一些双电荷离子的气相碎裂反应

借助质量分析离子动能谱和串联质谱研究了由电子轰击产生的双电荷离子的单分子亚稳碎裂及碰撞诱导分解过程,讨论了两种实验方法导致的差别因素.此外,根据质量分析离子动能谱提供的双电荷离子电荷分离反应的动能释放值计算了两电荷中心间距的最小值,以判别按不同电荷分离方式碎裂的双电荷离子的过渡态结构.     

Loss of charged versus neutral heme from gaseous holomyoglobin ions.

The dissociation of holomyoglobin ions ranging in charge state from +10 to +2 has been studied using collisional activation in a quadrupole ion trap. Collisional activation times and amplitudes were varied to investigate the effects of these variables on dissociation of the heme group from the holoprotein. The onset of neutral heme loss occurs at a lower activation amplitude than loss of charged heme. For solutions of ferri-myoglobin, charged heme loss was prominent for +10 to +4 holomyoglobin ions, while neutral heme loss product was found to be dominant for charge states +3 and +2. For any given charge state, activation of holomyoglobin ions from a solution containing primarily ferro-myoglobin yielded significantly more abundant neutral heme loss products than was observed for activation of ions from solutions containing primarily ferri-myoglobin. The relative concentrations of the two oxidation states were shown to be affected by redox chemistry within the nano-electrospray emitter used in this work. Results from a double activation experiment revealed that the precursor ions of a given charge state contained a mixture of two populations, with ferro-myoglobin giving rise to neutral heme loss upon dissociation and ferri-myoglobin yielding charged heme. No evidence for electron transfer upon collisional activation of ferri-myoglobin ions was observed. Furthermore, little or no evidence for electron transfer associated with ion/ion reactions with anions derived from perfluoro-1,3-dimethylcyclohexane was observed. Definitive results could not be drawn for the lowest precursor ion charge states (+3 and +2) due to low dissociation efficiencies.     

Electron Induced Dissociation of Singly Deprotonated Peptides

Dissociation of singly charged species is more challenging compared with that of multiply charged precursor ions because singly charged ions are generally more stable. In collision activated dissociation (CAD), singly charged ions also gain less kinetic energy in a fixed electric field compared with multiply charged species. Furthermore, ion–electron and ion–ion reactions that frequently provide complementary and more extensive fragmentation compared with CAD typically require multiply charged precursor ions. Here, we investigate electron induced dissociation (EID) of singly deprotonated peptides and compare the EID fragmentation patterns with those observed in negative ion mode CAD. Fragmentation induced upon electron irradiation and collisional activation is not specific and results in the formation of a wide range of product ions, including b-, y-, a-, x-, c-, and z-type ions. Characteristic amino acid side chain losses are detected in both techniques. However, differences are also observed between EID and CAD spectra of the same species, including formation of odd-electron species not seen in CAD, in EID. Furthermore, EID frequently results in more extensive fragmentation compared with CAD. For modified peptides, EID resulted in retention of sulfonation and phosphorylation, allowing localization of the modification site. The observed differences are likely due to both vibrational and electronic excitation in EID, whereas only the former process occurs in CAD.     

连续模型处理高剥离态离子在金属表面的能量获得

当高剥离态离子接近清洁金属表面时，表面上的电子由于强库仑相互作用会从金属表面转移到带有高电荷的离子上．这个电子转移过程可以用一速率方程确定，其中速率常数与高剥离态离子所带净电荷数，以及离子与金属表面的距离有关．由于金属表面的镜象势的作用，高剥离态离子在金属表面附近会受到加速作用而获得能量．本文采用经典镜象势，用数值方法处理了确定电子转移的速率方程，计算了高剥离态离子在金属表面的能量获得，计算结果与两组实验数据比较吻合．     

Internal energy distributions deposited in doubly and singly charged tungsten hexacarbonyl ions generated by charge stripping,electron impact,and charge exchange

