The geometrical factor of infinitely long cylindrical ICR cells for collision energy-resolved mass spectrometry: appearance energies of EI2+ (E = P, As, Sb, and Bi) from collision-induced dissociation of EI3+ and [EI2·ligand]+ complexes

Collision Cross-Section Measurements of Collision-Induced Dissociation Precursor and Product Ions in an FTICR-MS and an IM-MS: A Comparative Study

Sustained off-resonance irradiation-cross-sectional areas by Fourier transform ion cyclotron resonance mass spectrometry (SORI-CRAFTI) is an FTICR-MS strategy to collisionally activate precursor ions and then measure their ion-neutral collision cross sections, as well as those of selected products, at the same time. We benchmarked SORI-CRAFTI using protonated leucine-enkephalin, to excellent agreement (typically within 1-2%) with previous studies performed via collision-induced dissociation-ion mobility (CID-IMS). SORI-CRAFTI was then applied to alkali metal-cationized leucine-enkephalin and compared with CID-IMS via precursor/product cross-section ratios. Qualitative agreement between SORI-CRAFTI and CID-IMS was excellent (again, usually within 1-2%); however, neither SORI-CRAFTI nor CID-IMS could determine if metalated leucine-enkephalin was present in its canonical or zwitterionic form. When SORI-CRAFTI was used on [2.2.2]-cryptand+Cs⁺, SORI activation resulted in a 5% decrease in collision cross section, consistent with migration of the externally bound Cs⁺ into the cryptand's cavity and similar to the cross section observed when electrospraying from an isopropanol-rich solvent. Thus, SORI-CRAFTI is useful for studying gas-phase ion chemistry of small- to medium-sized molecules and host-guest systems.

Ion Mobility and Fourier Transform Ion Cyclotron Resonance Collision Cross Section Techniques Yield Long-Range and Hard-Sphere Results, Respectively

We determined collision cross section (CCS) values for singly and doubly charged cucurbit[n]uril (n = 5-7), decamethylcucurbit[5]uril, and cyclohexanocucurbit[5]uril complexes of alkali metal cations (Li⁺-Cs⁺). These hosts are relatively rigid. CCS values calculated using the projection approximation (PA) for computationally modeled structures of a given host are nearly identical for +1 and +2 complexes, with weak metal ion dependence, whereas trajectory method (TM) calculations of CCS for the same structures consistently yield values 7-10% larger for the +2 complexes than for the corresponding +1 complexes and little metal ion dependence. Experimentally, we measured relative CCS values in SF₆ for pairs of +1 and +2 complexes of the cucurbituril hosts using the cross-sectional areas by Fourier transform ion cyclotron resonance ("CRAFTI") method. At center-of-mass collision energies <∼30 eV, CRAFTI CCS values are sensitive to the relative binding energies in the +1 and +2 complexes, but at collision energies >∼40 eV (sufficient that ion decoherence occurs on essentially every collision) that dependence is not evident. Consistent with the PA calculations, these experiments found that the +2 complex ions have CCS values ranging between 94 and 105% of those of their +1 counterparts (increasing with metal ion size). In contrast, but consistent with the TM CCS calculations, ion mobility measurements of the same complexes at close to thermal energies in much less polarizable N₂ find the CCS of +2 complexes to be in all cases 9-12% larger than those of the corresponding +1 complexes, with little metal ion dependence.

Multi-CRAFTI: Relative Collision Cross Sections from Fourier Transform Ion Cyclotron Resonance Mass Spectrometric Line Width Measurements

