Differences between collisionally activated and electron-transfer dissociations found for CH(2)X(2)(X = Cl, Br, and I) by using alkali-metal targets

High-energy collisionally activated dissociation (HE-CAD) and high-energy electron- transfer dissociation (HE-ETD) on collisions with alkali-metal targets (Cs, K, and Na) were investigated for CH(2)X(2) (+) (X = Cl, Br, and I) ions by tandem mass spectrometry (MS/MS). In the HE-CAD spectra observed, peaks associated with CH(2)X(+) ions formed by a loss of a halogen atom are always predominant regardless of precursor ions and target metals. The observation of the predominant CH(2)X(+) ions is explained by the lowest energy levels of the fragments of CH(2)X(+) + X among the possible fragment energy levels and internal-energy distribution in HE-CAD. In the charge-inversion spectra, relative peak intensities of the negative ions formed by HE-ETD strongly depend on the precursor ions and the target metals. While the CHCl(2) (-) ion was predominant in the spectra of CH(2)Cl(2) (+) regardless of target species, the most intense peaks in those of CH(2)Br(2) (+) and CH(2)I(2) (+) were ascribed to either Br(-) or CH(2)Br(-) and either I(-) or I(2) (-), respectively, depending on the target metals. The dependence of the relative intensities of the fragment ions by HE-ETD on the precursor ions and target species are discussed on the basis of the energy levels of the neutral fragments and the narrow internal-energy distribution resulting from the near-resonant neutralization. It was demonstrated that HE-ETD using the alkali-metal targets provided rich information on the dissociation of the neutral species.     

Study of the dissociation of a charge-reduced phosphopeptide formed by electron transfer from an alkali metal target

Doubly protonated phosphopeptide (YGGMHRQET(p)VDC) ions obtained by electrospray ionization were collided with Xe and Cs targets to give singly and doubly charged positive ions via collision-induced dissociation (CID). The resulting ions were analyzed and detected by using an electrostatic analyzer (ESA). Whereas doubly charged fragment ions resulting from collisionally activated dissociation (CAD) were dominant in the CID spectrum with the Xe target, singly charged fragment ions resulting from electron transfer dissociation (ETD) were dominant in the CID spectrum with the Cs target. The most intense peak resulting from ETD was estimated to be associated with the charge-reduced ion with H2 lost from the precursor. Five c-type fragment ions with amino acid residues detached consecutively from the C-terminal were clearly observed without a loss of the phosphate group. These ions must be formed by N--Calpha bond cleavage, in a manner similar to the cases of electron capture dissociation (ECD) and ETD from negative ions. Although the accuracy in m/z of the CID spectra was about +/-1 Th because of the mass analysis using the ESA, it is supposed from the m/z values of the c-type ions that these ions were accompanied by the loss of a hydrogen atom. Four z-type (or y--NH3, or y--H2O) ions analogously detached consecutively from the N-terminal were also observed. The fragmentation processes took place within the time scale of 4.5 micros in the high-energy collision. The present results demonstrated that high-energy ETD with the alkali metal target allowed determination of the position of phosphorylation and the amino acid sequence of post-translational peptides.     

Effects of target gas in collision-induced dissociation using a double quadrupole mass spectrometer and radiofrequency

Collision-induced dissociation (CID) of polyatomic ions sampled from an rf-powered glow discharge is examined by using three target gases including atomic (Ar and Xe) and molecular species (N2). Collisions with these targets in the first quadrupole of the double quadrupole system result in the loss of discharge species by dissociation, symmetric and asymmetric charge exchange, and scattering, each to varying degrees. These processes are seen to be a function of the relative mass, size... and ionization potentials of the target species, as well as the collision center-of-mass energies. In light of the comparisons, xenon appears to be the best collision target for both CID and charge exchange because of its relatively low ionization potential and high dissociation efficiency of polyatornic species. Evidence for both symmetric and asymmetric charge exchange is presented for Ar and Xe target gases.     

