title: The shape of leucine in the gas phase.
abstract: Two structures of neutral leucine are detected in the jet-cooled rotational spectrum of a laser-ablation molecular-beam Fourier transform microwave (LA-MB-FTMW) experiment. The comparison between the experimental rotational and (14)N nuclear quadrupole coupling constants and those calculated ab initio provides conclusive evidence for the identification of the conformers. The most stable species is stabilized by a N-H...O=C intramolecular hydrogen bond and a cis-COOH interaction, while a higher-energy conformer exhibits a N...H-O intramolecular hydrogen bond and trans-COOH, as in lower aliphatic amino acids. The isobutyl side chain adopts the same configuration in the two conformers of leucine, characterized by a trans arrangement of the C'-C(alpha)-C(beta)-C(gamma)-C(delta) chain. The differences with the preferred side chain configurations observed in valine and isoleucine are discussed.
authors.affiliation: Grupo de Espectroscopía Molecular (GEM), Departamento de Química Física y Química Inorgánica, Facultad de Ciencias, Universidad de Valladolid, 47005 Valladolid, Spain.
authors.forename: Emilio J
authors.initials: EJ
authors.lastname: Cocinero
authors.forename: Alberto
authors.initials: A
authors.lastname: Lesarri
authors.forename: Jens-Uwe
authors.initials: JU
authors.lastname: Grabow
authors.forename: Juan C
authors.initials: JC
authors.lastname: López
authors.forename: José L
authors.initials: JL
authors.lastname: Alonso
mesh_terms: D005583:Fourier Analysis; D005740:Gases; D006860:Hydrogen Bonding; D007834:Lasers; D007930:Leucine / Q000737:chemistry*; D008872:Microwaves; D008958:Models, Molecular; D008968:Molecular Conformation; D013057:Spectrum Analysis; D013237:Stereoisomerism; D013816:Thermodynamics
chemical_list: D005740:Gases; D007930:Leucine