Determination of collision cross sections (CCS) using the cross-sectional areas by the Fourier transform ion cyclotron resonance (CRAFTI) technique is limited by the requirement that accurate pressures in the trapping cell of the mass spectrometer must be known. Experiments must also be performed in the energetic hard-sphere regime such that ions decohere after single collisions with neutrals; this limits application to ions that are not much more massive than the neutrals. To mitigate these problems, we have resonantly excited two (or more) ions of different m/z to the same center-of-mass kinetic energy in a single experiment, subjecting them to identical neutral pressures. We term this approach "multi-CRAFTI". This facilitates measurement of relative CCS without requiring knowledge of the pressure and enables determination of absolute CCS using internal standards. Experiments with tetraalkylammonium ions yield CCS in reasonable agreement with the one-ion-at-a-time CRAFTI approach and with ion mobility spectrometry (IMS) when differences in collision energetics are taken into account (multi-CRAFTI generally yields smaller CCS than does IMS due to the higher collision energies employed in multi-CRAFTI). Comparison of multi-CRAFTI and IMS results with CCS calculated from structures computed at the M06-2X/6-31+G* level of theory using projection approximation or trajectory method values, respectively, indicates that the computed structures have CCS increasingly smaller than the experimental CCS as m/z increases, implying the computational model overestimates interactions between the alkyl arms. For ions that undergo similar collisional decoherence processes, relative CCS reach constant values at lower collision energies than do absolute CCS values, suggesting a means of increasing the accessible upper m/z limit by employing multi-CRAFTI.

Halide Size-Selective Binding by Cucurbit[5]uril-Alkali Cation Complexes in the Gas Phase

We report data that suggest complexes with alkali cations capping the portals of cucurbit[5]uril (CB[5]) bind halide anions size-selectively as observed in the gas phase: Cl^- binds inside the CB[5] cavity, Br^- is observed both inside and outside, and I^- binds weakly outside. This is reflected in sustained off-resonance irradiation collision-induced dissociation (SORI-CID) experiments: all detected Cl^- complexes dissociate at higher energies, and Br^- complexes exhibit unusual bimodal dissociation behavior, with part of the ion population dissociating at very low energies and the remainder dissociating at significantly higher energies comparable to those observed for Cl^-. Decoherence cross sections measured in SF₆ using cross-sectional areas by Fourier transform ion cyclotron resonance techniques for [CB[5] + M₂X]⁺ (M = Na, X = Cl or Br) are comparable to or less than that of [CB[5] + Na]⁺ over a wide energy range, suggesting that Cl^- or Br^- in these complexes are bound inside the CB[5] cavity. In contrast, [CB[5] + K₂Br]⁺ has a cross section measured about 20% larger than that of [CB[5] + Na]⁺, suggesting external binding that may correspond with the weakly bound component seen in SORI. While I^- complexes with alkali cation caps were not observed, alkaline earth iodides with CB[5] yielded complexes with cross sections 5-10% larger than that of [CB[5] + Na]⁺, suggesting externally bound iodide. Geometry optimization at the M06-2X/6-31+G* level of ab initio theory suggests that internal anion binding is energetically favored by approximately 50-200 kJ mol^-1 over external binding; thus, the externally bound complexes observed experimentally must be due to large energetic barriers hindering the passing of large anions through the CB[5] portal, preventing access to the interior. Calculation of the barriers to anion egress using MMFF//M06-2X/6-31+G* theory supports this idea and suggests that the size-selective binding we observe is due to anion size-dependent differences in the barriers.

Ion traps in modern mass spectrometry

This review is devoted to trapping mass spectrometry wherein ions are confined by electromagnetic fields for prolonged periods of time within limited volume, with mass measurement taking place within the same volume. Three major types of trapping mass spectrometers are discussed, specifically radiofrequency ion trap, Fourier transform ion cyclotron resonance and Orbitrap. While these three branches are intricately interwoven with each other over their recent history, they also differ greatly in their fundamentals, roots and historical origin. This diversity is reflected also in the difference of viewpoints from which each of these directions is addressed in this review. Following the theme of the issue, we focus on developments mainly associated with the country of Germany but, at the same time, we use this review as an illustration of the rapidly increasing globalization of science and expanding multi-national collaborations.

