eV Collision Energy Research Articles

Reactive landing of peptide ions on self-assembled monolayer surfaces: an alternative approach for covalent immobilization of peptides on surfaces

Soft landing of mass-selected peptide ions onto reactive self-assembled monolayer surfaces (SAMs) was performed using a newly constructed ion deposition apparatus. SAM surfaces before and after soft landing were characterized ex situ using time-of-flight secondary-ion mass spectrometry (TOF-SIMS) and infrared reflection-absorption spectroscopy (IRRAS). We demonstrate that reactive landing (RL) results in efficient covalent linking of lysine-containing peptides onto the SAM of N-hydroxysuccinimidyl ester-terminated alkylthiol on gold (NHS-SAM). Systematic studies of the factors that affect the efficiency of RL revealed that the reaction takes place upon collision and is promoted by the kinetic energy of the ion. The efficiency of RL is maximized at ca. 40 eV collision energy. At high collision energies the RL efficiency decreases because of the competition with scattering of ions off the surface. The reaction yield is independent of the charge state of the projectile ions, suggesting that peptide ions undergo efficient neutralization upon collision. Chemical and physical properties of the SAM surface are also important factors that affect the outcome of RL. The presence of chemically reactive functional groups on the SAM surface significantly improves the reaction efficiency. RL of mass- and energy-selected peptide ions on surfaces provides a highly specific approach for covalent immobilization of biological molecules onto SAM surfaces.

Trajectory Surface Hopping Study of the O(3P) + Ethylene Reaction Dynamics

Spin-orbit coupling (SOC) induced intersystem crossing (ISC) has long been believed to play a crucial role in determining the product distributions in the O(3P) + C2H4 reaction. In this paper, we present the first nonadiabatic dynamics study of the title reaction at two center-of-mass collision energies: 0.56 eV, which is barely above the H-atom abstraction barrier on the triplet surface, and 3.0 eV, which is in the hyperthermal regime. The calculations were performed using a quasiclassical trajectory surface hopping (TSH) method with the potential energy surface generated on the fly at the unrestricted B3LYP/6-31G(d,p) level of theory. To simplify our calculations, nonadiabatic transitions were only considered when the singlet surface intersects the triplet surface. At the crossing points, Landau-Zener transition probabilities were computed assuming a fixed spin-orbit coupling parameter, which was taken to be 70 cm-1 in most calculations. Comparison with a recent crossed molecular beam experiment at 0.56 eV collision energy shows qualitative agreement as to the primary product branching ratios, with the CH3 + CHO and H + CH2CHO channels accounting for over 70% of total product formation. However, our direct dynamics TSH calculations overestimate ISC so that the total triplet/singlet ratio is 25:75, compared to the observed 43:57. Smaller values of SOC reduce ISC, resulting in better agreement with the experimental product relative yields; we demonstrate that these smaller SOC values are close to being consistent with estimates based on CASSCF calculations. As the collision energy increases, ISC becomes much less important and at 3.0 eV, the triplet to singlet branching ratio is 71:29. As a result, the triplet products CH2 + CH2O, H + CH2CHO and OH + C2H3 dominate over the singlet products CH3 + CHO, H2 + CH2CO, etc.

Doppler-free ion imaging of hydrogen molecules produced in bimolecular reactions

We report on the use of two-color Doppler-free [(1 + 1′) + 1/1′] resonance enhanced multiphoton ionization (REMPI) for three-dimensional imaging of state-selected product molecules from bimolecular reactions. We demonstrate the viability of this method by measuring differential cross sections for the reaction H + D 2 → D + HD( v′ = 1, j′ = 1,5,8) at 1.7 eV collision energy. We achieve higher resolution allowing us to observe oscillations that were not resolved by previous experiments; these oscillations agree closely with quantum mechanical calculations.

