IR-UV ion-dip spectroscopy

Keywords Biomolecules Conformation selective Free electron laser FTICR Ion trap IRMPD IR-UV ion dip spectroscopy Laser desorption Molecular beam OPO Tandem mass spectrometry... 

The types of action spectroscopy presented in this book include IR-UV ion dip spectroscopy for neutral species and Infrared Multiple Photon Dissociation... 

Fig. 2 Schematic representation of (a) a one-colour two-photon REMPI schane (b) the modus operand of IR-UV ion-dip spectroscopy (c) IR probing scheme for the excited state using UV— IR-UV spectroscopy (d) a general scheme for stimulated emission pumping for conformer A and B...

Fig. 3 a) mass selected one-color REMPI spectrum, b) IR-UV ion dip spectra, and c) IR-UV hole-bum spectra of the capped dlpeptide PheCys adapted from 8] The REMPI spectmm shows the vibrational progression of the electronic excited state of multiple conformations. The contributions of these conformers are visualized using the conformer selective IR-UV hole-burning method. The structure of the individual conformers can be probed by IR-UV ion-dip spectroscopy, yielding conformer specific ground-state IR spectra. 

Probing the Structure of the Electronic Ground State IR-UV Ion Dip Spectroscopy... 

Structure. As for ground state IR-UV ion dip spectroscopy, the depletion in the electronically excited state caused by IR absorption can also be probed via LIF. LIF detection should be equally applicable to ground state and excited states, but has more limited application as strongly fluorescent molecules are required [102, 104, 105]. 

In conclusion, laser desorption in combination with molecular beam high-resolution spectroscopic methods has enabled the structural characterization of gas-phase biomolecules of significant size. Moreover, IR-UV ion dip spectroscopy offers information about the number and types of conformers present. Applications to diverse classes of biomolecular systems such as peptides, carbohydrates and nucleobases, are described in the various chapters of this book. 

Robertson EG, Hockiidge MR, Jelfs PD, Simrais JP (2000) IR-UV ion-dip spectroscopy of N-benzylformamide clusters stepwise hydration of a model peptide. J Phys Chem A 104 11714... 

The conformer selectivity in IR-UV double resonance methods relies on the excitation bands of different conformers being resolvable in the UV spectrum. For tmresolved UV spectra, indicating either short excited state lifetimes (<1 ns) or strong conformational heterogeneity, the IR-UV ion dip scheme can be modified by introducing either a femtosecond ionization laser (IR femtosecond multiphoton ionization) [80,81] or an additional IR step (IR-IR-UV spectroscopy) [21,82-84]. 

Jaeqx S, Oomens J, Cimas A, Gaigeot M-P, Rijs AM (2014) Gas-phase peptide structures unraveled by far-IR spectroscopy combining IR-UV ion-dip experiments with Bom-Oppenheimer molecular dynamics simulations. Angew Chem hit Ed Engl 53(14) 3663-3666... 

This is demonstrated here with DFT-MD simulations to extract the vibrational spectrum of the Ac-Phe-Ala-NH2 peptide (capped peptide composed of phenyl and alanine residues, labeled FA in the remainder of the text see Fig. 9) in the far-IR domain, in relation to IR-UV ion-dip experiments [111]. With dynamical spectroscopy, one has to keep in mind that the length of the trajectories has to be commensurate with the domain investigated. Hence, to sample the vibrational modes and then-couplings in the 100-1,000 cm region, we have accumulated trajectories of 30 ps and make an average for the final theoretical specfrum over two ttajectories (i.e., 60 ps sampling in total). 

The study of complex mixtures of fragment isomers also requires enhanced gas-phase structural tools. The advent of cold spectroscopy techniques, such as IR-UV ion dip [53] or IR pre-dissociation [54] spectroscopy, offers much higher resolution, and can hence provide much more structural detail. In cold spectroscopy techniques, the ions are cooled to cryogenic temperatures (i.e., 4-20 K), resulting in frozen conformations and narrower, better-defined spectral bands. Another key advantage of cold spectroscopy methods is that particular conformers can be selectively studied. In IR-UV... 

Figure 12-1. Schematic diagram to illustrate double resonance techniques, (a) REMPI 2 photon ionization. The REMPI wavelength is scanned, while a specific ion mass is monitored to obtain a mass dependent SI <- SO excitation spectrum, (b) UV-UV double resonance. One UV laser is scanned and serves as a burn laser, while a second REMPI pulse is fired with a delay of about 100 ns and serves as a probe . The probe wavelength is fixed at the resonance of specific isomer. When the burn laser is tuned to a resonance of the same isomer it depletes the ground state which is recorded as a decrease (or ion dip) in the ion signal from the probe laser, (c) IR-UV double resonance spectroscopy, in which the burn laser is an IR laser. The ion-dip spectrum reflects the ground state IR transitions of the specific isomer that is probed by the REMPI laser, (d) Double resonance spectroscopy can also use laser induced fluorescence as the probe, however that arrangement lacks the mass selection afforded by the REMPI probe...

Recently, Ligare et al. confirmed the diketo character of the broad absorption of both U and T by IR-UV DRS. Their spectra showed ion gain rather than dips upon IR excitation. Double resonant spectroscopy relies on the fact that the bum laser changes the ground state vibrational distribution and thus the overall Franck-Condon (FC) factors. When the probe laser is timed to a strong resonance, this invariably leads to ion dip. However, in the case of a broad absorption, the consequences of modified FC factors are not always predictable and an ion dip may not necessarily occur. In the case of U and T, it is probable that there is a large... 

The conformational landscape of phenylalanine (R=CH2-C6Hs, m.p. = 270-275°C) has been widely investigated [11, 137-147]. Six conformational species were identified using laser induced fluorescence LIF, hole burning UV-UV, and ion dip IR-UV spectroscopy coupled with ab initio calculations [11, 137-141]. Lee et al. [141] carried out a definitive identification of the conformers of phenylalanine, based upon comparisons between the partially resolved ultraviolet band contours and that simulated by ab initio computations. The study of the rotational spectrum of phenylalanine by LA-MB-FTMW [153] showed rather weak spectra of only two conformers, Ila and Ilb (see Fig. 21). Both conformers exhibit a trans configuration in the COOH group, being stabilized by O-H- N and N-H- - -it... 

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