Quantitative Collision Cross-Sections from FTICR Linewidth Measurements: Improvements in Theory and Experiment

Two corrections to the equation used in the cross-sectional areas by Fourier transform ion cyclotron resonance ("CRAFTI") technique are identified. In CRAFTI, ion collision cross-sections are obtained from the pressure-dependent ion linewidths in Fourier transform mass spectra. The effects of these corrections on the accuracy of the cross-sections obtained using the CRAFTI technique are evaluated experimentally using the 20 biogenic amino acids and several crown ether complexes with protonated alkyl monoamines. Good absolute agreement is obtained between the CRAFTI cross-sections and the corresponding cross-sections obtained using both static drift ion mobility spectrometry and computational simulations. These results indicate that the CRAFTI cross-sections obtained using the updated equation presented here are quantitatively descriptive of the size and shape of the gas-phase ions. Cross-sections that differ by less than 3% are measured for the isobaric isomers n-butylamine and tert-butylamine complexed with the crown ethers. This level of precision is similar to what has been achieved previously using traveling wave ion mobility devices. These results indicate that CRAFTI can be used to probe subtle structural differences between ions with approximately the same precision as that achieved in traveling wave ion mobility devices. Graphical Abstract ᅟ.

Collision Cross Sections for 20 Protonated Amino Acids: Fourier Transform Ion Cyclotron Resonance and Ion Mobility Results

We report relative dephasing cross sections for the 20 biogenic protonated amino acids measured using the cross sectional areas by Fourier transform ion cyclotron resonance (CRAFTI) technique at 1.9 keV in the laboratory reference frame, as well as momentum transfer cross sections for the same ions computed from Boltzmann-weighted structures determined using molecular mechanics. Cross sections generally increase with increasing molecular weight. Cross sections for aliphatic and aromatic protonated amino acids are larger than the average trend, suggesting these side chains do not fold efficiently. Sulfur-containing protonated amino acids have smaller than average cross sections, reflecting the mass of the S atom. Protonated amino acids that can internally hydrogen-bond have smaller than average cross sections, reflecting more extensive folding. The CRAFTI measurements correlate well with results from drift ion mobility (IMS) and traveling wave ion mobility (TWIMS) spectrometric measurements; CRAFTI results correlate with IMS values approximately as well as IMS and TWIMS values from independent measurements correlate with each other. Both CRAFTI and IMS results correlate well with the computed momentum transfer cross sections, suggesting both techniques provide accurate molecular structural information. Absolute values obtained using the various methods differ significantly; in the case of CRAFTI, this may be due to errors in measurements of collision gas pressure, measurement of excitation voltage, and/or dependence of cross sections on kinetic energy. Graphical Abstract ᅟ.

Collision cross sectional areas from analysis of Fourier transform ion cyclotron resonance line width: a new method for characterizing molecular structure

We demonstrate a technique for determining molecular collision cross sections via measuring the variation of Fourier transform ion cyclotron resonance (FTICR) line width with background damping gas pressure, under conditions where the length of the FTICR transient is pressure limited. Key features of our method include monoisotopic isolation of ions, the pulsed introduction of damping gas to a constant pressure using a pulsed leak valve, short excitation events to minimize collisions during the excitation, and proper choice of damping gas (Xe is superior to He). The measurements are reproducible within a few percent, which is sufficient for distinguishing between many structural possibilities and is comparable to the uncertainty in cross sections calculated from computed molecular structures. These techniques complement drift ion mobility measurements obtained on dedicated instruments. They do not require a specialized instrument, but should be easily performed on any FTICR mass spectrometer equipped with a pulsed leak valve.

Hydrogen bonding in phosphonate cavitands: investigation of host-guest complexes with ammonium salts

The H-bonding in alkylammonium complexes of phosphonate cavitands were studied by mass spectrometric methods and theoretical calculations. The alkylammonium ions included primary, secondary, and tertiary methyl- and ethylammonium ions. Their complexation with mono-, tetra-, and two di-phosphonate cavitands, which differ according to the number and position of H-bond acceptor P = O groups, was evaluated by using different competition experiments, energy-resolved CID, gas-phase H/D-exchange, and ligand-exchange reactions, together with ab initio theoretical optimization of the complexes. The phosphonate cavitands with two or more adjacent P = O groups were found to be selective towards secondary alkylammonium ions, due to simultaneous formation of two stable hydrogen bonds. In the ion-molecule reactions (both H/D- and ligand-exchange), the formation of two stable hydrogen bonds was observed either to slow down the reaction or to completely prevent it. This was, however, limited to situations where two hydrogen bonds are formed between the H-bond donor sites of the alkyl ammonium ion and the vicinal H-bond acceptor sites of the cavitand.