Tandem Time-of-Flight Mass Spectrometer with High Precursor Ion Selectivity Employing Spiral Ion Trajectory and Improved Offset Parabolic Reflectron

In this study, we have developed a tandem time-of-flight mass spectrometry (TOF/TOF) technique involving the use of a matrix-assisted laser desorption/ionization ion source that exhibits high precursor ion selectivity. An ion optical system with a 17 m spiral ion trajectory was used in the first time-of-flight mass spectrometer. High precursor ion selectivity was achieved by realizing a 15 m flight path, which is considerably longer than that of the conventional MALDI-TOF/TOF before the precursor ion selection by an ion gate; monoisotopic ions could be selected properly up to m/z 2500. Furthermore, the first time-of-flight mass spectrometer was composed of electrostatic sectors and could eliminate post-source decay (PSD) ions. Precursor ions with 20 keV kinetic energy were selected and injected into a collision cell, leading to the generation of fragment ions by high-energy collision-induced dissociation (HE-CID). The optimized second time-of-flight mass spectrometer included a post-acceleration region and an offset parabolic reflectron to record product ion spectra in the entire mass range. Our system could generate a simple HE-CID product ion spectrum because each fragment pathway could be observed as a single peak by the selection of monoisotopic ions of all precursor ions and HE-CID fragment pathways could be predominantly observed by the PSD ion elimination.     

Rapid sequencing of a peptide containing a single disulfide bond using high-energy collision-induced dissociation

A peptide containing a single disulfide bond was sequenced using high-energy collision-induced dissociation (HE-CID) in conjunction with a high mass resolution time-of-flight tandem mass spectrometer equipped with a matrix-assisted laser desorption/ionization source. This mass spectrometer, which has spiral ion trajectory, allowed both high mass resolution and high precursor ion selectivity. It is difficult to obtain sufficient product ions from peptides containing disulfide bonds using HE-CID due to the single collision in the gas phase. To compensate for insufficient dissociation, the disulfide bond was cleaved via an in-source reduction process using 1,5-diaminonaphthalene, a reducing matrix. After applying the reduction in the ionization, subsequent sequencing using HE-CID provided the detailed structural information of the peptide containing the single disulfide bond.     

Selective detection of phosphopeptides in complex mixtures by electrospray liquid chromatography/mass spectrometry

A mass spectrometry-based method that does not involve the use of radiolabeling was developed for selective detection of phosphopeptides in complex mixtures. Mixtures of phosphorylated and nonphosphorylated peptides at the low picomole level are analyzed by negative ion electrospray liquid chromatography/mass spectrometry using C-18 packed fused-silica columns (≤320-μm i.d.). Peptides and phosphopeptides in the chromatographic eluant undergo collision-induced dissociation in the free-jet expansion region prior to the mass analyzing quadrupole. Using relatively high collisional excitation potentials, phospho|peptides containing phosphoserine, phosphothreonine, and phosphotyrosine fragment to yield diagnostic ions at m/z 63 and 79 corresponding to PO₂ ^?; and PO₃ ^?, respectively. Chromatographic peaks containing phosphopeptides are indicated where these diagnostic ions maximize. The highest sensitivity for phosphopeptide detection is obtained using selected-ion monitoring for m/z 63 and 79. Full-scan mass spectra that exhibit the diagnostic phosphopeptide fragment ions, together with pseudomolecular ions, may be obtained by stepping the collisional excitation potential from a high value during the portion of each scan in which the low-mass-to-charge ratio diagnostic marker ions are being detected to a lower value while the upper mass-to-charge ratio range is being scanned. Good sensitivity for phosphopeptide detection was achieved using standard trifluoroacetic acid containing mobile phases for reversed-phase high-performance liquid chromatography. Data illustrating the selectivity and sensitivity of the approach are presented for mixtures of peptides and phosphopeptides containing the three commonly phosphorylated amino acids.     