The effects of collision energy, vibrational mode, and vibrational angular momentum on energy transfer and dissociation in NO2+–rare gas collisions: An experimental and trajectory study

A combined experimental and trajectory study of vibrationally state-selected NO2+ collisions with Ne, Ar, Kr, and Xe is presented. Ne, Ar, and Kr are similar in that only dissociation to the excited singlet oxygen channel is observed; however, the appearance energies vary by approximately 4 eV between the three rare gases, and the variation is nonmonotonic in rare gas mass. Xe behaves quite differently, allowing efficient access to the ground triplet state dissociation channel. For all four rare gases there are strong effects of NO2+ vibrational excitation that extend over the entire collision energy range, implying that vibration influences the efficiency of collision to internal energy conversion. Bending excitation is more efficient than stretching; however, bending angular momentum partially counters the enhancement. Direct dynamics trajectories for NO2+ + Kr reproduce both the collision energy and vibrational state effects observed experimentally and reveal that intracomplex charge transfer is critical for the efficient energy transfer needed to drive dissociation. The strong vibrational effects can be rationalized in terms of bending, and to a lesser extent, stretching distortion enhancing transition to the Kr+ -NO2 charge state.

Exact quantum dynamics study of the O++H2(v=,j=)→OH++H ion-molecule reaction and comparison with quasiclassical trajectory calculations

The close-coupling hyperspherical (CCH) exact quantum method was used to study the title barrierless reaction up to a collision energy (E(T)) of 0.75 eV, and the results compared with quasiclassical trajectory (QCT) calculations to determine the importance of quantum effects. The CCH integral cross section decreased with E(T) and, although the QCT results were in general quite similar to the CCH ones, they presented a significant deviation from the CCH data within the 0.2-0.6 eV collision energy range, where the QCT method did not correctly describe the reaction probability. A very good accord between both methods was obtained for the OH(+) vibrational distribution, where no inversion of population was found. For the OH(+) rotational distributions, the agreement between the CCH and QCT results was not as good as in the vibrational case, but it was satisfactory in many conditions. The kk(') angular distribution showed a preferential forward character, and the CCH method produced higher forward peaks than the QCT one. All the results were interpreted considering the potential energy surface and plots of a representative sampling of reactive trajectories.

LC-MS-based method for the qualitative and quantitative analysis of complex lipid mixtures

A simple and robust LC-MS-based methodology for the investigation of lipid mixtures is described, and its application to the analysis of human lipoprotein-associated lipids is demonstrated. After an optional initial fractionation on Silica 60, normal-phase HPLC-MS on a YMC PVA-Sil column is used first for class separation, followed by reversed-phase LC-MS or LC-tandem mass spectrometry using an Atlantis dC18 capillary column, and/or nanospray MS, to fully characterize the individual lipids. The methodology is applied here for the analysis of human apolipoprotein B-associated lipids. This approach allows for the determination of even low percentages of lipids of each molecular species and showed clear differences between lipids associated with apolipoprotein B-100-LDL isolated from a normal individual and those associated with a truncated version, apolipoprotein B-67-containing lipoproteins, isolated from a homozygote patient with familial hypobetalipoproteinemia. The methods described should be easily adaptable to most modern MS instrumentation.

Electron Transfer from Sodium to Oriented Nitromethane, CH3NO2: Probing the Spatial Extent of Unoccupied Orbitals

Beams of sodium atoms with energies of a few eV are crossed with a beam of oriented CH3NO2 molecules to study the effect of collision energy and orientation on electron transfer. The electron transfer produces Na+ ions and free electrons, parent negative ions (CH)NO2-), and fragmentation ions NO2- and O- in proportions that depend on the collision energy. The steric asymmetry is very small or zero and suggests that production of all of the ions is favored by sideways attack with respect to the permanent dipole along the C-N axis. In these experiments, the electron appears to be transferred into the 2B1 state of the anion comprising mainly the pi*NO LUMO, producing a valence-bound state rather than a dipole-bound state.