[Nepsilon-(gamma-glutamyl) lysine] as a potential biomarker in neurological diseases: new detection method and fragmentation pathways

Protein aggregates are characteristic of a number of diseases of the central nervous system such as diseases of polyQ expansion. Covalent bonds formed by the action of transglutaminase are thought to participate in the stabilization of these aggregates. Transglutaminase catalyzes the formation of cross-links between the side chains of glutaminyl and lysyl residues of polypeptides. Identification of the isodipeptide N(epsilon)-(gamma-glutamyl) lysine (iEK) in terminal proteolytic digests of neuronal aggregates would demonstrate participation of transglutaminase in neurological diseases. In order to identify and quantify the iEK present in the brain of patients with neurological disease, a method combining liquid chromatography and multistep mass spectrometry was developed. Because isobaric peptides of iEK could be present in the digest of aggregated proteins, the choice of fragment diagnostic ions was crucial. These ions were identified by mass spectrometry on sodiated iEK, which was derivatized on the carboxylic functions and terminal amines in order to improve sensitivity. Deuterated molecules as well as (13)C(6)- and (15)N(2)-isotopomers were used to derive filiations in the multistep fragmentations. The main fragmentation patterns have been identified, so that two ions (m/z 396 [MH - 56-42 u](+) and 350 [MH - 56-88 u](+)) are shown to be adequate markers for quantitation experiments. In order to gain a better understanding of the fragmentation processes, detailed quantum chemical calculations have been performed at levels which are expected to provide good accuracy. A thorough study has been carried out with a reduced model in which only the 'active' part of the molecule is retained. This allowed obtaining full mechanistic details on the pathways leading to a number of observed fragments. In particular, it has been shown that losses of 87 and 88 u from A(+) = [MH - 56 u](+) are competitive. Computations on the entire derivatized isodipeptide have been used to validate the use of the smaller model in order to obtain reliable energetics and mechanisms.

Cationic P-S-X cages (X=Br, I)

The first condensed-phase preparation of ternary P-Ch-X cations (Ch=O-Te, X=F-I) is reported: [P5S3X2]+, [P5S2X2]+, and [P4S4X]+ (X=Br, I). [P5S3X2]+ is formed from the reaction of the Ag+/PX3 reagent with P4S3. The [P5S3X2]+ ions have a structure that is related to P4S5 by replacing P=S by P+--X and S in the four-membered ring by P(X). We provide evidence that the active ingredient of the Ag+/PX3 reagent is the (H2CCl2)Ag-X-PX2+ cation. The latter likely reacts with the HOMO of P4S3 in a concerted HOMO-LUMO addition to give the P5S3X2+ ion as the first species visible in situ in the low-temperature 31P NMR spectrum. The [P5S3X2]+ ions are metastable at -78 degrees C and disproportionate at slightly higher temperatures to give [P5S2X2]+ and [P4S4X]+, probably with the extrusion of 1/n (PX)n (X=Br, I). All six new cage compounds have been characterized by multinuclear NMR spectroscopy and, in part, by IR or Raman spectroscopy. The [P5S2X2]+ salts have a nortricyclane skeleton and were also characterized by X-ray crystallography. The structure of the [P4S4X]+ ion is related to that of P4S5 in that the exo-cage P=S bond is replaced by an isoelectronic P+--X moiety.