Identification of Proteins and Phosphoproteins Using Pulsed Q Collision Induced Dissociation (PQD)

Pulsed Q collision induced dissociation (PQD) was developed to facilitate detection of low-mass reporter ions from labeling reagents (e.g., iTRΑQ) in peptide quantification using an LTQ mass spectrometer (MS). Despite the large number of linear ion traps worldwide, the use and optimization of PQD for protein identification have been limited, in part due to less effective ion fragmentation relative to the collision induced dissociation (CID). PQD expands the m/z coverage of fragment ions to the lower m/z range by circumventing the typical low mass cut-off of an ion trap MS. Since database searching relies on the matching between theoretical and observed spectra, it is not clear how ion intensity and peak number might affect the outcomes of a database search. In this report, we systematically evaluated the attributes of PQD mass spectra, performed intensity optimization, and assessed the benefits of using PQD on the identification of peptides and phosphopeptides from an LTQ. Based on head-to-head comparisons between CID (higher intensity) and PQD (better m/z coverage), peptides identified using PQD generally have Xcorr scores lower than those using CID. Such score differences were considerably diminished by the use of 0.1% m-nitrobenzyl alcohol (m-NBA) in mobile phases. The ion intensities of both CID and PQD were adversely affected by increasing m/z of the precursor, with PQD more sensitive than CID. In addition to negating the 1/3 rule, PQD enhances direct bond cleavage and generates patterns of fragment ions different from those of CID, particularly for peptides with a labile functional group (e.g., phosphopeptides). The higher energy fragmentation pathway of PQD on peptide fragmentation was further compared to those of CID and the quadrupole-type activation in parallel experiments.     

Dipole-Guided Electron Capture Causes Abnormal Dissociations of Phosphorylated Pentapeptides

Electron transfer and capture mass spectra of a series of doubly charged ions that were phosphorylated pentapeptides of a tryptic type (pS,A,A,A,R) showed conspicuous differences in dissociations of charge-reduced ions. Electron transfer from both gaseous cesium atoms at 100 keV kinetic energies and fluoranthene anion radicals in an ion trap resulted in the loss of a hydrogen atom, ammonia, and backbone cleavages forming complete series of sequence z ions. Elimination of phosphoric acid was negligible. In contrast, capture of low-energy electrons by doubly charged ions in a Penning ion trap induced loss of a hydrogen atom followed by elimination of phosphoric acid as the dominant dissociation channel. Backbone dissociations of charge-reduced ions also occurred but were accompanied by extensive fragmentation of the primary products. z-Ions that were terminated with a deaminated phosphoserine radical competitively eliminated phosphoric acid and H₂PO₄ radicals. A mechanism is proposed for this novel dissociation on the basis of a computational analysis of reaction pathways and transition states. Electronic structure theory calculations in combination with extensive molecular dynamics mapping of the potential energy surface provided structures for the precursor phosphopeptide dications. Electron attachment produces a multitude of low lying electronic states in charge-reduced ions that determine their reactivity in backbone dissociations and H- atom loss. The predominant loss of H atoms in ECD is explained by a distortion of the Rydberg orbital space by the strong dipolar field of the peptide dication framework. The dipolar field steers the incoming electron to preferentially attach to the positively charged arginine side chain to form guanidinium radicals and trigger their dissociations.     

Phosphorylated serine and threonine residues promote site‐specific fragmentation of singly charged,arginine‐containing peptide ions