The distribution Pε of internal energies deposited in W(CO)₆ ^+?. ions upon charge stripping (that is, electron detachment to yield the doubly charged ion in the course of a single kiloelec-tronvolt energy collision) was estimated by a thermochemical method from the measured relative abundances of the doubly charged fragment ions produced. The thermochemical information needed to estimate P/ge was obtained by measuring the threshold translational energy losses associated with charge stripping of the singly charged fragment ions, W(CO) _n ⁺ (n = 0-5). The P(/ge) curve falls exponentially with increasing internal energy. The average energy transferred to W(CO)₆ ^+? upon a 7.8-keV collision with O₂ is 19 eV, yielding W(CO)₆ ^2? ions with an average of 4 eV of internal energy. In its general appearance, the P(ε) distribution associated with charge stripping is similar to the curves obtained from simple collisional activation of either W(CO) ₆ ^+?. or W(CO)₆ ^2+? in kiloelectronvolt energy gaseous collisions. Given that charge stripping occurs by way of an electronic excitation process, this similarity in the energy deposition function is taken to indicate that electronic excitation is also the major mechanism for simple collisional activation in this system at zero scattering angle in the kiloelectronvolt energy regime. The internal energy distribution associated with a related charge-stripping process, charge inversion from the metal carbonyl anions to yield the corresponding cations, was also recorded. This reaction shows a large (～7 eV) average internal energy deposition with a distribution that indicates near-zero probability of formation of unexcited ions. These data are tentatively interpreted in terms of vibrationalelectron detachment. The internal energy distribution associated with an exothermic process, charge exchange [W(CO)₆ ^2+? + O₂ → W(CO) ₊ ^6?+O₂ ^+?], was also characterized. Unexpectedly strong coupling of translational to internal energy is observed, and there is a large probability of depositing internal energies in excess of 10 eV, even though the exothermicity is only 3 eV. Finally, the internal energy distributions associated with the formation of doubly charged W(CO)₆ ^2+? ions by electron ionization have been measured. Unlike the distribution for charge stripping, but like that for singly charged ions generated by electron impact, this distribution shows considerable structure, presumably due to Franck-Condon factors.     

A multiconfigurational time-dependent Hartree-Fock method for excited electronic states. II. Coulomb interaction effects in single conjugated polymer chains

Conjugated polymers have attracted considerable attention in the last few decades due to their potential for optoelectronic applications. A key step that needs optimisation is charge carrier separation following photoexcitation. To understand better the dynamics of the exciton prior to charge separation, we have performed simulations of the formation and dynamics of localised excitations in single conjugated polymer strands. We use a nonadiabatic molecular dynamics method which allows for the coupled evolution of the nuclear degrees of freedom and of multiconfigurational electronic wavefunctions. We show the relaxation of electron-hole pairs to form excitons and oppositely charged polaron pairs and discuss the modifications to the relaxation process predicted by the inclusion of the Coulomb interaction between the carriers. The issue of charge photogeneration in conjugated polymers in dilute solution is also addressed.     

Direct observation of ion emission from charged aqueous nanodrops: effects on gaseous macromolecular charging

Mechanistic information about how gaseous ions are formed from charged droplets has been difficult to establish because direct observation of nanodrops in a size range relevant to gaseous macromolecular ion formation by optical or traditional mass spectrometry methods is challenging owing to their small size and heterogeneity. Here, the mass and charge of individual aqueous nanodrops between 1–10 MDa (15–32 nm diameter) with ∼50–300 charges are dynamically monitored for 1 s using charge detection mass spectrometry. Discrete losses of minimally solvated singly charged ions occur, marking the first direct observation of ion emission from aqueous nanodrops in late stages of droplet evaporation relevant to macromolecular ion formation in native mass spectrometry. Nanodrop charge depends on the identity of constituent ions, with pure water nanodrops charged slightly above the Rayleigh limit and aqueous solutions containing alkali metal ions charged progressively below the Rayleigh limit with increasing cation size. MS2 capsid ions (∼3.5 MDa; ∼27 nm diameter) are more highly charged from aqueous ammonium acetate than from its biochemically preferred, 100 mM NaCl/10 mM Na phosphate solution, consistent with ion emission reducing the nanodrop and resulting capsid charge. The extent of charging indicates that the capsid partially collapses inside the nanodrops prior to the charging and formation of the dehydrated gaseous ions. These results demonstrate that ion emission can affect macromolecular charging and that conformational changes to macromolecular structure can occur in nanodrops prior to the formation of naked gaseous ions.