ESI-FTICR mass spectrometric study of alcohol complexation properties of mono- and diphosphonate-bridged cavitands

Alcohol complexation properties of eight mono- and diphenyl phosphonate-bridged cavitands were studied by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR) and theoretical calculations. The cavitands varied in number and position of phenyl phosphonate bridges and their orientation with respect to the cavity, length of the lower rim alkyl chains, and substituents at apical positions of the resorcarene skeleton. The specificities of the different cavitands toward primary, secondary, and tertiary alcohols varying long of the alkyl chain were investigated, together with the stabilities of the formed complexes. The number, position, and orientation of the P = O moieties affected the complex formation of the cavitands and stability of the complexes dramatically. Methyl groups at apical positions of the resorcarene skeleton also affected the complexation properties. Although length and branching of the alkyl chain of the alcohol influenced the complex formation, the effect on stability of the complexes was negligible.

Electron transfer collisions between isolated fullerene dianions and SF6

Electron transfer collisions of trapped doubly charged fullerene anions C76(2-), C(78)2-, and C84(2-) with SF6 are studied in a Fourier transform ion cyclotron resonance mass spectrometer at center-of-mass collisional energies ranging from thermal energy to 77 eV. Collision energy dependencies manifest threshold energies for (nominally exoergic) single electron transfer onto SF6 of 1.46+/-0.3 eV, 1.56+/-0.3 eV, and 1.63+/-0.3 eV for C(76)2-, C78(2-), and C(84)2-, respectively. Kinetics studies reveal charge-transfer cross sections of up to 430+/-200 A2 for C84(2-) at a collision energy of 77 eV. The mechanism and the energetics are discussed in terms of classical electrostatic model calculations. Additionally, we rationalize the collision energy dependencies of the charge-transfer cross sections using the two-state Landau-Zener formalism to describe the associated resonant electron tunneling probability.

Elementary supramolecular chemistry in the gas phase: ligand exchange reactions of proton-bound clusters of aliphatic amides and diamides with amines

The ligand exchange reactions of the proton-bound trimer [(DMF)2...H+...H2NCH3] of dimethylformamide (DMF) and methylamine and of proton-bound dimers [diamide...H+...amine] of three aliphatic primary diamides, succinic diamide, adipinic diamide, and maleic diamide, and of three tertiary diamides N,N,N',N'-tetramethyl succinic diamide, N,N,N',N'-tetramethyl adipinic diamide, and N,N,N,N-tetramethyl bis-endo-bicyclo[2.2.1]heptane-2,5-diamide (N,N,N,N'-tetramethyl bis-endo-BCH-2,5-diamide) were investigated by FT-ICR spectrometry. The proton-bound clusters were generated in the external CI ion source of the instrument using the appropriate amine as the CI reagent and primary ionization of the CI gas by electron ionization. The ions were transferred to the FT-ICR cell for measuring the kinetics of the exchange reaction with a reactant base which was present at a constant background pressure in the cell. The trimer [(DMF)2...H...H2NCH3] displays fast exchange reactions with all amines used with exception of pyridine as well as with DMF and DMSO used as polar reactants. The proton-bound trimer shows a "role-specificity" of the components: the amine ligand is exchanged specifically by a more basic amine without observation of any intermediate, while the polar reactants substitute the two DMF ligands of the cluster in two reaction steps. Like proton-bound trimers, the proton-bound dimers [diamide...H+...amine] contain two amide groups interacting with a proton and an amine. In the case of n-propylamine as amine ligand, exchange by a more basic amine is always efficient. However, clusters [diamide...H+...amine] containing trimethylamine show reduced reactivity, which is attributed to a steric hindrance of the proton transfer to the incoming base. Such steric effects can be maximized by generating proton-bound dimers of N,N,N',N'-tetramethyl bis-endo-BCH-2,5-diamide with tertiary amines or amines containing bulky alkyl groups. Thus, the cluster [N,N,N',N'-tetramethyl bis-endo-BCH-2,5-diamide...H+...di-isopropylamine] exhibits a small efficiency of only 0.35% for the exothermic exchange reaction with di-sec-butylamine. The results are used to propose a solvation model for the structure of these proton-bound clusters, in which the ammonium group of the protonated amine is solvated by two amide groups.

