Conformations of alpha-aminobutyric acid in the gas phase

A laser-ablation molecular-beam Fourier transform microwave (LA-MB-FTMW) spectrometer has been successfully applied to the structural study of a-aminobutyric acid. Three neutral conformers have been identified in the gas phase by comparing their experimental rotational and N-14 nuclear quadrupole coupling parameters with those predicted by ab initio calculations at the MP2/6-311+ +G(d,p) level. The most stable conformer is stabilized by a bifurcated amine-to-carbonyl hydrogen bond (N-(HO)-O-...=C) and a cis COOH group, and the side-chain adopts a configuration with a torsion angle tau(C-gamma C-beta C-alpha C) of about 180 degrees. The second most stable conformer exhibits the same configuration for the amino acid skeleton but adopts a different orientation for the side chain with tau(C gamma C beta C alpha C) approximate to 60 degrees. In the third conformer an intramolecular hydrogen bond is established between the hydroxyl group and the nitrogen atom ((NH)-H-...-O), with a side-chain orientation similar to that of the most stable conformer.