The structure of deprotonated tri-alanine and its a(3)(-) fragment anion by IR spectroscopy

We present the first infrared spectra of a mass-selected deprotonated peptide anion (AlaAlaAla) and its decarboxylated fragment anion formed by collision induced dissociation. Spectra are obtained by IRMPD spectroscopy using an FTICR mass spectrometer in combination with the free electron laser FELIX. Spectra have been recorded over the 800-1800 cm(-1) spectral range and compared with density functional theory calculated spectra at the B3LYP/6-31+ +G(d,p) level for different isomeric structures. These experiments suggest a carboxylate anion for [M and an amide deprotonated (amidate) structure for the a(3) fragment anion [M - H - CO2](-). The frequency for the amidate carbonyl stretch occurring around 1555 +/- 5 cm(-1) has been confirmed by additional spectroscopic studies of the conjugated base of N-methylacetamide, which serves as a simple model system for the deprotonated amide linkage in a peptide anion

Gas-phase deprotonation of p-hydroxybenzoic acid investigated by IR spectroscopy: Solution-phase structure is retained upon ESI

The gas-phase structure of the conjugate base of p-hydroxybenzoic acid (and related compounds) and the influence of the solvent used in its generation by electrospray ionization have recently been under debate. While the phenoxide structure is known to be lower in energy in the gas phase, the carboxylate structure is favored in aqueous solution, fuelling the controversy. Here we probe the structure of this gas-phase anion by IR spectroscopy and show that its structure is determined by the protic or aprotic nature of the solvent, which suggests that it is the solution-phase structure that is transferred to the gas phase

Spectroscopically resolved competition between dissociation and detachment from nitrobenzene radical anion

We report the vibrational spectrum of the gas-phase isolated nitrobenzene radical anion. The spectrum has been acquired by infrared multiple-photon absorption induced dissociation and electron detachment using the FT mass spectrometer coupled to the infrared free-electron laser FELIX. Upon wavelength-dependent multiple-photon absorption of intense IR irradiation, the vibrational spectrum acquired by on-resonance dissociation to NO2− was shown to correlate with the more sensitive electron detachment channel which is indirectly observed by using SF6 as electron scavenger. The spectrum is compared to previous spectroscopic studies and novel DFT calculations. The frequency and intensity changes of the vibrational bands for the radical anion with respect to the neutral are interpreted with the aid of molecular orbital calculations and mode projection analysis. The vibrations of the neutral and the anion are interpreted in terms of the component benzene modes. The anion shows a reversal of the familiar strongly deactivating meta-directing electrophilic aromatic substitution effect of the neutral due to a resonance effect placing electron density at the ortho- and para-positions, resulting in a structure of distonic character. The greater abundance of the electron detachment channel over the NO2− loss dissociation channel is interpreted in terms of statistical models of the energy-dependent unimolecular rates. The anion and neutral vibrational frequencies employed in a quasi-equilibrium theory (QET) model of electron detachment compare favorably to previous experimental results of metastable anion autodetachment lifetimes. The ratio of dissociation to detachment is investigated as a function of FEL power and the competition between these channels is in agreement with a statistical model

Oxazolone Versus Macrocycle Structures for Leu-Enkephalin b(2)-b(4): Insights from Infrared Multiple-Photon Dissociation Spectroscopy and Gas-Phase Hydrogen/Deuterium Exchange