In order to investigate gas‐phase fragmentation reactions of phosphorylated peptide ions, matrix‐assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI) tandem mass (MS/MS) spectra were recorded from synthetic phosphopeptides and from phosphopeptides isolated from natural sources. MALDI‐TOF/TOF (TOF: time‐of‐flight) spectra of synthetic arginine‐containing phosphopeptides revealed a significant increase of y ions resulting from bond cleavages on the C‐terminal side of phosphothreonine or phosphoserine. The same effect was found in ESI‐MS/MS spectra recorded from the singly charged but not from the doubly charged ions of these phosphopeptides. ESI‐MS/MS spectra of doubly charged phosphopeptides containing two arginine residues support the following general fragmentation rule: Increased amide bond cleavage on the C‐terminal side of phosphorylated serines or threonines mainly occurs in peptide ions which do not contain mobile protons. In MALDI‐TOF/TOF spectra of phosphopeptides displaying N‐terminal fragment ions, abundant b–H₃PO₄ ions resulting from the enhanced dissociation of the pSer/pThr–X bond were detected (X denotes amino acids). Cleavages at phosphoamino acids were found to be particularly predominant in spectra of phosphopeptides containing pSer/pThr–Pro bonds. A quantitative evaluation of a larger set of MALDI‐TOF/TOF spectra recorded from phosphopeptides indicated that phosphoserine residues in arginine‐containing peptides increase the signal intensities of the respective y ions by almost a factor of 3. A less pronounced cleavage‐enhancing effect was observed in some lysine‐containing phosphopeptides without arginine. The proposed peptide fragmentation pathways involve a nucleophilic attack by phosphate oxygen on the carbon center of the peptide backbone amide, which eventually leads to cleavage of the amide bond. Copyright © 2009 John Wiley & Sons, Ltd.     

Activated Ion ETD Performed in a Modified Collision Cell on a Hybrid QLT-Oribtrap Mass Spectrometer

We describe the implementation and characterization of activated ion electron transfer dissociation (AI-ETD) on a hybrid QLT-Orbitrap mass spectrometer. AI-ETD was performed using a collision cell that was modified to enable ETD reactions, in addition to normal collisional activation. The instrument manifold was modified to enable irradiation of ions along the axis of this modified cell with IR photons from a CO₂ laser. Laser power settings were optimized for both charge (z) and mass to charge (m/z) and the instrument control firmware was updated to allow for automated adjustments to the level of irradiation. This implementation of AI-ETD yielded 1.6-fold more unique identifications than ETD in an nLC-MS/MS analysis of tryptic yeast peptides. Furthermore, we investigated the application of AI-ETD on large scale analysis of phosphopeptides, where laser power aids ETD, but can produce b- and y-type ions because of the phosphoryl moiety’s high IR adsorption. nLC-MS/MS analysis of phosphopeptides derived from human embryonic stem cells using AI-ETD yielded 2.4-fold more unique identifications than ETD alone, demonstrating a promising advance in ETD sequencing of PTM containing peptides.

Generation and characterization of ionic and neutral [HS-P-OH]<Superscript>+/·</Superscript> and S=P(OH)<Stack><Subscript>2</Subscript><Superscript>+/·</Superscript></Stack> in the Gas Phase by Tandem Mass Spectrometry and Computational Chemistry

Dissociative electron ionization of diethyl dithiophosphate (I) and O,O′-diethyl methylphosphonothioate (II) generates moderately abundant m/z 81 ions of composition [P, O, S, H₂]⁺. From tandem mass spectrometry experiments and theoretical calculations at the B3LYP/6-31G(d,p), G2, and G2 (MP2) levels it is concluded that the majority of the ions have the structure of HS-P-OH⁺ (1a ⁺) and it is separated by high-energy barriers from its isomers P(= S)OH₂⁺ (1b ⁺), P(= O)SH₂⁺ (1c ⁺), HP(= S)OH⁺ (1d ⁺), and HP(= O)SH⁺ (1e ⁺). Low-energy (metastable) ions 1a ⁺ dissociate via losses of H₂O and H₂S to yield m/z 63 (PS⁺) and m/z 47 (PO⁺) product ions, respectively. These reactions involve isomerization of 1a ⁺ into the stable isomers 1b ⁺ and 1c ⁺. Neutralization-reionization experiments confirm the theoretical prediction that radical 1a ^· is a stable species in the gas-phase. Variable-time NR experiments indicated that only a small fraction of metastable 1a ^· radicals dissociate in the 0.4–4.6 μs time window, while most dissociations occurred on a shorter time scale. RRKM calculations were performed to investigate unimolecular dissociation kinetics of 1a ^· which were found to be in agreement with the fragmentation observed in the NR spectrum. The 70-eV electron ionization of (I) and diethyl chlorothiophosphate (III) yields m/z 97 ions, predominantly of the structure S = P(OH)₂⁺ (2a ⁺). This conclusion follows from tandem mass spectrometry experiments and theoretical calculations. The calculations predict that (2a ⁺) is separated by high-energy barriers from its isomers O = P(SH)OH⁺ (2b ⁺), S = P(= O)OH₂⁺ (2c ⁺), and O = P(= O)SH₂⁺ (2d ⁺). Neutralization-reionization experiments confirmed that 2a ^· radical is a kinetically stable species on the time scale of up to 5 μs, which is in agreement with ab initio calculations. However, owing to a mismatch of Franck-Condon factors a large fraction of 2a ^· dissociates by loss of SH^· yielding O=P-OH.     

