunneling motion arising from the two equivalent methyl groups in the molecule. The observation of a single spectrum at the midpoint between those predicted for the two 13C of the methyl groups has been explained by considering a double‐minimum potential function with a low‐energy interconversion barrier for a large amplitude internal motion. This singular fact has been corroborated by the anomalous centrifugal distortion effects determined in conformer Aib‐II. Maxing and relaxing: Three conformers of the non‐proteinogenic amino acid α‐aminoisobutyric acid (Aib) have been revealed in a supersonic expansion by laser ablation and high‐resolution Fourier transform microwave techniques. For the first time, a conformer showing the N−H⋅⋅⋅O−H intramolecular interaction has been detected and characterized (see figure). This study deepens our understanding of the relaxation process. [ABSTRACT FROM AUTHOR]