Activation of large ions in FT-ICR mass spectrometry

The advent of soft ionization techniques, notably electrospray and laser desorption ionization methods, has enabled the extension of mass spectrometric methods to large molecules and molecular complexes. This both greatly extends the applications of mass spectrometry and makes the activation and dissociation of complex ions an integral part of these applications. This review emphasizes the most promising methods for activation and dissociation of complex ions and presents this discussion in the context of general knowledge of reaction kinetics and dynamics largely established for small ions. We then introduce the characteristic differences associated with the higher number of internal degrees of freedom and high density of states associated with molecular complexity. This is reflected primarily in the kinetics of unimolecular dissociation of complex ions, particularly their slow decay and the higher energy content required to induce decomposition--the kinetic shift (KS). The longer trapping time of Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) significantly reduces the KS, which presents several advantages over other methods for the investigation of dissociation of complex molecules. After discussing general principles of reaction dynamics related to collisional activation of ions, we describe conventional ways to achieve single- and multiple-collision activation in FT-ICR MS. Sustained off-resonance irradiation (SORI)--the simplest and most robust means of introducing the multiple collision activation process--is discussed in greatest detail. Details of implementation of this technique, required control of experimental parameters, limitations, and examples of very successful application of SORI-CID are described. The advantages of high mass resolving power and the ability to carry out several stages of mass selection and activation intrinsic to FT-ICR MS are demonstrated in several examples. Photodissociation of ions from small molecules can be effected using IR or UV/vis lasers and generally requires tuning lasers to specific wavelengths and/or utilizing high flux, multiphoton excitation to match energy levels in the ion. Photodissociation of complex ions is much easier to accomplish from the basic physics perspective. The quasi-continuum of vibrational states at room temperature makes it very easy to pump relatively large amounts of energy into complex ions and infrared multiphoton dissociation (IRMPD) is a powerful technique for characterizing large ions, particularly biologically relevant molecules. Since both SORI-CID and IRMPD are slow activation methods they have many common characteristics. They are also distinctly different because SORI-CID is intrinsically selective (only ions that have a cyclotron frequency close to the frequency of the excitation field are excited), whereas IRMPD is not (all ions that reside on the optical path of the laser are excited). There are advantages and disadvantages to each technique and in many applications they complement each other. In contrast with these slow activation methods, the less widely appreciated activation method of surface induced dissociation (SID) appears to offer unique advantages because excitation in SID occurs on a sub-picosecond time scale, instantaneously relative to the observation time of any mass spectrometer. Internal energy deposition is quite efficient and readily adjusted by altering the kinetic energy of the impacting ion. The shattering transition--instantaneous decomposition of the ion on the surface--observed at high collision energies enables access to dissociation channels that are not accessible using SORI-CID or IRMPD. Finally, we discuss some approaches for tailoring the surface to achieve particular aims in SID.

Reactions of [NH3+*, H2O] with carbonyl compounds: a McLafferty rearrangement within a complex?

The reactions of the water solvated ammonia radical cation [NH(3)(+*), H(2)O] with a variety of aldehydes and ketones were investigated. The reactions observed differ from those of low energy aldehydes and ketones radical cations, although electron transfer from the keto compound to ionized ammonia is thermodynamically allowed within the terbody complexes initially formed. The main process yields an ammonia solvated enol with loss of water and an alkene. This process corresponds formally to a McLafferty fragmentation within a complex. With aldehydes, another reaction can take place, namely the transfer of the hydrogen from the CHO group to ammonia, leading to the proton bound dimer of ammonia and water, and to the NH(4)(+) cation. Comparison between the available experimental results leads to the conclusion that the McLafferty fragmentation occurs within the terbody complex initially formed, with no prior ligand exchange, the water molecule acting as a spectator partner.