The elusive methene ylides CH2CIH,CH2FH and CH2OH2

The possible existence of the ylides CH₂CIH, CH₂₅FH and CH₂OH₂ as stable neutral species in the gas phase has been investigated by the neutralization–reionization (NR) mass spectrometry of their radical cations using a double-focusing mass spectrometer of reversed geometry. The experiments were, for the most part, performed under single-collision conditions with Xe as the neutralization target gas and He and O₂ as reionization agents. For each ylidion a peak was observed in their NR mass spectrum which indicated that the neutral ylide had apparently been produced. However for CH₂FH⁺˙ and CH₂OH₂⁺˙ the m/z 34 and m/z 32 peaks, respectively, were attribut-able to interferences from the natural isotopic abundance of ions of lower mass. For CH₂CIH⁺˙, the NR recovery signal was found to arise from the presence of CH₃CI⁺˙ as an impurity in the ylidion flux. This was proved by examination of the collisional activation mass spectra of the [C, H₃, CI]⁺˙ ions produced in the NR mass spectra of the conventional ions and ylidions, an experiment performed using a triple-sector mass spectrometer.     

Identification of Antifungal Peptides from Bacillus subtilis BS-918

Bacillus subtilis BS-918 shows strong activity against a broad spectrum of plant and postharvest pathogenic fungi. Antifungal compounds produced by BS-918 were isolated by extraction and gel chromatography, and structural identification was performed by electrospray ionization coupled with collision induced dissociation mass spectrometry. Two kinds of precursor ions belonging to iturin and fengycin were revealed. The first precursor ion at m/z 1071.7 was analyzed by collision induced dissociation mass spectrometry and was identified to be iturin A with a C₁₆ β-amino fatty acid chain. Collision induced dissociation mass spectrometry of the second precursor showed the presence of two fengycins. Ions at m/z 1449.5 and m/z 1463.9 were identified to homologs of fengycin A with C₁₅ and C₁₆ β-hydroxy fatty acid chains, whereas ions at m/z 1477.8 and m/z 1491.9 were homologs of fengycin B with C₁₅ and C₁₆ fatty acid chains.     

Evidence for the Amino–Claisen rearrangement occurring in the molecular ions of N-allylaniline

One of the most intense peaks in the mass spectrum of N-allylaniline is at m/z 106 (97%). High resolution analysis and collision-induced dissociation studies confirm that this peak contains mostly [C₇H₈N]⁺ ions having the anilinomethene structure, but also a small contribution is seen from [C₈H₁₀]⁺ ions which result from the loss of the elements of HCN from molecular ions, following an Amino–Claisen rearrangement. The occurrence of a thermal rearrangement in the sample molecules cannot, however, be completely ruled out. Studies on metastable molecular ions of N-allylaniline and collision-induced dissociation of the m/z 106 ions formed from these show that, in the case of molecular ions with energies closer to threshold, the rearrangement reaction competes much more effectively with the direct cleavage reaction.     