Vibrational mode and collision energy effects on reaction of H2CO+ with CO2

The effects of collision energy (Ecol) and five different modes of H2CO+ vibration on the title reaction have been studied over the center-of-mass Ecol range from 0.1 to 3.2 eV, including measurements of product ion recoil velocity distributions. Electronic structure and Rice-Ramsperger-Kassel-Marcus calculations were used to examine properties of various complexes and transition states that might be important along the reaction coordinate. Two product channels are observed, corresponding to Hydrogen Transfer (HT) and Proton Transfer (PT). Both channels are endothermic with similar onset energies of approximately 0.9 eV; however, HT dominates over the entire Ecol range and accounts for 70-85% of the total reaction cross section. Both HT and PT occur by direct mechanisms over the entire Ecol range, and have similar dependence on reactant vibrational and collision energy. Despite these similarities, and the fact that the two channels are nearly isoenergetic and differ only in which product moiety carries the charge, their dynamics appear quite different. PT occurs primarily in large impact parameter stripping collisions, where most of the available energy is partitioned to product recoil. HT, in contrast, results in internally hot products with little recoil energy and a more forward-backward symmetric product velocity distribution. Vibration is found to affect the reaction differently in different collision energy regimes. The appearance thresholds are found to depend only on total energy, i.e., all modes of vibration are equivalent to Ecol. With increasing Ecol, vibrational energy becomes increasingly effective, relative to Ecol, at driving reaction. For HT, this transition occurs just above threshold, while for PT it begins at roughly twice the threshold energy.

Vibrational mode and collision energy effects on reaction of H2CO+ with C2H2: Charge state competition and the role of Franck-Condon factors in endoergic charge transfer

The effects of collision energy (E(col)) and six different H(2)CO(+) vibrational states on the title reaction have been studied over the center-of-mass E(col) range from 0.1 to 2.6 eV, including measurements of product ion recoil velocity distributions. Ab initio and Rice-Ramsperger-Kassel-Marcus calculations were used to examine the properties of complexes and transition states that might be important in mediating the reaction. Reaction is largely direct, despite the presence of multiple deep wells on the potential surface. Five product channels are observed, with a total reaction cross section at the collision limit. The competition among the major H(2) (+) transfer, hydrogen transfer, and proton transfer channels is strongly affected by E(col) and H(2)CO(+) vibrational excitation, providing insight into the factors that control competition and charge state "unmixing" during product separation. One of the more interesting results is that endoergic charge transfer appears to be controlled by Franck-Condon factors, implying that it occurs at large inter-reactant separations, contrary to the expectation that endoergic reactions should require intimate collisions to drive the necessary energy conversion.

Mass spectrometric identification and quantification of glycosyl flavonoids, including dihydrochalcones with neutral loss scan mode

We developed a strategy for determination and quantification of glycosyl flavonoids using liquid chromatography–triple quadrupole mass spectrometry with neutral loss scan at 15 and 30 eV collision energy in the positive ion mode. The fragmentation patterns of glycosyl flavonoids at 15 and 30 eV showed that fragmentation of sugar moiety depended on the type of glycosidic bond to aglycone, the site of C-glycosylation, and the type of aglycone. C-Glycosyl dihydrochalcones especially stood out because they produced [M+H−162] + even at 15 eV such as O-glycoside in spite of C-glycoside. C-Glycosides were classified according to (i) the intensity ratio A of [M+H−150] + to [M+H−120] + at 30 eV and (ii) the intensity ratio B of [M+H−120] + at 15 eV to one at 30 eV. The 8- C-glycosides were A < 1 and B < 1, the 6- C-glycosides were A > 1 and B < 1, and the C-glycosyl dihydrochalcones were A > 1 and B ≫ 1. Therefore, the intensity ratios of the neutral loss scan of 120 and 150 Da at 30 eV and those of 120, 162, and 308 Da at 15 eV allowed sequential distinction among these three types of C-glycosides as well as between O- and C-glycosides. Our method was applied for analysis of Rooibos tea, and the identified glycosides could be quantified specifically by the selected reaction monitoring method.

