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Conformationalpreferences have been surveyed for divalent metalcation complexes with the dipeptide ligands AlaPhe, PheAla, GlyHis,and HisGly. Density functional theory results for a full set of complexesare presented, and previous experimental infrared spectra, supplementedby a number of newly recorded spectra obtained with infrared multiplephoton dissociation spectroscopy, provide experimental verificationof the preferred conformations in most cases. The overall structuralfeatures of these complexes are shown, and attention is given to comparisonsinvolving peptide sequence, nature of the metal ion, and nature ofthe side-chain anchor. A regular progression is observed as a functionof binding strength, whereby the weakly binding metal ions (Ba2+to Ca2+) transition from carboxylate zwitterion(ZW) binding to charge-solvated (CS) binding, while the stronger bindingmetal ions (Ca2+to Mg2+to Ni2+)transition from CS binding to metal-ion-backbone binding (Iminol)by direct metal–nitrogen bonds to the deprotonated amide nitrogens.Two new sequence-dependent reversals are found between ZW and CS bindingmodes, such that Ba2+and Ca2+prefer ZW bindingin the GlyHis case but prefer CS binding in the HisGly case. The overallbinding strength for a given metal ion is not strongly dependent onthe sequence, but the histidine peptides are significantly more stronglybound (by 50–100 kJ mol–1) than thephenylalanine peptides. [ABSTRACT FROM AUTHOR]

Complexes of glutamine (Gln) cationizedwith Zn2+andCd2+were examined by infrared multiple photon dissociation(IRMPD) action spectroscopy using light generated from a free-electronlaser. Electrospray ionization yielded complexes of deprotonated Glnwith Zn2+, [Zn(Gln–H)]+, and intact Glnwith CdCl+, CdCl+(Gln). For each complex, thespectra obtained were compared with those for low-energy conformersfound using quantum chemical calculations to identify the structurespresent experimentally. Calculations were performed at the B3LYP/6-311+G(d,p)level for [Zn(Gln–H)]+and at the B3LYP/def2-TZVPlevel with an SDD effective core potential on cadmium for CdCl+(Gln). The main binding motif observed for the Cd2+complex was a charge-solvated, tridentate [N,CO,COsc]structure in which the metal binds to the backbone amino group andthe carbonyl oxygens of the carboxylic acid and side-chain amide groups.The Zn2+system similarly preferred a [N,CO–,COsc] binding motif, where binding was observed at thecarboxylate site along with the backbone amino and side-chain carbonylgroups. In both cases, the theoretically determined lowest-energyconformers explain the experimental [Zn(Gln–H)]+and CdCl+(Gln) spectra well. [ABSTRACT FROM AUTHOR]

The capability of infrared multiple photon dissociation (IRMPD) spectroscopy to gain structural information on surfactant-based supramolecular aggregates has been exploited to elucidate intermolecular interactions and local organization of positively charged sodium bis(2-ethylhexyl)sulfosuccinate (AOTNa) aggregates in the gas phase. A detailed analysis of the stretching modes of the AOTNa CO and SO3−head groups allows one to directly probe their interactions with sodium counterions and to gain insight in their organization within the aggregate. Similarities and differences of the IRMPD spectra as compared to the infrared absorption spectrum of micellized AOTNa in CCl4have been analyzed. They strongly suggest a reverse micelle-like organization of AOTNa charged aggregates in the gas phase. Apart from low-abundance fragmentation channels of the AOTNa (molecule) itself, the main dissociation pathway of singly charged surfactant aggregates is the loss of neutral surfactant molecules, while doubly charged aggregates dissociate preferentially by charge separation forming singly charged species. In both cases, decomposition leads to the formation of the most energetically stable charged fragments. [ABSTRACT FROM AUTHOR]