Measuring internal energy deposition in collisional activation using hydrated ion nanocalorimetry to obtain peptide dissociation energies and entropies

The internal energy deposited in both on- and off-resonance collisional activation in Fourier transform ion cyclotron resonance mass spectrometry is measured with ion nanocalorimetry and is used to obtain information about the dissociation energy and entropy of a protonated peptide. Activation of Na⁺(H₂O)₃₀ results in sequential loss of water molecules, and the internal energy of the activated ion can be obtained from the abundances of the product ions. Information about internal energy deposition in on-resonance collisional activation of protonated peptides is inferred from dissociation data obtained under identical conditions for hydrated ions that have similar m/z and degrees-of-freedom. From experimental internal energy deposition curves and Rice-Ramsperger-Kassel-Marcus (RRKM) theory, dissociation data as a function of collision energy for protonated leucine enkephalin, which has a comparable m/z and degrees-of-freedom as Na⁺(H₂O)₃₀, are modeled. The threshold dissociation energies and entropies are correlated for data acquired at a single time point, resulting in a relatively wide range of threshold dissociation energies (1.1 to 1.7 eV) that can fit these data. However, this range of values could be significantly reduced by fitting data acquired at different dissociation times. By measuring the internal energy of an activated ion, the number of fitting parameters necessary to obtain information about the dissociation parameters by modeling these data is reduced and could result in improved accuracy for such methods.     

Effect of internal excitation on the collision-induced dissociation and reactivity of Co 2 +

The kinetic energy dependence of collision-induced dissociation (CID) of dicobalt ion (Co ₂ ⁺ ) with He, Ar, and Xe has been investigated using guided ion-beam mass spectrometry. The change in efficiency of CID as the target gas is changed is in general agreement with previous CID studies of other systems: the cross section with Ar is 0.5 that with Xe, and no product ions are found with He. By varying the conditions under which the reactant ions are formed, the degree of internal excitation of the dicobalt ions is changed. The internal energies can be characterized by a Maxwell-Boltzmann distribution. We find that CID and reactions with O₂ and CO are very sensitive to Co ₂ ⁺ internal energy. The bond-dissociation energy derived from this work is D^o(Co ₂ ⁺ )=2.75±0.10 eV (63.4±2.3 kcal/mol). The Co ₂ ⁺ results are compared with a previous study of Fe ₂ ⁺ .     

Protonated Dipeptide Losses from b 5 and b 4 Ions of Side Chain Hydroxyl Group Containing Pentapeptides

In this study, C-terminal protonated dipeptide eliminations were reported for both b ₅ and b ₄ ions of side chain hydroxyl group (–OH) containing pentapeptides. The study utilized the model C-terminal amidated pentapeptides having sequences of XGGFL and AXVYI, where X represents serine (S), threonine (T), glutamic acid (E), aspartic acid (D), or tyrosine (Y) residue. Upon low-energy collision-induced dissociation (CID) of XGGFL (where X?=?S, T, E, D, and Y) model peptide series, the ions at m/z 279 and 223 were observed as common fragments in all b ₅ and b ₄ ion (except b ₄ ion of YGGFL) mass spectra, respectively. By contrast, peptides, namely S_MeGGFL-NH₂ and E_OMeGGFL-NH₂, did not show either the ion at m/z 279 or the ion at m/z 223. It is shown that the side chain hydroxyl group is required for the possible mechanism to take place that furnishes the protonated dipeptide loss from b ₅ and b ₄ ions. In addition, the ions at m/z 295 and 281 were detected as common fragments in all b ₅ and b ₄ ion (except b ₄ ion of AYVYI) mass spectra, respectively, for AXVYI model peptide series. The MS⁴ experiments exhibited that the fragment ions at m/z 279, 223, 295, and 281 entirely reflect the same fragmentation behavior of [M?+?H]⁺ ion generated from commercial dipeptides FL-OH, GF-OH, YI-OH, and VY-OH. These novel eliminations reported here for b ₅ and b ₄ ions can be useful in assigning the correct and reliable peptide sequences for high-throughput proteomic studies.

