Preformed Ion Research Articles

Electron acceleration to GeV energy by an axicon Gaussian laser pulse in a preformed ion channel

Electron acceleration to GeV energy in a preformed ion channel by an axicon laser pulse is investigated. Due to a striking inherent symmetry and strong axial electric field of axicon laser pulse with radial Gaussian intensity profile; the electrons get accelerated effectively along the propagation direction. The electrostatic space charge field of preformed ion channel facilitates electrons to remain trapped and confined in the accelerating phase and owing to betatron resonance phenomenon, the electron’s energy can be enhanced effectively up to GeV-level of energies. The existence of a preformed ion channel serves as an alternative to the external magnetic field for effective acceleration.

Electron Acceleration by a Radially Polarized Laser Pulse in an Ion Channel

The presence of a preformed ion channel in a laser accelerator enhances the oscillatory velocity of electrons due to the confined motions that leads to increase the energy gain from acceleration. The unique properties of radially polarized laser beam are utilized which leads to improvement in trapping and acceleration of electron so that an electron can be accelerated further to a very high energy level. The enhancement in electron energy is due to fact that the radial field vanishes on axis, but only axial field survives which accelerates the electron longitudinally. The electrostatic field generated by an ion channel stops the electrons to escape from the interaction region due to the transverse oscillations and thus causes a resonance between the electrons and the electric field of laser pulse. Our results show that the combined role of radial polarization and the effect of an ion channel can enhance the electron energy gain significantly.

ChemInform Abstract: Formation of Gas‐Phase Peptide Ions and Their Dissociation in MALDI: Insights from Kinetic and Ion Yield Studies

AbstractReview: 130 refs.

Formation of gas-phase peptide ions and their dissociation in MALDI: insights from kinetic and ion yield studies.

Insights on mechanisms for the generation of gas-phase peptide ions and their dissociation in matrix-assisted laser desorption ionization (MALDI) gained from the kinetic and ion yield studies are presented. Even though the time-resolved photodissociation technique was initially used to determine the dissociation kinetics of peptide ions and their effective temperature, it was replaced by a simpler method utilizing dissociation yields from in-source decay (ISD) and post-source decay (PSD). The ion yields for a matrix and a peptide were measured by repeatedly irradiating a region on a sample and collecting ion signals until the sample in the region was completely depleted. Matrix- and peptide-derived gas-phase cations were found to be generated by pre-formed ion emission or by ion-pair emission followed by anion loss, but not by laser-induced ionization. The total number of ions, that is, matrix plus peptide, was found to be equal to the number of ions emitted from a pure matrix. A matrix plume was found to cool as it expanded, from around 800-1,000 K to 400-500 K. Dissociation of peptide ions along b/y channels was found to occur statistically, that is, following RRKM behavior. Small critical energy (E0 = 0.6-0.7 eV) and highly negative critical entropy (ΔS(‡) = -30 to -25 eu) suggested that the transition structure was stabilized by multiple intramolecular interactions.

In situ generation of electrochemical gradients across pore-spanning membranes

Silicon substrates with cavities in the micrometre range were micro-fabricated and appropriately functionalized to allow for the generation of pore-spanning membranes (PSMs) sealing the pore cavities. PSMs were either formed by applying lipids dissolved in organic solvent (painting technique) on a hydrophobically functionalized silicon surface followed by ‘solvent freeze-out’, or solvent-free PSMs were prepared by spreading of giant unilamellar vesicles on hydrophilically functionalized substrates. The geometry of the silicon cavities in conjunction with three dimensional confocal laser scanning microscopy images enabled us to simultaneously monitor the PSMs and a pH-sensitive dye entrapped into the picolitre-sized cavities. The excellent sealing properties of both PSM types allowed an in situ generation of proton gradients across these membranes. In the presence of nigericin, a proton/potassium-antiporter, a preformed potassium ion gradient was transformed into a stable proton gradient across the PSMs, which was visualized by the pH-sensitive dye pyranine entrapped in the silicon cavities in a time resolved manner by means of confocal laser scanning fluorescence microscopy.

