Infrared multiple photon dissociation spectroscopy of cationized methionine: effects of alkali-metal cation size on gas-phase conformation

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, [CO2-], 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.