Ion evaporation from aqueous nanodrops is measured for the first time using charge detection mass spectrometry, and the origin of solute ion dependent charging of large (MDa) macromolecules is revealed.     

双电荷离子［C_（12）H_（12）N_2O］~（2＋）和［C_（12）H_（12

双电荷离子［Ｃ_（１２）Ｈ_（１２）Ｎ_２Ｏ］~（２＋）和［Ｃ_（１２）Ｈ_（１２）Ｎ_２Ｓ］~（２＋）的气相单分子分解反应研究任达，贾维平，李智立，刘淑莹（中国科学院长春应用化学研究所，长春，１３００２２）关键词双电荷离子，质量分析离子动能谱，串联质谱，４...     

双电荷离子[C12H12N2O]2+和[C12H12N2S]2+的气相单分子分解反应研究

The unimolecular dissociation reactions of doubly charged ions were reported,which resulted from a tandem mass spectrometer and a reversed geometry double focusing mass spectrometer by electron impact.Mass analyzed ion kinetic energy spectrometry(MIKES) was used to obtain the kinetic energy releases in charge separation reactions of doubly charged ions.The intercharge distances between the two charges at transition states can be calculated from the kinetic energy releases.Transition structures of unimolecular dissociation reactions were infered from MIKES and MS/MS.     

Investigation of resonant inner-shell processes with an electron beam ion trap

The collision processes of highly charged ions with electrons have been studied with an electron beam ion trap. Resonant inner-shell processes such as dielectronic recombination and resonant excitation double autoionization were investigated by observing the number ratio of extracted ions with adjacent charge states.     

A Mechanism for Ionization of Nonvolatile Compounds in Mass Spectrometry: Considerations from MALDI and Inlet Ionization

Mechanistic arguments relative to matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) address observations that predominately singly charged ions are detected. However, recently a matrix assisted laser ablation method, laserspray ionization (LSI), was introduced that can use the same sample preparation and laser as MALDI, but produce highly charged ions from proteins. In MALDI, ions are generated from neutral molecules by the photon energy provided to a matrix, while in LSI ions are produced inside a heated inlet tube linking atmospheric pressure and the first vacuum region of the mass spectrometer. Some LSI matrices also produce highly charged ions with MALDI ion sources operated at intermediate pressure or high vacuum. The operational similarity of LSI to MALDI, and the large difference in charge states observed by these methods, provides information of fundamental importance to proposed ionization mechanisms for LSI and MALDI. Here, we present data suggesting that the prompt and delayed ionization reported for vacuum MALDI are both fast processes relative to producing highly charged ions by LSI. The energy supplied to produce these charged clusters/droplets as well as their size and time available for desolvation are determining factors in the charge states of the ions observed. Further, charged droplets/clusters may be a common link for ionization of nonvolatile compounds by a variety of MS ionization methods, including MALDI and LSI.     

Fragmentation dynamics of methane by few-cycle femtosecond laser pulses

The fragmentation pattern of CH4 was experimentally studied at an intensity of approximately 10(14) W/cm2 with laser durations varying from 8 to 110 fs. When the laser duration was 8 fs, only the primarily fragmental CH3+ ion was observed in addition to the parent CH4+ ion. When the laser duration was 30 fs, small fragmental CH2+ and H+ ions appeared. When the laser duration was 110 fs, some doubly charged ions were also observed in addition to the abundant singly charged ions. The large mass spectra difference demonstrated that the pulse duration had a strong effect on the fragmentation of the parent ion produced in the single ionization. The effect of laser intensity on the fragmentation of CH4+ was also studied for few-cycle femtosecond laser pulses. The results demonstrated that the first-return recollision between the rescattered electron and the parent ion played a significant role in the fragmentation dynamics of the parent ion. Depending on the ion-electron impact energy, the recollision excited the parent ion to a dissociated state or doubly charged state. The experimentally observed singly charged fragmental ions resulted from the recollision-induced dissociation of CH4+ or the Coulomb explosion of CH(4)2+.     