Ion Yields for Some Salts in MALDI: Mechanism for the Gas-Phase Ion Formation from Preformed Ions

Preformed ion emission is the main assumption in one of the prevailing theories for peptide and protein ion formation in matrix-assisted laser desorption ionization (MALDI). Since salts are in preformed ion forms in the matrix-analyte mixture, they are ideal systems to study the characteristics of preformed ion emission. In this work, a reliable method to measure the ion yield (IY) in MALDI was developed and used for a solid salt benzyltriphenylphosphonium chloride and two room-temperature ionic liquids 1-butyl-3-methylimidazolium hexafluorophosphate and trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate. IY for the matrix (α-cyano-4-hydroxycinnamic acid, CHCA) was also measured. Taking 1 pmol salts in 25 nmol CHCA as examples, IYs for three salts were similar, (4-8) × 10(-4), and those for CHCA were (0.8-1.2) × 10(-7). Even though IYs for the salts and CHCA remained virtually constant at low analyte concentration, they decreased as the salt concentrations increased. Two models, Model 1 and Model 2, were proposed to explain low IYs for the salts and the concentration dependences. Both models are based on the fact that the ion-pair formation equilibrium is highly shifted toward the neutral ion pair. In Model 1, the gas-phase analyte cations were proposed to originate from the same cations in the solid that were dielectrically screened from counter anions by matrix neutrals. In Model 2, preformed ions were assumed to be released from the solid sample in the form of neutral ion pairs and the anions in the ion pairs were assumed to be eliminated via reactions with matrix-derived cations.

Polarization dependent fragmentation of ions produced by laser desorption from nanopost arrays

Tailored silicon nanopost arrays (NAPA) enable controlled and resonant ion production in laser desorption ionization experiments and have been termed nanophotonic ion sources (Walker et al., J. Phys. Chem. C, 2010, 114, 4835-4840). As the post dimensions are comparable to or smaller than the laser wavelength, near-field effects and localized electromagnetic fields are present in their vicinity. In this contribution, we explore the desorption and ionization mechanism by studying how surface derivatization affects ion yields and fragmentation. We demonstrate that by increasing the laser fluence on derivatized NAPA with less polar surfaces that have decreased interaction energy between the structured silicon substrate and the adsorbate, the spectrum changes from exhibiting primarily molecular ions to showing a growing variety and abundance of fragments. The polarization angle of the laser beam had been shown to dramatically affect the ion yields of adsorbates. For the first time, we report that by rotating the plane of polarization of the desorption laser, the internal energy of the adsorbate can also be modulated resulting in polarization dependent fragmentation. This polarization effect also resulted in selective fragmentation of vitamin B(12). To explore the internal energy of NAPA generated ions, the effect of the post aspect ratios on the laser desorption thresholds and on the internal energy of a preformed ion was studied. Elevated surface temperatures and enhanced near fields in the vicinity of high aspect ratio posts are thought to contribute to desorption and ionization from NAPA. Comparison of the fluence dependence of the internal energies of ions produced from nanoporous silicon and NAPA substrates indicates that surface restructuring or transient melting by the desorption laser is a prerequisite for the former but not for the latter.

A Comparative Study of In- and Post-Source Decays of Peptide and Preformed Ions in Matrix-Assisted Laser Desorption Ionization Time-of-Flight Mass Spectrometry: Effective Temperature and Matrix Effect

In-source decay (ISD) and post-source decay (PSD) of a peptide ion ([Y(6) + H](+)) and a preformed ion (benzyltriphenylphosphonium, BTPP) generated by matrix-assisted laser desorption ionization (MALDI) were investigated with time-of-flight mass spectrometry. α-Cyano-4-hydroxycinammic acid (CHCA) and 2,5-dihydroxybenzoic acid (DHB) were used as matrices. For both ions, ISD yield was unaffected by delay time, indicating rapid termination of ISD. This was taken as evidence for rapid expansion cooling of hot "early" plume formed in MALDI. CHCA was hotter than DHB for [Y(6) + H](+) while the matrix effect was insignificant for BTPP. The "early" plume temperature estimated utilizing previous kinetic results was 800-900 K, versus 400-500 K for "late" plume. The results support our previous finding that the temperature of peptide ions interrogated by tandem mass spectrometry was lower than most rough estimates of MALDI temperature.

