New approach for studying slow fragmentation kinetics in FT-ICR: Surface-induced dissociation combined with resonant ejection
Introduction
Tandem mass spectrometry (MS/MS) is a powerful tool both for structural characterization and for studying the energetics and mechanisms of gas-phase fragmentation of complex ions [1], [2]. In MS/MS experiments, mass-selected ions are excited by photon absorption, electron capture/transfer, or collisions with a neutral gas or a surface. Vibrational excitation of the precursor ion is followed by unimolecular dissociation with a microcanonical rate constant described by the Rice–Ramsperger–Kassel–Marcus (RRKM) theory [3], [4]. According to RRKM, the rate constant increases almost exponentially with increase in ion internal energy. Several techniques have been developed for measuring dissociation thresholds and internal energy-dependent microcanonical rate constants, k(E) [4], [5], [6]. These include time-resolved single-photon photoionization [7] and photodissociation [8], photoelectron-photoion coincidence [6], and threshold collision-induced dissociation (TCID) under single-collision conditions [9].
Gas-phase fragmentation of complex ions is associated with the kinetic shift, defined as the amount of internal energy necessary for the ion to fragment on the time scale of the instrument [10]. The kinetic shift increases with increase in the number of vibrational degrees of freedom of the ion, making it impossible to observe dissociation at the thermochemical threshold. It follows that efficient fragmentation of large gaseous ions requires deposition of a larger amount of energy that may be difficult to achieve using traditional approaches. Several techniques have been developed for studying the energetics and dynamics of fragmentation of large ions.
Most of these techniques have been implemented on Fourier transform ion cyclotron resonance mass spectrometers (FT-ICR MS) [11], [12]. Blackbody infrared radiative dissociation (BIRD) [13], [14] has been extensively used for studying thermal kinetics of relatively small molecules [13], [15] and large peptide [14], [16], protein [17], [18], and cluster ions [19]. In BIRD, ions are maintained at a desired temperature through interaction with blackbody radiation generated by the vacuum chamber walls [13]. Temperature-dependent experiments enable direct determination of Arrhenius dissociation parameters for large ions [20]. Thermal kinetics have been also examined using infrared multiphoton dissociation (IRMPD) [21], [22], [23], in which slow absorption of multiple photons results in an energy deposition function (EDF) closely approximated by a Boltzmann distribution. Sustained off-resonance CID (SORI-CID) [24], in which ions are excited through multiple collisions with neutral molecules, once it has been calibrated against other methods, is a useful technique for studying the energetics of ion fragmentation [25], [26], [27].
It has been demonstrated that surface-induced dissociation (SID) [28], [29], [30] in FT-ICR MS has particular advantages for studying energy and entropy effects in dissociation of large ions [31]. Rapid vibrational excitation of hyperthermal (<100 eV) ions in collisions with surfaces [30], [32], [33] results in a quasi-thermal EDF [34] that is adequately described by a relatively simple analytical expression. RRKM modeling of time- and collision energy-resolved FT-ICR SID data [26], [35] enables accurate determination of energy and entropy effects in the gas-phase fragmentation of large ions including even- [31], [36], [37], [38], [39], [40], [41], [42], [43], [44] and odd-electron peptide ions [45], [46], [47], [48], [49], non-covalent complexes [50], [51], [52], [53], [54], and organometallic complexes [55]. In the present study, we introduce a new approach for examining the kinetics of large ion fragmentation using FT-ICR SID, which is closely related to earlier studies of metastable ion lifetimes in FT-ICR [56], [57]. In this approach, SID is coupled with resonant ejection of selected fragment ions [58], [59]. By using a fairly short ejection pulse (5 ms) and varying the delay time between ion activation and resonant ejection of a fragment ion while keeping the total reaction time constant, we directly probe the kinetics of fragment ion formation on a timescale of >1 ms. RRKM modeling of the kinetics plots provides information on the energy and entropy effects governing the dissociation of large ions.
Section snippets
Experimental
Angiotensin III and 1-dodecanethiol were purchased from Sigma–Aldrich (St. Louis, MO) and used as received. Angiotensin III was dissolved in a 70:30 (v/v) methanol:water solution with 1% acetic acid. The self-assembled monolayer surface of 1-dodecanethiol (HSAM) was prepared on a single gold crystal (Monocrystals Co., Richmond Heights, OH). The target was cleaned in an ultraviolet (UV) cleaner (Model 135,500, Boekel Industries Inc., Feasterville, PA) for 10 min and allowed to stand in a solution
SID combined with resonant ejection of selected fragment ions
In this study, we examined fragmentation kinetics of singly protonated angiotensin III (RVYIHPF) following collision with an HSAM surface. The kinetics were probed by applying a 5 ms duration excitation pulse at a specified delay time, td, for selective ejection of a fragment ion of interest after gated trapping while keeping the delay between ion trapping and detection at 150 ms. Under these conditions, fragment ions formed prior to the ejection pulse are eliminated from the spectrum, while
Conclusions
Kinetics of slow peptide ion fragmentation have been examined using a new approach introduced in this study that combines SID in FT-ICR MS with resonant ejection of a selected fragment ion at a variable delay time. All ions at a particular m/z formed before the ejection pulse are removed from the spectrum while ions at the same m/z formed after the ejection pulse are detected. Fragment ions formed in fast dissociation channels are eliminated from the spectrum even at very short delay times.
Acknowledgments
This work was supported by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences, Chemical Sciences, Geosciences & Biosciences Division. The research was performed using EMSL, a national scientific user facility sponsored by the DOE's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is operated by Battelle for DOE under Contract DE-AC05-76RL01830.
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