氯代甲苯双电荷离子的单分子解离反应研究

研究了在70 eV电子轰击电离条件下，氯代甲苯及氯化苄产生的双电荷离子[C_7H_7Cl]~(2＋)、[C_7H_6Cl]~(2＋·)和[C_7H_5Cl]~(2＋)为母体的两种类型单分子解离反应．主要讨论了亚稳双电荷离子的异构化反应、失H解高的“偶电子规则”以及单分子电荷分离过渡态的结构．     

Charge and energy dynamics in photo-excited poly(para-phenylenevinylene) systems

We report results from simulations of charge and energy dynamics in poly(para-phenylenevinylene) (PPV) and PPV interacting with C60. The simulations were performed by solving the time-dependent Schrodinger equation and the lattice equation of motion simultaneously and nonadiabatically. The electronic system and the coupling of the electrons to the lattice were described by an extended three-dimensional version of the Su-Schrieffer-Heeger model, which also included an external electric field. Electron and lattice dynamics following electronic excitations at different energies have been simulated. The effect of additional lattice energy was also included in the simulations. Our results show that both exciton diffusion and transitions from high to lower lying excitations are stimulated by increasing the lattice energy. Also field induced charge separation occurs faster if the lattice energy is increased. This separation process is highly nonadiabatic and involves a significant rearrangement of the electron distribution. In the case of PPV coupled to C60, we observe a spontaneous charge separation. The separation time is in this case limited by the local concentration of C60 molecules close to the PPV chain.     

Cross sections and ion kinetic energy analysis for the electron impact ionization of acetylene

Using a Nier-type electron impact ion source in combination with a double focusing two sector field mass spectrometer, partial cross sections for electron impact ionization of acetylene are measured for electron energies up to 1000 eV. Discrimination factors for ions are determined using the deflection field method in combination with a three-dimensional ion trajectory simulation of ions produced in the ion source. Analysis of the ion yield curves obtained by scanning the deflectors allows the assignment of ions with the same mass-to-charge ratio to specific production channels on the basis of their different kinetic energy distributions. This analysis also allows to determine, besides kinetic energy distributions of fragment ions, partial cross sections differential in kinetic energy. Moreover a charge separation reaction, the Coulomb explosion of the doubly charged parent ions C2H2++ into the fragment ions C2H+ and H+, is investigated and its mean kinetic energy release (KER=3.88 eV) is deduced.     

Laserspray ionization (LSI) ion mobility spectrometry (IMS) mass spectrometry

A simple device is described for desolvation of highly charged matrix/analyte clusters produced by laser ablation leading to multiply charged ions that are analyzed by ion mobility spectrometry-mass spectrometry. Thus, for example, highly charged ions of ubiquitin and lysozyme are cleanly separated in the gas phase according to size and mass (shape and molecular weight) as well as charge using Tri-Wave ion mobility technology coupled to mass spectrometry. This contribution confirms the mechanistic argument that desolvation is necessary to produce multiply charged matrix-assisted laser desorption/ionization (MALDI) ions and points to how these ions can be routinely formed on any atmospheric pressure mass spectrometer.     

Where does the electron go? Electron distribution and reactivity of peptide cation radicals formed by electron transfer in the gas phase

