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Glutathione is a biologically abundant and redox active tripeptide that coordinates with metals. This study examines the change in binding conformation with relevant dications; Zn²⁺, Cu²⁺, and Fe²⁺. 

Using light generated by an infrared free electron laser, action spectroscopy was performed on doubly charged complexes of the metalated dipeptide histidyl-histidine (HisHis). Metal cations used were zinc, cadmium, and copper. Molecular dynamics and quantum-chemical calculations were used to screen a large number of conformers, whose theoretical infrared spectra were compared to the recorded action spectra of these metalated complexes. The zinc and cadmium spectra display dominant features associated with an iminol binding motif of the HisHis ligand, where the metal ion coordinates with both pros () nitrogens of the imidazole sidechains, the terminal carbonyl oxygen, and the backbone nitrogen for which the hydrogen ordinarily bound here has migrated to a carbonyl. The copper complex was difficult to assign to a single species, because a few predicted bands are absent from the experimental spectrum. The theoretical single point energies were also calculated for all structures examined, and DFT methods were found to describe the ion conformer populations in the case of the zinc and cadmium chelates better than the MP2 prediction. 

Cesiated complexes of the aliphatic amino acids (Gly, Ala, hAla, Val, Leu, and Ile) were examined by infrared multiple photon dissociation (IRMPD) action spectroscopy utilizing light from a free-electron laser (FEL). To identify structures, the experimental spectra were compared to linear spectra calculated at the B3LYP-GD3BJ/def2-TZVP level of theory. Relative energies at 0 and 298 K for various possible conformers of all complexes were calculated at B3LYP, B3LYP-GD3BJ, and MP2(full) levels using the def2-TZVP basis set. Spectral comparison for all complexes indicates that the dominant conformation has the cesium cation binding to the carbonyl and hydroxyl oxygens, [CO,OH]. This conclusion contrasts with previous work for Cs+(Gly), which suggested that the [CO] binding motif was prevalent. This dichotomy is explored theoretically in detail using coupled-cluster calculations with single, double, and perturbative triple excitations, CCSD(T), as well as advanced density functional theory (DFT) approaches. The comparisons show that the [CO,OH] – [CO] double-well potential found for most DFT approaches disappears at the higher level of theory with only the [CO,OH] well remaining. An exploration of this effect indicates that electron correlation is critically important and that DFT approaches incorrectly handle the internal hydrogen bonding in these molecules, thereby over-delocalizing the charges on the amino acid ligands. 

A combination of infrared multiple-photon dissociation (IRMPD) action spectroscopy and quantum chemical calculations was employed to investigate the [M,C,2H]⁺(M = Ru and Rh) species. 

Threshold collision‐induced dissociation of conformations selected by ion mobility spectrometry is described as a method to determine their absolute relative energies. Here, the method is demonstrated with thecisandtransconformers of the protonated tetrapeptide, Gly‐Pro‐Gly‐Gly, which differ in the orientation of the peptide bond at the proline residue. Thetransconformation was found to be more stable than thecisconformation by 4.5 ± 2.5 kJ mol⁻¹at 0 K. Heating the molecule anneals it to thecisconformer, indicating it has a lower Gibbs energy at higher temperatures. These results are compared to theoretical values calculated here and from the literature. This experimental analysis is the first quantitative measurement of the relative stability of the conformers of a protonated peptide in the gas phase, which has far‐reaching impacts on the selection of theoretical methods to describe the energetics of these systems.