| Abstract
| - The solid α-amino acid isoleucine has been vaporized by laser ablation and expanded in asupersonic jet, where the molecular conformations of the isolated molecule were probed using Fouriertransform microwave spectroscopy. Two conformers of neutral isoleucine have been detected in gas phase,the most stable being stabilized by an intramolecular hydrogen bond N−H···OC and a cis-COOHarrangement. The higher energy form is stabilized by an intramolecular hydrogen bond N···H−O. Thesec-butyl side chain of the amino acid adopts the same configuration in the two observed conformers, witha staggered configuration at Cβ similar to that observed in valine and a trans arrangement of Cα and Cδ.Ab initio calculations at MP2/6-311++G(d,p) level reproduce satisfactorily the experimental results.
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