Manipulating internal energy of protonated biomolecules in electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry

The internal energy of protonated leucine enkephalin has been manipulated in electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry with two newly designed pump-probe experiments. Blackbody infrared radiation was applied to pump an ion population into a well-defined internal energy distribution below the dissociation threshold. Following this pumping stage, the internal energy distribution was probed using on-resonance collisional activation to dissociate the ions. These pump-probe experiments were carried out in two different ways: (a) using on-resonance collisional activation with variable kinetic energies to dissociate the ions at a constant initial ion temperature (determining the precursor ion survival percentage as a function of kinetic energy) and (b) using on-resonance collisional activation with a constant kinetic energy to dissociate the ions at variable initial ion temperatures (to investigate the ion survival yield-initial ion temperature dependence). Using this approach, a detailed study of the effects of the initial ion temperature, the probing kinetic energy and the internal energy loss rate on the effective conversion efficiency of (laboratory-frame) kinetic energy to internal energy was conducted. This conversion efficiency was found to be dependent on the initial ion temperature. Depending on the experimental conditions the conversion efficiency (for collisions with argon) was estimated to be about 4.0 +/- 1.7%, which agrees with that obtained from a theoretical modeling. Finally, the reconstructed curves of the ion survival yield versus the mode of the (final) total internal energy distribution of the activated ion population (after pump and probe events) at different pump-probe conditions reveal the internal energy content of the activated ions.

Improvement of electron capture efficiency by resonant excitation

A novel pulse sequence improving the efficiency for electron capture dissociation (ECD) of an unmodified Fourier transform ion cyclotron resonance (FTICR) mass spectrometer by more than an order of magnitude is presented. Commercially available FTICR instruments are usually equipped with a filament-based electron source producing an electron beam that has a rather small cross section. An ideal overlap between the rotating ion cloud and the electron beam appears to be a prerequisite for a high ECD efficiency. A reduced interception of the ion cloud and the electron beam is probably due to the contribution of the magnetron motion to the trajectory of the ions, resulting in a precession about the z-axis of the instrument. By increasing the kinetic energy and therefore increasing the cyclotron radii of the precursor ions by resonant excitation, the overlap of the rotating ion cloud with the electron beam is improved. By use of this protocol the efficiency of electron capture is substantially increased and consequently the acquisition time of ECD spectra is reduced significantly. The capability of resonant excitation of the precursor ions during the irradiation with electrons is demonstrated for standard peptides. This approach is particularly valuable for analysis and characterization of O-glycosylated peptides. In addition to amino acid sequence information, the attachment site of the labile glycan moiety is determined, and also radical-site-induced fragmentations of the glycosidic bonds are observed.

Reactions of CH(3)CHO(.+) and of CH(3)COH(.+) with water upon Fourier transform ion cyclotron resonance conditions

The reactions of CH(3)CHO(+) and of CH(3)COH(+) with water yield the same products, at almost the same rate. It is shown, by using a characteristic reaction of the carbene structure, that a molecule of water converts CH(3)COH(+) into its more stable isomer CH(3)CHO(+), which is a new example of catalyzed 1,2-H transfer. The dominant product is the proton-bound dimer of water which, in fact, comes from the [H(2)OH(+)...CH(3)(.)] and [H(2)OH(+)...CO] primary products whose observed abundances are poor. In a related system, ionized formamide/water, a water molecule catalyzes the 1,3-transfer leading from the solvated carbene to the [H(2)O...H(+)...H(2)N-C=O)] stable intermediate, which eliminates CO without back energy. In contrast, such a process does not take place in the studied system since the cleavage of the so formed [H(2)OH(+)...CH(3)CO] transient intermediate involves a high back energy; this is explained by the charge repartition within this intermediate. In fact, a different pathway takes place. The solvated acetaldehyde ion isomerizes into a terbody intermediate in which protonated water is bonded to a CO molecule on the one hand and to a methyl radical on the other hand. Simple cleavages of this complex yield the observed products.

