Structure of protonated tryptophan dimer in the gas phase investigated by IRPD spectroscopy and theoretical calculations

Indication of 310-Helix Structure in Gas-Phase Neutral Pentaalanine

We investigate the gas-phase structure of the neutral pentaalanine peptide. The IR spectrum in the 340-1820 cm^-1 frequency range is obtained by employing supersonic jet cooling, infrared multiphoton dissociation, and vacuum-ultraviolet action spectroscopy. Comparison with quantum chemical spectral calculations suggests that the molecule assumes multiple stable conformations, mainly of two structure types. In the most stable conformation theoretically found, the N-terminus forms a C5 ring and the backbone resembles that of an 3₁₀-helix with two β-turns. Additionally, the conformational preferences of pentaalanine have been evaluated using Born-Oppenheimer molecular dynamics, showing that a nonzero simulation time step causes a systematic frequency shift.

Structural Diversity of Di-Metalized Arginine Evidenced by Infrared Multiple Photon Dissociation (IRMPD) Spectroscopy in the Gas Phase

Although metal cations are prevalent in biological media, the species of multi-metal cationized biomolecules have received little attention so far. Studying these complexes in isolated state is important, since it provides intrinsic information about the interaction among them on the molecular level. Our investigation here demonstrates the unexpected structural diversity of such species generated by a matrix-assisted laser desorption ionization (MALDI) source in the gas phase. The photodissociation spectroscopic and theoretical study reflects that the co-existing isomers of [Arg+Rb+K−H]⁺ can have energies ≥95 kJ/mol higher than that of the most stable one. While the result can be rationalized by the great isomerization energy barrier due to the coordination, it strongly reminds us to pay more attention to their structural diversities for multi-metalized fundamental biological molecules, especially for the ones with the ubiquitous alkali metal ions.

IRMPD Spectroscopy of Homo- and Heterochiral Asparagine Proton-Bound Dimers in the Gas Phase

We investigate gas-phase structures of homo- and heterochiral asparagine proton-bound dimers with infrared multiphoton dissociation (IRMPD) spectroscopy and quantum-chemical calculations. Their IRMPD spectra are recorded at room temperature in the range of 500-1875 and 3000-3600 cm-1. Both varieties of asparagine dimers are found to be charge-solvated based on their IRMPD spectra. The location of the principal intramolecular H-bond is discussed in light of harmonic frequency analyses using the B3LYP functional with GD3BJ empirical dispersion. Contrary to theoretical analyses, the two spectra are very similar.

Versatile Double-Beam Confocal Laser System Combined with a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer for Photodissociation Mass Spectrometry and Spectroscopy

Both infrared multiphoton dissociation (IRMPD) and ultraviolet photodissociation (UVPD) play important roles in tandem mass spectrometry and the action spectroscopy of organic and biological molecules. A flexible combination of the two methods may provide researchers with more versatile and powerful ion activation/dissociation choices for structural characterization and spectroscopic studies. Here, we report the integration of two tunable lasers with a Fourier transform ion cyclotron resonance mass spectrometer in a confocal mode, which offers multiple capabilities for photon activation/dissociation experiments. The two overlapped beams can be introduced into the cell individually, sequentially, or simultaneously, providing highly flexible and diverse activation schemes. The setup can also measure the UVPD or IRMPD action spectra of fragment ions generated by previous photon dissociation processes. In addition, the multistage tandem-in-time mass spectrometry performance up to MS⁴, including three different activation methods in a single cell, has also been demonstrated.

Structure of Proton-Bound Methionine and Tryptophan Dimers in the Gas Phase Investigated with IRMPD Spectroscopy and Quantum Chemical Calculations

The structures of three proton-bound dimers (Met₂H⁺, MetTrpH⁺, and Trp₂H⁺) are investigated in the gas phase with infrared multiple photon disassociation (IRMPD) spectroscopy in combination with quantum chemical calculations. Their IRMPD spectra in the range of 600–1850 cm^–1 are obtained experimentally using an FT-ICR mass spectrometer and the CLIO free electron laser as an IR light source. The most abundant conformers are elucidated by comparing the IRMPD spectra with harmonic frequencies obtained at the B3LYP-GD3BJ/6-311++G** level of theory. Discrepancies between the experimental and theoretical data in the region of 1500–1700 cm^–1 are attributed to the anharmonicity of the amino bending modes. We confirm the result of a previous IRMPD study that the structure of gas-phase Trp₂H⁺ is charge-solvated but find that there are more stable structures than originally reported (Feng, R.; Yin, H.; Kong, X. Rapid Commun. Mass Spectrom.2016, 30, 24–28). In addition, gas-phase Met₂H⁺ and MetTrpH⁺ have been revealed to have charge-solvated structures. For all three dimers, the most stable conformer is found to be of type A. The spectrum of Met₂H⁺, however, cannot be explained without some abundance of type B charge-solvated conformers as well as salt-bridged structures.

Infrared Multiphoton Dissociation Spectroscopy with Free-Electron Lasers: On the Road from Small Molecules to Biomolecules

Infrared multiphoton dissociation (IRMPD) spectroscopy is commonly used to determine the structure of isolated, mass-selected ions in the gas phase. This method has been widely used since it became available at free-electron laser (FEL) user facilities. Thus, in this Minireview, we examine the use of IRMPD/FEL spectroscopy for investigating ions derived from small molecules, metal complexes, organometallic compounds and biorelevant ions. Furthermore, we outline new applications of IRMPD spectroscopy to study biomolecules.

The structures and properties of anionic tryptophan complexes

IRMPD spectroscopy and electronic structure calculations are employed to identify π–π interactions in ionic tryptophan clusters.