title: Infrared spectroscopy of phenylalanine Ag(I) and Zn(II) complexes in the gas phase.
abstract: Infrared multiple-photon dissociation (IR-MPD) spectroscopy has been applied to singly-charged complexes involving the transition metals Ag(+) and Zn(2+) with the aromatic amino acid phenylalanine. These studies are complemented by DFT calculations. For [Phe+Ag](+) the calculations favor a tridentate charge solvation N/O/ring structure. The experimental spectrum strongly supports this as the predominant binding geometry and, in particular, rules out a significant presence of the salt-bridge conformation. Zn(2+) forms a deprotonated dimer complex with Phe, [Zn+Phe(2)-H](+), in which the +2 oxidation state serves as a useful biomimetic model for zinc protein sites. A number of low-energy conformations were located, of which the lowest-energy conformer predicted by the calculations involves a Phe ligand deprotonated on the carboxylic acid, while the other Phe ligand is in the tridentate charge solvation conformation. The calculated IR spectrum of this conformer gives a close fit to the experimental spectrum, strongly supporting this as the predominant binding geometry. This most stable calculated complex is characterized by N/ O/ring metal chelation with a tetrahedral-type coordination core of Zn(2+) to N and O of both ligands. Another similar tightly chelated structure shows a square-planar-type coordination core, but this structure is computed to be less stable and gives a less satisfactory match to the experimental spectrum. This preference for the tetrahedral geometry of the Lewis-basic atomic ligands parallels the common Zn(II) coordination geometry in proteins. The number of clearly identifiable peaks resolved in the IR-MPD spectra as well as the much-improved matches between the observed spectra and the DFT-calculated spectra of the most stable geometries compared to previous studies are noteworthy for systems of this size and complexity. These results demonstrate that IR spectroscopy of transition metal-amino acid complexes in combination with DFT calculations is a very powerful structural tool, readily applicable to biomimetic systems that model, for example, the reaction centers of proteins in the solvent-free environment. In addition, we present a novel ion-capturing method for Fourier transform ion cyclotron resonance mass spectrometry which removes the necessity of a buffer gas pulse, while allowing ion trapping at moderate voltages with apparently reduced collisional excitation of the ions.
authors.affiliation: FOM-Institute for Plasmaphysics Rijnhuizen, MN Nieuwegein, The Netherlands.
authors.forename: Nick C
authors.initials: NC
authors.lastname: Polfer
authors.forename: Jos
authors.initials: J
authors.lastname: Oomens
authors.forename: David T
authors.initials: DT
authors.lastname: Moore
authors.forename: Gert
authors.initials: G
authors.lastname: von Helden
authors.forename: Gerard
authors.initials: G
authors.lastname: Meijer
authors.forename: Robert C
authors.initials: RC
authors.lastname: Dunbar
mesh_terms: D005583:Fourier Analysis; D005740:Gases; D008958:Models, Molecular; D010649:Phenylalanine / Q000737:chemistry*; D012834:Silver / Q000737:chemistry*; D021241:Spectrometry, Mass, Electrospray Ionization; D013055:Spectrophotometry, Infrared; D015032:Zinc / Q000737:chemistry*
chemical_list: D005740:Gases; D012834:Silver; D010649:Phenylalanine; D015032:Zinc