title: Infrared multiple photon dissociation spectroscopy of cationized  methionine: effects of alkali-metal cation size on gas-phase  conformation.
abstract: The gas-phase structures of alkali-metal cation complexes of the amino acid methionine (Met) as well as protonated methionine are investigated using infrared multiple photon dissociation (IRMPD) spectroscopy utilizing light generated by a free electron laser. Spectra of Li(+)(Met) and Na(+)(Met) are similar and relatively simple, whereas the spectra of K(+)(Met), Rb(+)(Met), and Cs(+)(Met) include distinctive new bands. Measured IRMPD spectra are compared to spectra calculated at the B3LYP/6-311+G(d,p) level of theory to identify the conformations present in the experimental studies. For Li(+) and Na(+) complexes, the only conformation present is a charge-solvated, tridentate structure that binds the metal cation to the amine and carbonyl groups of the amino acid backbone and the sulfur atom of the side chain, [N,CO,S]. In addition to the [N,CO,S] conformer, bands corresponding to alkali-metal cation binding to a bidentate zwitterionic structure, [CO(2)(-)], are clearly present for the K(+), Rb(+), and Cs(+) complexes. Theoretical calculations of the lowest energy conformations of Rb(+) and Cs(+) complexes suggest that the experimental spectra could also include contributions from two additional charge-solvated structures, tridentate [COOH,S] and bidentate [COOH]. For H(+)(Met), the IRMPD action spectrum is reproduced by multiple low-energy [N,CO,S] conformers, in which the protonated amine group hydrogen bonds to the carbonyl oxygen atom and the sulfur atom of the amino acid side chain. These [N,CO,S] conformers only differ in their side-chain orientations.
authors.affiliation: Department of Chemistry, University of Utah, Salt Lake City, UT 84112, USA.
authors.forename: Damon R
authors.initials: DR
authors.lastname: Carl
authors.forename: Theresa E
authors.initials: TE
authors.lastname: Cooper
authors.forename: Jos
authors.initials: J
authors.lastname: Oomens
authors.forename: Jeff D
authors.initials: JD
authors.lastname: Steill
authors.forename: P B
authors.initials: PB
authors.lastname: Armentrout
mesh_terms: D000468:Alkalies / Q000737:chemistry*; D002412:Cations; D005740:Gases; D008670:Metals / Q000737:chemistry*; D008715:Methionine / Q000737:chemistry*; D017785:Photons; D013055:Spectrophotometry, Infrared / Q000379:methods*
chemical_list: D000468:Alkalies; D002412:Cations; D005740:Gases; D008670:Metals; D008715:Methionine