Formation of cation–radical anion pairs derived from carboxybenzophenone–tetrabutylammonium salts. Pulse radiolysis studies

Pulse radiolysis of acetonitrile solutions of tetra-n-butyl ammonium salts of 2- and 4-carboxybenzophenones [BP-COO−···N+(C4H9)4] were performed in order to generate directly the reduced forms of the benzophenone moieties within pre-formed ion pairs. In earlier studies on photochemical electron transfer reactions, ion pairs containing a tetraalkyl ammonium cation and a benzophenone radical anion were formed in an electron transfer to the triplet BP from a quencher consisting of a tetraalkyl ammonium salt of (phenylthio)acetic acid. In the current work, the [BP•−COO−···N+(C4H9)4] ion pairs were formed by direct reduction of the salts without the complication of a third moiety, i.e., the (phenylthio)acetic anion. The spectra and kinetic parameters of the radiolytically-reduced salts were compared to the behavior of reduced forms of the 2- and 4-COOH substituted benzophenones. The results from the pulse radiolysis and photochemistry were compared and explained in terms of the different structures of the ion pairs.

Substrate-Assisted Cysteine Deprotonation in the Mechanism of Dimethylargininase (DDAH) from Pseudomonas aeruginosa

The enzyme dimethylargininase (also known as dimethylarginine dimethylaminohydrolase or DDAH; EC 3.5.3.18) catalyzes the hydrolysis of endogenous nitric oxide synthase inhibitors, N(omega)-methyl-l-arginine and N(omega),N(omega)-dimethyl-l-arginine. Understanding the mechanism and regulation of DDAH activity is important for developing ways to control nitric oxide production during angiogenesis and in many cases of vascular endothelial pathobiology. Several possible physiological regulation mechanisms of DDAH depend upon the presence of an active-site cysteine residue, Cys249 in Pseudomonas aeruginosa (Pa) DDAH, which is proposed to serve as a nucleophile in the catalytic mechanism. Through the use of pH-dependent ultraviolet and visible (UV-vis) difference spectroscopy and inactivation kinetics, the pK(a) of the active-site Cys249 in the resting enzyme was found to be unperturbed from pK(a) values of typical noncatalytic cysteine residues. In contrast, the pH dependence of k(cat) values indicates a much lower apparent pK(a) value. UV-vis difference spectroscopy between wild-type and C249S DDAH shows absorbance changes consistent with Cys249 deprotonation to the anionic thiolate upon binding positively charged ligands. The proton from Cys249 is lost either to the solvent or to an unidentified general base. A mutation of the active-site histidine residue, H162G, does not eliminate cysteine nucleophilicity, further arguing against a pre-formed ion pair with Cys249. Finally, UV-vis and X-ray absorption spectroscopy revealed that inhibitory metal ions can bind at these two active-site residues, Cys249 and His162, and also stabilize the anionic form of Cys249. These results support a proposed substrate-assisted mechanism for Pa DDAH in which ligand binding modulates the reactivity of the active-site cysteine.

ULTRASOUND ACCELERATED REDUCTIVE COUPLING OF IMINE OR IMINIUM ION GENERATED IN 5 M LITHIUM PERCHLORATE SOLUTION BY LITHIUM METAL

A novel one-pot reductive coupling of imine or iminium ion generated in the concentrated ethereal lithium perchlorate solution was achieved in the presence of lithium metal. The yield of the reaction depends on the reactivity preformed imine or iminium ion. Diamines were formed with high diastereoselectivity in some cases in about 5 h. The reaction time was doubled without the use of ultrasound acceleration.

Distinction between pre-formed cluster emission and heavy ion decay by fission

For studying cluster radioactivity in the actinide region as well as trans-tin region two types of models are used: the pre-cluster formation model and the unified fission model. In the case of the actinide region, the cluster-like shapes are preferred for very high asymmetry while fissioning shapes are more suitable for less asymmetry and symmetry (the line of demarcation being around Ac=31). In this work this line of demarcation is studied in the case of the trans-tin region. The results of this study show that the transition from cluster mode to fission mode takes place at Ac=16.

Fast Atom Bombardment Mass Spectrometry:Ionization Mechanism.