The collision-induced dissociation (CID) products

Encapsulation of metal cations by the PhePhe ligand: a cation-pi ion cage

Structures and binding thermochemistry are investigated for protonated PhePhe and for complexes of PhePhe with the alkaline-earth ions Ba2+ and Ca2+, the alkali-metal ions Li+, Na+, K+, and Cs+, and the transition-metal ion Ag+. The two neighboring aromatic side chains open the possibility of a novel encapsulation motif of the metal ion in a double cation−π configuration, which is found to be realized for the alkaline-earth complexes and, in a variant form, for the Ag+ complex. Experimentally, complexes are formed by electrospray ionization, trapped in an FT-ICR mass spectrometer, and characterized by infrared multiple photon dissociation (IRMPD) spectroscopy using the free electron laser FELIX. Interpretation is assisted by thermochemical and IR spectral calculations using density functional theory (DFT). The IRMPD spectrum of protonated PhePhe is reproduced with good fidelity by the calculated spectrum of the most stable conformation, although the additional presence of the secondmost stable conformation is not excluded. All metal-ion complexes have charge-solvated binding modes, with zwitterion (salt bridge) forms being much less stable. The amide oxygen always coordinates to the metal ion, as well as at least one phenyl ring (cation−π interaction). At least one additional chelation site is always occupied, which may be either the amino nitrogen or the carboxy carbonyl oxygen. The alkaline-earth complexes prefer a highly compact caged structure with both phenyl rings providing cation−π stabilization in a "sandwich" configuration (OORR chelation). The alkali-metal complexes prefer open-cage structures with only one cation−π interaction, except perhaps Cs+ . The Ag+ complex shows a unique preference for the closed-cage amino-bound NORR structure. Ligand-driven perturbations of normal-mode frequencies are generally found to correlate linearly with metal-ion binding energy

Encapsulation of Metal Cations by the PhePhe Ligand: A Cation-pi Ion Cage

Structures and binding thermochemistry are investigated for protonated PhePhe and for complexes of PhePhe with the alkaline-earth ions Ba2+ and Ca2+, the alkali-metal ions Li+, Na+, K+, and Cs+, and the transition-metal ion Ag+. The two neighboring aromatic side chains open the possibility of a novel encapsulation motif of the metal ion in a double cation-pi configuration, which is found to be realized for the alkaline-earth complexes and, in a variant form, for the Ag+ complex. Experimentally, complexes are formed by electrospray ionization, trapped in an FT-ICR mass spectrometer, and characterized by infrared multiple photon dissociation (IRMPD) spectroscopy using the free electron laser FELIX. Interpretation is assisted by thermochemical and IR spectral calculations using density functional theory (DFT). The IRMPD spectrum of protonated PhePhe is reproduced with good fidelity by the calculated spectrum of the most stable conformation, although the additional presence of the secondmost stable conformation is not excluded. All metal-ion complexes have charge-solvated binding modes, with zwitterion (salt bridge) forms being much less stable. The amide oxygen always coordinates to the metal ion, as well as at least one phenyl ring (cation-pi interaction). At least one additional chelation site is always occupied, which may be either the amino nitrogen or the carboxy carbonyl oxygen. The alkaline-earth complexes prefer a highly compact caged structure with both phenyl rings providing cation-pi stabilization in a "sandwich" configuration (OORR chelation). The alkali-metal complexes prefer open-cage structures with only one cation-pi interaction, except perhaps Cs+. The Ag+ complex shows a unique preference for the closed-cage amino-bound NORR structure. Ligand-driven perturbations of normal-mode frequencies are generally found to correlate linearly with metal-ion binding energy

Cationized phenylalanine conformations characterized by IRMPD and computation for singly and doubly charged ions

Electrospray ionization produces phenylalanine (Phe) complexes of the alkali metal ion series, plus Ag+ and Ba2+. Infrared multiple photon dissociation (IRMPD) spectroscopy using the FELIX free electron laser light source is used to characterize the conformations of the ions, in conjunction with density functional theory (DFT) calculations giving thermochemical information and computed infrared spectra for likely candidate conformations. For complexes of small, singly charged ions (Li+, Na+, K+ and Ag+) a single tridentate, charge-solvated conformational theme (N/O/Ring) binding amino nitrogen, carbonyl oxygen and the aromatic ring to the metal ion accounts for all the observations. The larger alkalis Rb+ and Cs+ show clear spectroscopic evidence of mixed populations, containing substantial fractions of both tridentate and also bidentate chelation. For Rb+ the bidentate fraction is assigned as the (O/Ring) chelation pattern, while for Cs+ a mixture of (O/Ring) and (O/O) chelation patterns seems likely. All of the smaller ions with high positive charge density have a clear preference for cation-pi interaction with the side-chain aromatic ring, but for the larger ions Rb+ and particularly Cs+ this interaction becomes sufficiently weak to allow conformations having the metal ion remote from the pi system. The Ba2+ complex is unique in showing clear evidence of a major fraction of salt-bridge (zwitterionic) ions along with charge-solvated conformations. Plots of the frequency shifts of the two highly perturbed ligand vibrational modes (C=O stretch and NH2 frustrated inversion) give good linear correlations with the binding energy of the metal to the ligand