Shedding Light on the Frontier of Photodissociation

Abstract

The development of new ion activation/dissociation methods is motivated by the need for more versatile ways to characterize structures of ions, especially in the growing arena of biological mass spectrometry in which better tools for determining sequences, modifications, interactions, and conformations of biopolymers are essential. Although most agree that collision-induced dissociation (CID) remains the gold standard for ion activation/dissociation, recent inroads in electron-and photon-based activation methods have cemented their role as outstanding alternatives. This article will focus on the impact of photodissociation, including its strengths and drawbacks as an analytical tool, and its potential for further development in the next decade. Moreover, the discussion will emphasize photodissociation in quadrupole ion traps, because that platform has been used for one of the greatest arrays of new applications over the past decade.Key words: Photodissociation, Ion activation, Peptide, Tandem mass spectrometry, Ion trap T he field of ion activation/dissociation remains incredibly vibrant due to the ongoing quest to improve the sequencing of biopolymers and map their modifications, to increase the sensitivity to small structural differences, to enhance the ability to differentiate isomers, to facilitate highthroughput applications, and to advance the understanding of fragmentation mechanisms for better automated spectral interpretation. CID has proven to be enormously robust and readily implemented, but insufficient energy deposition and/ or low efficiency for certain fragmentation pathways has stimulated the hunt for other options. There have been several reviews and insightful discussions of electron-based activation methods [1-6], including electron-capture dissociation (ECD) and electron-transfer dissociation (ETD), so these methods will not be extensively addressed in the present article. Examples of some of the more recent photodissociation studies are cited herein, but the citations are by no means comprehensive.Photodissociation describes the process in which ions are exposed to photons of a selected wavelength, resulting in an accumulation of internal energy that leads to dissociation [7][8][9][10]. The activation process may entail the absorption of one or more high-energy UV or visible photons (∼2-10 eV/ photon) or tens/hundreds of lower energy IR photons (ca. 0.1 eV/photon). Both pulsed and continuous-wave lasers have been used for photodissociation. The energization period may range from nanoseconds to hundreds of milliseconds based on the photon flux of the laser, the competition between ion activation and collisional cooling, and the energy deposition per photon. The ability to vary photon flux, the total irradiation period, and the selected wavelength endows photodissociation with a high degree of tunability. In fact, it is precisely this type of tunability and the increasing availability of wavelength-tunable lasers coupled to ion trapping mass spectrometers, including Fourier transform i...

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“…In fact, both UVPD and to an even greater extent UVPD/HCD generated a number of fragment ions that exhibited hydrogen migration suggesting that they were generated via a radical mechanism, which has been reported in other UVPD studies of other biopolymers. 41,43–66 Although UVPD/HCD did not provide any new structural information over UPVD, the dissociation efficiency of UVPD/HCD was greater than UVPD alone and the array of observed fragment ions exceeded that of HCD alone, thus providing information about both the lipid A and inner core regions of Kdo 2 -Lipid A. One notable difference between CID, HCD, UVPD and UVPD/HCD is the variation in the relative abundances of doubly charged fragment ions.…”

“…As a promising alternative, UVPD is a higher energy activation method that has demonstrated advantages over CID for characterization of a wide range of biopolymers (including lipids, proteins, peptides, nucleic acids, and glycans). 41,43–66 Hybrid activation methods consisting of UVPD in conjunction with CID or electron transfer dissociation (ETD) have also been developed as a means to yield even richer fragmentation patterns. 44,46 We have explored UVPD at 193 nm for characterization of glycolipids, including lipid A, and found that UV photoexcitation promotes extensive C-C, C-N, C-O, glycosidic and cross-ring cleavages which contribute to the highly informative fragmentation patterns observed upon UVPD.…”

UV Photodissociation Action Spectroscopy of Haloanilinium Ions in a Linear Quadrupole Ion Trap Mass Spectrometer

“…Many mass spectrometry-based experiments, not just ion traps, have since exploited the use of coherent laser light sources for photodissociation spectroscopy -both UV and IR -as well as for photodissocation of molecules for structural characterization. [23][24][25][26][27] UV-vis PD action spectroscopy of ions is becoming increasingly prevalent due to advances and commercial availability of both ion trapping technologies and tunable laser sources. The linear quadrupole ion trap [28,29] is particularly well suited to PD experiments [30] as it provides a trapped ion ensemble with large number densities, located in a tight cylindrical ion packet confined around the ion-trap principal axis (z-axis), [31] other well-suited configurations include multipole traps [32,33] and storage rings.…”

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“…For example, photodissociation methods for MS/MS have achieved some success in this regard [116,117]. Irradiation of trapped ions with infrared photons to bring about IRMPD is one such method [118].…”