Some ideas have been presented to understand ion formation under matrix fast atom bombardment (FAB) conditions. A state of hydrogen-bonding hydrophilic solvation, named “quasi-preformed state”, has been introduced to explain the ion formation reflecting condensed-phase, and the both “quasi-preformed state” and a state of “preformed ion” have been defined on the basis of the Bjerrum's proposition for ion-pairs in solution. Some examples of preformed ion have been described. The extents of electronic excitation leading to the formation of molecular ions M+·, under matrix FAB conditions, have been examined by detecting FAB-induced fluorescence from a thin layer of alkali-halides (LiCl, NaCl, KCl, and CsCl), and by using some compounds of which ionization energies are known. The results obtained indicate that matrix FAB conditions are sufficient in energy for M+· formation (by about 10 eV electronic excitation) and are insufficient for M2+ formation (by about 20 eV electronic excitation). The mechanisms of the formation of various ions, [M+H]+, [M+Na]+, [M-H]+, M+·, M-·, and [M-H]-, under matrix FAB conditions have been described. A model for FAB ionization, named “cavity” model, has been introduced to deal quantitatively with the rate of ion formation or ion yields from matrix surface. The “cavity” model requires that the ion formation occurs in both processes, i) the collision cascade reflecting condensed-phase and ii) the ion/molecule reactions reflecting gas-phase. The influence of fast atom species (He, Ar, and Xe) on the ion yields which may directly relate to the size of cavity or crater has been examined by comparing with the spectral patterns in the gas-phase FAB mass spectra of m-nitrobenzyl alcohol. It has been roughly estimated that a “cavity” formed with Ar or Xe beam contains several 100 molecules.

Ionization in liquid secondary ion mass spectrometry (LSIMS)

The ionization processes in fast atom bombardment (FAB) are reviewed. The subjects covered include the ionization processes in the collision cascade, the nature of preformed ions in the liquid surface, the desorption process, the distinction between prompt and delayed chemistry, reactions of secondary electrons, ion-ion recombination and ionic cluster formation, ion-molecule reactions in the selvedge and preformed ion formation. Finally, the ejection of multiply charged ions and prompt reduction processes are briefly discussed. It is considered that the initial collision cascade ionization processes in FAB are similar to those observed in radiation chemistry

Synthesis and ion-binding properties of an immobilized bis-cysteine peptide

Techniques of solid-phase peptide segment coupling have been applied for the first time to the unambiguous grafting of a preformed ion complexing peptide to a solid support. The immobilized peptides forms complexes with sodium ions in trifluoroethanol with high affinity and considerable selectivity.

The acid effect in fast atom bombardment

AbstractThe effects on fast atom bombardment (FAB) mass spectra of analyte bases in glycerol on adding acids were investigated. The MH+ ion intensity usually increased but sometimes decreases were also found. For the practical use of FAB, it is importaat to clarify the reasons for this acid effect. By critical evaluation, a variety of different explanations were found, e.g. changes in solubility, surface activity, volatility, radiation‐induced chemistry and desorption energetics. No unambiguous examples of efficient preformed ion formation were found. However, it is still possible that preformed ion formation may be important in the uppermost surface layer. The mechanistic implications of these findings are briefly discussed.

Eliminations promoted by weak bases. IX. Stereochemistry of olefin formation from trans-2-Phenylcyclopentyl p-bromobenzenesulfonate promoted by lithium chloride in acetone

Parker, Winstein, and their coworkers have previously established that in the E2C elimination of trans-2-phenylcyclopentyl p-bromobenzenesulfonate induced by Bu4NCl in acetone some 9% of the olefinic product is produced by a syn-elimination. In view of the current idea that syn-eliminations in solution are assisted by association of the base with its counterion, the stereochemistry of the reaction induced by lithium chloride in acetone has been studied. There is no increase in the amount of syn-elimination, and kinetic analysis reveals that lithium chloride ion pairs are completely unreactive. 1-Phenylcyclopentene is not produced by rate-limiting attack of chloride ion on a preformed symmetrical phenonium ion pair. These results do not serve to distinguish between two alternative models of the E2C transition state.