Proton bound homodimers and heterodimers of amides and amines in the gas phase. Equilibrium studies by Fourier transform ion cyclotron resonance spectrometry

By injection of the proton bound homodimer [DMF.H+.DMF] of N,N-dimethylformamide (DMF) generated in an external ion source into a mixture of DMF and a second base within the cell of a Fourier transform ion cyclotron resonance (FT-ICR) spectrometer the equilibria between [DMF.H+.DMF] and the other possible proton bound dimers [DMF.H+.base] and [base.H+.base] have been studied for 13 different bases. Strongly polar bases like aliphatic amides and dimethyl sulfoxide (DMSO) exchange both DMF in [DMF.H+.DMF] by a two step process, while the almost non-polar amines exchange only one DMF. If the base is a primary or secondary amine, the proton bound heterodimer [DMF.H+.amine] reacts further by the addition of one DMF to create a proton bound trimer [(DMF)2.H+.amine]. The affinity deltaG(DMFH+) of the bases towards protonated DMF relative to neutral DMF depends linearly on the difference deltaGB of the gas phase basicity of DMF and the other base, but different correlation lines are obtained for polar and non-polar ligands (deltaGDMFH+ = 0.44GB(base)-375 [kJ/mol] (r = 0.97) and deltaGDMFH+ = 0.46GB(base)-397 [kJ/mol] (r = 0.99), respectively). This different behavior is explained by a different character of the proton bridge in the heterodimers containing only polar ligands and those incorporating a non-polar ligand besides DMF. The former dimers contain a more or less symmetric proton bridge while the latter can be viewed as a protonated base solvated by DMF. The available data have been used to calculate the molecular pair gas phase basicity of DMF and the 13 bases used and to estimate the dissociation energies of the bonds of the proton bridge in various proton bound heterodimers.

Insights into the C(4)H(8)(+.) potential energy surface: fourier transform ion cyclotron resonance studies of ion-molecule reactions of D-labeled partners

Reactions of [ethylene](+.) with ethylene and of [acetylene](+.) with ethane were studied by Fourier transform ion cyclotron resonance spectrometry using labeled reactants. The results confirm and clarify the different steps of the mechanism proposed previously and elaborated with other methods. The [[acetylene](+.), ethane] system can either dissociate to give the ethyl cation product, or isomerize into [[ethylene](+.), ethylene]. The latter system can either dissociate to yield ionized ethylene or convert into ionized but-2-ene, which undergoes a complete H-exchange prior to dissociation, leading to methyl radical, hydrogen radical and ethylene losses. The transfers of labeled atoms and the existence of H-exchange prior to formation of the products were used as a probe to check the different steps of the mechanism. The influence of the initial energy of the system on the reaction pathway is discussed. Copyright 1999 John Wiley & Sons, Ltd.

Analysis of nisin A and some of its variants using Fourier transform ion cyclotron resonance mass spectrometry

The lantibiotic nisin and some of its variants and degradation products have been characterized, using a 9.4-T Fourier transform ion cyclotron resonance mass spectrometer and electrospray ionization. The abundances of all products in the sample (i.e., major component, variants, degradation products, and adducts) have been measured quantitatively. The mass resolution obtained in the electrospray ionisation mass spectra was approximately 100,000 over the measured range. The resulting mass accuracy, better than 0.7 ppm (or within 0.001 Da) allowed the molecular masses and in many cases chemical formulae of most components in the mixture to be identified unambiguously. Additionally, amino acid sequence information on nisin and a variant [nisin + 18 Da] was obtained using sustained off-resonance irradiation collisional activated decomposition (SORI-CAD) of mass-selected precursor ions. Even after introducing collision gas into the mass analyser for the SORI-CAD experiments, the mass accuracy in the fragment ion mass spectra was approximately 5 ppm. It was established that the [nisin + 18 Da] molecule, present as a minor component in the mixture, was a species formed predominantly via hydration of nisin at position 33, i.e., [Ser33]nisin, with a small contribution due to hydration at position 5,[2-hydroxy-Ala5]nisin.