We report the first detailed analysis at correlated levels of ab initio theory of experimentally studied peptide cations undergoing charge reduction by collisional electron transfer and competitive dissociations by loss of H atoms, ammonia, and N-C alpha bond cleavage in the gas phase. Doubly protonated Gly-Lys, (GK + 2H) (2+), and Lys-Lys, (KK + 2H) (2+), are each calculated to exist as two major conformers in the gas phase. Electron transfer to conformers with an extended lysine chain triggers highly exothermic dissociation by loss of ammonia from the Gly residue, which occurs from the ground ( X ) electronic state of the cation radical. Loss of Lys ammonium H atoms is predicted to occur from the first excited ( A ) state of the charge-reduced ions. The X and A states are nearly degenerate and show extensive delocalization of unpaired electron density over spatially remote groups. This delocalization indicates that the captured electron cannot be assigned to reduce a particular charged group in the peptide cation and that superposition of remote local Rydberg-like orbitals plays a critical role in affecting the cation-radical reactivity. Electron attachment to ion conformers with carboxyl-solvated Lys ammonium groups results in spontaneous isomerization by proton-coupled electron transfer to the carboxyl group forming dihydroxymethyl radical intermediates. This directs the peptide dissociation toward NC alpha bond cleavage that can proceed by multiple mechanisms involving reversible proton migrations in the reactants or ion-molecule complexes. The experimentally observed formations of Lys z (+*) fragments from (GK + 2H) (2+) and Lys c (+) fragments from (KK + 2H) (2+) correlate with the product thermochemistry but are independent of charge distribution in the transition states for NC alpha bond cleavage. This emphasizes the role of ion-molecule complexes in affecting the charge distribution between backbone fragments produced upon electron transfer or capture.     

Charge separation and ion-pair formation in cations subsequent to electron-impact

Investigation of metastable ion transitions shows, that spontaneous charge separations (loss of an electron from a cation) and ion-pair formations (loss of negative ions from positive ones)are taking place in conventional mass spectrometry. Such processes may be important factors in formation of doubly charged ions.     

Visible and ultraviolet spectroscopy of gas phase protein ions

Optical spectroscopy has contributed enormously to our knowledge of the structure and dynamics of atoms and molecules and is now emerging as a cornerstone of the gas phase methods available for investigating biomolecular ions. This article focuses on the UV and visible spectroscopy of peptide and protein ions stored in ion traps, with emphasis placed on recent results obtained on protein polyanions, by electron photodetachment experiments. We show that among a large number of possible de-excitation pathways, the relaxation of biomolecular polyanions is mainly achieved by electron emission following photo-excitation in electronically excited states. Electron photodetachment is a fast process that occurs prior to relaxation on vibrational degrees of freedom. Electron photodetachment yield can then be used to record gas phase action spectra for systems as large as entire proteins, without the limitation of system size that would arise from energy redistribution on numerous modes and prevent fragmentation after the absorption of a photon. The optical activity of proteins in the near UV is directly related to the electronic structure and optical absorption of aromatic amino acids (Trp, Phe and Tyr). UV spectra for peptides and proteins containing neutral, deprotonated and radical aromatic amino acids were recorded. They displayed strong bathochromic shifts. In particular, the results outline the privileged role played by open shell ions in molecular spectroscopy which, in the case of biomolecules, is directly related to their reactivity and biological functions. The optical shifts observed are sufficient to provide unambiguous fingerprints of the electronic structure of chromophores without the requirement of theoretical calculations. They constitute benchmarks for calculating the absorption spectra of chromophores embedded in entire proteins and could be used in the future to study biochemical processes in the gas phase involving charge transfer in aromatic amino acids, such as in the mediation of electron transfer or redox reactions. We then addressed the important question of the sensitivity of protein optical spectra to the intrinsic properties of protein ions, including conformation, charge state, etc., and to environmental factors. We report optical spectra for different charge states of insulin, for ubiquitin starting from native and denaturated solutions, and for apo-myoglobin protein. All these spectra are compared critically to spectra recorded in solution, in order to assess solvent effects. We also report the spectra of peptides complexed with metal cations and show that complexation gives rise to new optical transitions related to charge transfer types of excitation. The perspectives of this work include integrative approaches where UV-Vis spectroscopy could, for example, be combined with ion mobility spectrometry and high level calculations for protein structural characterization. It could also be used in spectroscopy to probe biological processes in the gas phase, with different light sources including VUV radiation (to probe different types of excitations) and ultra short pulses with time and phase modulation (to probe and control the dynamics of de-excitation or charge transfer events), and with the derivatization of proteins with chromophores to modulate their optical properties. We also envision that photo-excitation will play an important role in the future to produce intermediates with new chemical and reactive properties. Another promising route is to conduct activated electron photodetachment dissociation experiments.