ChemInform Abstract: MERCURY(II)‐ AND NITROSYL‐INDUCED AQUATION OF ANIONOPENTA‐AMINECOBALT(III) SPECIES. REACTIONS OF THE T‐(CO(N(CH2CH2NH2)3)(NH3)X)(2+) IONS (X = CL(‐) OR N3(‐)) AND THE IMPORTANCE OF ION PAIRS

Evidence is presented which demonstrates that the Hg2+- and NO+-induced aquations of t-[Co(tren)(NH3)Cl]2+,(1), and t-[Co(tren)(NH3)(N3)]2+, (2), respectively, follow different paths; i.e. that a common intermediate of the type [Co(tren)(NH3)]3+ is not involved [tren = 2,2′,2″-triaminotriethylamine, N(CH2CH2NH2)3]. Saturation entry of NO3– to form t-[Co(tren)(NH3)(NO3)]2+(3) occurs as [NO3–] is raised to 1 mol dm–3[48% for (1); 35% for (2)] and this limiting condition is interpreted in terms of pre-formed ion pairs (Kipca. 5 dm3 mol–1). Second-order rate constants for the Hg2+-induced reaction of p-[Co(tren)(NH3)Cl]2+ and t-[Co(tren)(NH3)Cl]2+, are 5.6 ± 0.2 and 9.7 ± 0.1 dm3 mol–1s–1 respectively.

Mercury(II)- and nitrosyl-induced aquation of anionopenta-aminecobalt-(III) species. Reactions of the t-[Co{N(CH2CH2NH2)3}(NH3)X]2+ions (X = Cl–or N3–) and the importance of ion pairs

Evidence is presented which demonstrates that the Hg2+- and NO+-induced aquations of t-[Co(tren)(NH3)Cl]2+,(1), and t-[Co(tren)(NH3)(N3)]2+, (2), respectively, follow different paths; i.e. that a common intermediate of the type [Co(tren)(NH3)]3+ is not involved [tren = 2,2′,2″-triaminotriethylamine, N(CH2CH2NH2)3]. Saturation entry of NO3– to form t-[Co(tren)(NH3)(NO3)]2+(3) occurs as [NO3–] is raised to 1 mol dm–3[48% for (1); 35% for (2)] and this limiting condition is interpreted in terms of pre-formed ion pairs (Kipca. 5 dm3 mol–1). Second-order rate constants for the Hg2+-induced reaction of p-[Co(tren)(NH3)Cl]2+ and t-[Co(tren)(NH3)Cl]2+, are 5.6 ± 0.2 and 9.7 ± 0.1 dm3 mol–1s–1 respectively.

Base hydrolysis of acidopentaaminecobalt(III) ions and the question of common intermediates. Hydrolysis of some t-[Co(tren)(NH3)X]2+/3+ ions (X = NO3-, MeSO3-, Cl-, Me2SO, N3-)

The preparations, and characterizations, of the complex cations t-[Co(tren)(NH3)X]2+/3+* (X = NO3-, Me2SO, H2O, Cl-, N3-) and p-[Co(tren)(NH3)X]2+/3+ (X = N3-, Cl-, H2O) are described. Base hydrolysis of t-[Co(tren)(NH3)X]2+/3+ (X = MeSO3-, Me2SO, NO3-, N3-) and of p- [Co(tren)(NH3)N3]2+ follows the rate law kobs = k[OH-] over the [OH-] range 1.0-10-4 mol dm-3 except for the 3+ ion t-[Co(tren)(NH3)Me2SO]3+ where some curvature is observed at [OH-] > 0.025 mol dm-3. The products for the t-[Co(tren)(NH3)X]2+/3+ ions (X = Cl-, NO3-, MeSO3-, Me2SO) depend on X, and on the presence or absence of supporting electrolyte (1.0 mol dm-3 NaClO4 or NaN3) varying from 12.1 % retention of the t-configuration for X = NO3- in the absence of electrolyte to c. 26% for X = Me2SO in 1.0 mol dm-3 NaN3. Significantly more retention is observed in the azido products (75-80% t-[Co(tren)(NH3)N3]2+). The results are interpreted in terms of important contributions to the mechanism from pre-formed ion pairs.