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The gas-phase structures of cationized glycine (Gly), including complexes with Li+, Na+, K+, Rb+, and Cs+, are examined using infrared multiple-photon dissociation (IRMPD) spectroscopy utilizing light generated by a free electron laser, in conjunction with ab initio calculations. To identify the structures present in the experimental studies, measured IRMPD spectra are compared to spectra calculated at B3LYP/6-311+G(d,p) for the Li+, Na+, and K+ complexes and at B3LYP/def2TZVP for the Rb+ and Cs+ complexes. Single-point energy calculations were carried out at the B3LYP, B3P86, and MP2(full) levels using the 6-311+G(2d,2p) basis set for Li+, Na+, K+ and the def2TZVPP basis set for Rb+ and Cs+. The Li+ and Na+ complexes are identified as metal cation coordination to the amino nitrogen and carbonyl oxygen, [N,CO]-tt, although Na+(Gly) may have contributions from additional structures. The heavier metal cations coordinate to either the carbonyl oxygen, [CO]-cc, or the carbonyl oxygen and hydroxy oxygen, [CO,OH]-cc, with the former apparently preferred for Rb+ and Cs+ and the latter for K+. These two structures reside in a double-well potential and different levels of theory predict very different relative stabilities. Some experimental evidence is provided that MP2(full) theory provides the most accurate relative energies.

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Metallated gas-phase structures consisting of an intact and deprotonated histidine (His) ligand, M(His-H)(His)+, where M = Ca and Ni, were examined using infrared multiple photon dissociation (IRMPD) action spectroscopy utilizing light from a free-electron laser (FEL). In parallel, ab initio quantum-chemical calculations identified several low-energy isomers for each complex. Experimental action spectra were compared to linear absorption spectra calculated at the B3LYP level of theory, using the 6-311+G(d,p) basis set. Single-point energies were calculated at B3LYP, B3LYP-GD3BJ, B3P86, and MP2(full) levels using the 6-311+G(2d,2p) basis set. For Ca(His-H)(His)+, the dominant structure has the metal center coordinating with the π nitrogen of the imidazole ring (Nπ) and both oxygen atoms of the carboxylate group of the deprotonated His ligand while coordinating with the backbone amine (Nα), Nπ, and the carbonyl oxygen of the carboxylic acid of the intact His ligand. The Ni(His-H)(His)+ species coordinates the metal ion through Nα, Nπ, and the carbonyl oxygen for both the deprotonated and intact His ligands, but also shows evidence for a minor secondary structure where the deprotonated His coordinates the metal at Nα, Nπ, and the deprotonated carbonyl oxygen and the intact His ligand is zwitterionic, coordinating the metal with both carboxylate oxygens. Different levels of theory predict different ground structures, highlighting the need for utilizing multiple levels of theory to help identify the gas-phase structure actually observed experimentally.

Methionine (Met) cationized with Zn2+, forming Zn (Met-H)+(ACN) where ACN = acetonitrile, Zn (Met-H)+, and ZnCl+(Met), as well as Cd2+, forming CdCl+(Met), were examined by infrared multiple photon dissociation (IRMPD) action spectroscopy using light generated from the FELIX free electron laser. A series of low-energy conformers for each complex was found using quantum-chemical calculations in order to identify the structures formed experimentally. For all four complexes, spectral comparison indicated that the main binding motif observed is a charge solvated, tridentate structure where the metal center binds to the backbone amino group nitrogen, backbone carbonyl oxygen (where the carboxylic acid is deprotonated in two of the Zn2+ complexes), and side-chain sulfur. For all species, the predicted ground structures reproduce the experimental spectra well, although low-lying conformers characterized by similar binding motifs may also contribute in each system. The current work provides valuable information regarding the binding interaction between Met and biologically relevant metals. Further, the comparison between the current work and previous analyses involving alkali metal cationized Met as well as cysteine (the other sulfur containing amino acid) cationized with Zn2+ and Cd2+ allows for the elucidation of important metal dependent trends associated with physiologically important metal-sulfur binding.

Metallated gas-phase structures consisting of a deprotonated and an intact histidine (His) ligand, yielding M(His-H)(His)+, where M = Zn and Cd, were examined with infrared multiple photon dissociation (IRMPD) action spectroscopy utilizing light from a free-electron laser (FEL). In parallel, quantum chemical calculations identified several low-energy isomers for each complex. Experimental action spectra were compared to linear spectra calculated at the B3LYP level of theory using the 6-311+G(d,p) and def2-TZVP basis sets for the zinc and cadmium complexes, respectively. For both Zn and Cd species, the definitive assignment is complicated by conflicting relative energetics, which were calculated at B3LYP, B3LYP-GD3BJ, B3P86, and MP2(full) levels. Spectral comparison for both species indicates that the dominant conformation, [Nα, Nπ, CO-][CO2-](NπH+), has the deprotonated His chelating the metal at the amine nitrogen, π nitrogen of the imidazole ring, and the deprotonated carbonyl oxygen and that the intact His ligand adopts a salt-bridge bidentate binding motif, coordinating the metal with both carboxylate oxygens. There is also evidence for a conformation where the deprotonated His coordination is maintained, but the intact His ligand adopts a more canonical structure, coordinating with the metal atom at the amine nitrogen and π nitrogen, [Nα, Nπ, CO-][Nα, Nπ]gtgg. For both metallated species, B3LYP, B3P86, and B3LYP-GD3BJ levels of theory appear to describe the relative stability of the dominant zwitterionic species more accurately than the MP2(full) level.

The present work explores the structure of the gold carbene cation, AuCH