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When Mass Spectrometry meets Spectroscopy: A Different Perspective on Ion Photodissociation
Gabelica, Valérie
20077th Igler MS-Tage
 

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Keywords :
mass spectrometry; spectroscopy; DNA; photodissociation; photodetachment; electron; porphyrin; laser; UV; visible; electronic; MS/MS
Abstract :
[en] Photodissociation methods are more and more widely used in analytical mass spectrometry as an alternative to collision-induced dissociation. A well known example is infrared multiphoton dissociation (IRMPD) with CO2 lasers (wavelength 10.6 µm), which is often preferred to SORI-CID in FTICR mass spectrometers. IR photons are absorbed by the vibrational modes of the ion, and this vibrational energy is redistributed all over the molecule. The fragmentation channels observed are usually the same as for CID. Recently, new possibilities offered by UV lasers of fixed wavelength (i.e. 157 nm or 193 nm) for peptide fragmentation were described. On the contrary to IRMPD, irradiation at these UV wavelengths gives rise to fragmentation channels which are unprecedented in CID, suggesting that photochemical reactions take place. The prerequisite for ion photodissociation is photon absorption. In the infrared, photon absorption results in the excitation of vibrational modes. Anharmonic coupling of the excited vibrational modes with other vibrational modes of the ion results in a rapid intramolecular vibrational energy redistribution (IVR). The energy of an IR photon is too low for the ion to dissociate (0.117 eV/photon for a 10.6 µm CO2 laser). Multiple cycles of absorption and IVR are necessary for IR photodissociation to occur (hence the term multiphoton). Because the first step in photodissociation is photon absorption, the recording of the photodissociation yield as a function of the photon wavenumber gives information on the vibrational modes of the ion. This is the basic principle of infrared spectroscopy experiments inside the mass spectrometer. This technique will be illustrated by a recent example on DNA single strands and quadruplex structures. The shifting of the vibrational frequencies corresponding to guanine C=O stretch gives information on the base pairing in the DNA ion in the gas phase. The situation is somewhat different with UV photons, which may lead to either electronic excitation of the ions or direct electron ejection. Electronic excited states may evolve in different ways: (1) Internal conversion to a vibrationally excited electronic ground state. Resulting fragments would then be similar to CID and IRMPD. This process can be multiphotonic and could be coined UVMPD. (2) Electron autodetachment, leading to the formation of radicals that can be further fragmented to obtain ECD-like or EDD-like fragments. (3) Photochemical reactions giving rise to unprecedented fragmentation channels. In all three cases, product ion yield is conditioned by photon absorption by an electronic state, meaning that UV spectroscopy of gas-phase ions can be performed with a mass spectrometer. We recently explored the effects of photon absorption on multiply charged DNA anions. Depending on the photon wavelength and on the chromophores, all three situations could be observed. Electron photodetachment is the major channel observed when using wavelengths corresponding to DNA base absorption.1,2 When irradiated at proper wavelength, chromophoric DNA ligands also give rise to electron photodetachment. UV-visible spectra of the DNA duplexes and complexes were recorded and compared with solution-phase spectra. Finally, a porphyrin ligand gives rise to specific fragmentation pathways when irradiated at 450 nm, while photodetachment is observed at 260 nm. The most recent results will be presented and discussed. 1. Gabelica, V.; Tabarin, T.; Antoine, R.; Rosu, F.; Compagnon, I.; Broyer, M.; De Pauw, E.; Dugourd, P. Electron Photodetachment Dissociation of DNA Polyanions in a Quadrupole Ion Trap Mass Spectrometer. Anal. Chem. 2006, 78 (18), 6564. 2. Gabelica, V.; Rosu, F.; Tabarin, T.; Kinet, C.; Antoine, R.; Broyer, M.; De Pauw, E.; Dugourd, P. Base-Dependent Electron Photodetachment from Negatively Charged DNA Strands upon 260-nm Laser Irradiation. J. Am. Chem. Soc. 2007, accepted for publication.
Research center :
Giga-Systems Biology and Chemical Biology - ULiège
Disciplines :
Chemistry
Author, co-author :
Gabelica, Valérie ;  Université de Liège - ULiège > Département de chimie (sciences) > GIGA-R : Laboratoire de spectrométrie de masse (L.S.M.)
Language :
English
Title :
When Mass Spectrometry meets Spectroscopy: A Different Perspective on Ion Photodissociation
Publication date :
27 February 2007
Event name :
7th Igler MS-Tage
Event organizer :
Kathrin Breuker
Event place :
Obergurgl, Austria
Event date :
February 25-28, 2007
By request :
Yes
Audience :
International
Funders :
F.R.S.-FNRS - Fonds de la Recherche Scientifique [BE]
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since 12 February 2011

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