Single series peptide fragment ion spectra generated by two-stage collision-induced dissociation in a triple quadrupole

By using nanoelectrospray ionization and a triple quadrupole analyzer, simplified fragment ion spectra of peptides have been recorded by combining skimmer collision-induced dissociation with precursor ion scanning or neutral loss scanning. These pseudo-MS³ scan modes are characterized by two-stage collision-induced dissociation and have been termed sCID/precursor and sCID/neutral loss scan, respectively. By these scan modes, peptide fragment ion spectra can be generated that predominantly show signals of a single fragment ion series, such as the B or Y″ series. Skimmer collision-induced dissociation combined with scanning for neutral loss of 28 generates spectra showing B ions, whereas combination with precursor ion scanning for the Y″₁ ion results in spectra showing Y″ ions for tryptic peptides (Y″₁=m/z 147 for C-terminal lysine, Y″₁=m/z 175 for C-terminal arginine). Sequence information including the direction of the sequence is easily extracted from the simplified fragment ion spectra generated by two-stage collision-induced dissociation, because the scan mode defines the type of fragments observed. The analytical results reported are similar to those that have been achieved in MS³ experiments using a hybrid BEQQ or a pentaquadrupole mass spectrometer (Schey, K. L.; Schwartz, J. C.; Cooks, R. G. Rapid Commun. Mass Spectrom. 1989, 3, 305–309). The pseudo-MS³ technique used in this study has some limitations with respect to sample purity, because there is no step of mass selection before the first stage of collisional activation; however, it has the advantage that a standard triple quadrupole instrumentation can be used.     

Intriguing Differences in the Gas-Phase Dissociation Behavior of Protonated and Deprotonated Gonyautoxin Epimers

The aim of this study was to investigate the unusual gas-phase dissociation behavior of two epimer pairs of protonated gonyautoxins (GTX) following electrospray ionization in comparison to their deprotonated counterparts. The chemical structures of the investigated GTX1-4 variants vary in their substitution pattern at N-1 and the stereochemical orientation of the hydroxysulfate group at C-11 (11α for GTX1/2 versus 11β for GTX3/4). The direct comparison of mass spectra in positive and negative ion modes illustrated two distinct features: first, an intriguing difference between protonated 11α and 11β species, where 11α conformations exhibited almost complete dissociation of [M + H]⁺ ions via facile SO₃ elimination, while 11β species remained mostly intact as [M + H]⁺; and second, the lack of such differences for the deprotonated counterparts. In this study, we propose an acid-catalyzed elimination mechanism from density functional theory calculations, initiated by a proton transfer of a guanidinium proton to the hydroxysulfate group with simultaneous SO₃ release, which is only possible for the 11α conformation based on intramolecular distances. The same mechanism explains the lack of a comparable SO₃ loss in the negative ion mode. CID experiments supported this proposed mechanism for GTX1 and GTX2. Computational modeling of product ions seen in the CID spectra of GTX3 and GTX4 established that the lowest energy dissociation pathway for the 11β epimers is elimination of water with the possibility for further SO₃ release from the intermediate product. Experimental data for structurally analogous decarbamoyl gonyautoxins confirmed the evidence for the GTX compounds as well as the proposed elimination mechanisms.     

Quadrupole ion trap mass spectrometry of fullerenes

The fullerenes C₆₀ and C₇₀ can be ionized by desorption from a liquid matrix upon bombardment by Cs⁺ ions of 7 keV kinetic energy. The resulting radical cations, when activated in the ion trap by collisions with Xe target, in the presence of helium, undergo extensive dissociation by loss of multiple C₂ units. Large internal energies are deposited into these molecular ions and the dissociation efficiency is in excess of 60%.