Infrared multiple photon dissociation (IRMPD) kinetics measured with tunable laser radiation from a free electron laser (FEL) are used to probe the relative populations of and interconversions between energetically competitive isomers of gas-phase ions at 298 K. On-resonance IRMPD kinetics of monoisomeric benzoate anion and anilinium (protonated aniline) are measured to determine the extent of overlap of the laser beam with the precursor ion population (∼93%). IRMPD kinetics indicating different photodissociation behavior for different isomers obtained at isomer-specific resonances are used to determine relative populations of salt bridge and charge-solvated isomers for ArgGly·Na+, Ser·Cs+, and Arg·Na+. These values and Gibbs free energy differences obtained from them for thermal precursor populations are compared to values reported using other, less direct population probes. Rapid interconversion of two charge-solvated isomers occurs for ArgGly·Li+, precluding population analysis for this ion. ArgGly·Na+, ArgGly·Li+, and Arg·Na+exhibit IRMPD induction periods lasting many seconds for some isomers at the laser photon energies and power used, indicating that IRMPD relative spectral intensities are time-dependent for these ions and that the relative band intensities in IRMPD spectra measured with short irradiation times may not provide meaningful information about relative isomer populations. These results constitute the first direct probe of ion isomer populations using IRMPD kinetics obtained with a FEL and illustrate a number of caveats in interpreting IRMPD spectra measured with just a single irradiation time. These results also indicate that more complete overlap of the laser beam with the ions will be highly advantageous in future instrument designs for IRMPD kinetics and spectroscopy experiments. [ABSTRACT FROM AUTHOR]

Structures of trivalent lanthanide metal cations La3+, Ho3+, and Eu3+with deprotonated Alan(n= 2−5) or Leu-enk (Tyr-Gly-Gly-Phe-Leu) are investigated with infrared multiple photon dissociation (IRMPD) spectroscopy between 900 and 1850 cm−1and theory. In all of these complexes, a salt bridge is formed in which the metal cation coordinates to the carboxylate group of the peptide, resulting in a limited conformational space and many sharp IRMPD spectral bands. The IRMPD spectra clearly indicate that all carbonyl groups solvate the metal cation in each of the Alancomplexes. Due to strong vibrational coupling between the carbonyl groups, a sharp, high-energy amide I band due to in-phase stretching of all of the amide carbonyl groups bound to the metal cation is observed that is separated by ∼50 cm−1from a strong, lower-energy amide I band. This extent of carbonyl coupling, which is sometimes observed in condensed-phase peptide and protein IR spectroscopy, has not been reported in IRMPD spectroscopy studies of other cationized peptide complexes. Intense bands due to carbonyl groups not associated with the metal cation are observed for Leu-enk complexes, indicating that a side chain group, such as the Tyr or Phe aromatic ring, prevents complete carbonyl coordination of the metal cation. Substitution of smaller lanthanide cations for La3+in these peptide complexes results only in minor structural changes consistent with the change in metal cation size. These are the first IRMPD spectra reported for lanthanide metal cationized peptides, and comparison to previously reported protonated and alkali metal or alkaline earth metal cationized peptide complexes reveals many trends consistent with the higher charge state of the lanthanide cations. [ABSTRACT FROM AUTHOR]

Gas-phase coronene cations () can be sequentially hydrogenated with up to 24 additional H atoms, inducing a gradual transition from a planar, aromatic molecule toward a corrugated, aliphatic species. The mass spectra of hydrogenated coronene cations show that molecules with odd numbers of additional hydrogen atoms (nH) are dominant, with particularly high relative intensity for “magic numbers” nH = 5, 11, and 17, for which hydrogen atoms have the highest binding energies. Reaction barriers and binding energies strongly affect the hydrogenation sequence and its site specificity. In this contribution, we monitor this sequence experimentally by the evolution of infrared multiple-photon dissociation (IRMPD) spectra of gaseous with nH = 3–11, obtained using an infrared free electron laser coupled to a Fourier transform ion cyclotron mass spectrometer. For weakly hydrogenated systems (nH = 3, 5) multiple-photon absorption mainly leads to loss of H atoms (and/or H2). With increasing nH, C2H2 loss becomes more relevant. For nH = 9, 11, the carbon skeleton is substantially weakened and fragmentation is distributed over a large number of channels. A comparison of our IRMPD spectra with density functional theory calculations clearly shows that only one or two hydrogenation isomers contribute to each nH. This confirms the concept of hydrogenation occurring along very specific sequences. Moreover, the atomic sites participating in the first 11 steps of this hydrogenation sequence are clearly identified. [ABSTRACT FROM AUTHOR]