Theoretical investigation of nonadiabatic and internal temperature effects on the collision-induced multifragmentation dynamics of Na5+ cluster ions

In a continued effort to disentangle adiabatic, nonadiabatic, and internal temperature effects in the collision-induced multifragmentation of alkali-metal cluster ions at moderate energies, we report a theoretical study of this process for the Na5++He encounter in the 100 eV (center-of-mass) collision energy range. The investigation makes use of a diatomics-in-molecules based nonadiabatic molecular-dynamics (NAMD) method. All of the ten electronic 1A' molecular states of the cluster that can be formed by assembling ground-state monomers are considered explicitly. Cross sections for the corresponding 12 possible fragmentation channels are determined. As in the Na4++He case, we find that a few-channel characteristic of adiabatic fragmentation in the electronic ground state dominates. This owes primarily to the dominance of impulsive adiabatic mechanisms. Nonetheless, two significant nonadiabatic transitions take place: electronic excitation during the collision and electronic deexcitation in the postcollision stage. A large amount of the electronic excitation subsequently relaxes into the electronic ground state during the postcollision stage. This important intramolecular vibrational relaxation (IVR)-type mechanism enhances the population of channels characteristic of adiabatic fragmentation in the electronic ground state. The populations of the fragmentation channels are quite sensitive to the internal cluster temperature. This is discussed in terms of the conditions of occurrence of the fragmentation mechanisms and their competition. Comparisons with experimental results are presented and discussed.

Ab initio molecular dynamics study of ion-surface interactions

Ion-surface collisions have been investigated theoretically using ab initio molecular dynamics within density functional theory. The temporal evolution of the position of the bombarding ion, as well as its nearest neighbors, was studied for initial kinetic energies of 0 and 3.5 eV (0 and 5 km/s, respectively). Also investigated was the ion-surface interaction prior to collision and the following energy transfer, as indicated by changes in ion velocity. At 3.5 eV collision energy, the calculation results suggest the formation of local structural disorder within the simulation time frame studied. These results are of fundamental importance for an increased understanding of the ion-surface interaction during a collision event, with resulting changes in atomic level structure.

Branching ratios and absolute cross sections of dissociative recombination processes of N2O+

We have investigated the dissociative recombination of the N2O+ ion using the CRYRING heavy-ion storage ring at the Manne Siegbahn laboratory in Stockholm, Sweden. The dissociative recombination branching ratios were determined at minimal (approximately 0 eV) collision energy, showing that the dominating pathways involved two-body fragmentation: N2 + O (48%) and NO + N (36%). The branching ratio of the three-body break-up 2N + O was 16%. The overall thermal rate coefficient of the title reaction follows the expression k(T) = 3.34 +/- 0.75 x 10(-7) (T/300) (-0.57+/- 0.03 cm3 s(-1)), which correlates perfectly with earlier flowing afterglow studies on the same process.

Isotopomer distribution computation from tandem mass spectrometric data with overlapping fragment spectra

We present a method for determination of the isotopomer distributions of metabolites from the data generated by a tandem mass spectrometer. The method is an improvement over existing method as it is able to deal with overlapping fragments in the spectra. Our experiments indicate that the new method surpasses its predecessors in separating isotopomers from each other. When using the daughter ion scanning (collision induced dissociation) mode, the method was shown to be able to constrain the isotopomer distribution of different amino acids better than two existing methods. In particular, the isotopomer distributions of three amino acids, glycine, alanine and serine, can be fully uncovered with the method. However, due to the imperfect fragmentation of molecules in the tandem mass spectrometer, isotopomer distributions of larger amino acids still cannot be fully uncovered. In tests with isotope-labelled alanine, most accurate results were obtained using multiple reaction monitoring and 15 eV collision energy. The meausured isotopomer frequecies were in the range 99?106% of the theoretical value and the deviation between repetitions was in the range 1?10%.

Vibrational dependence in the dissociative recombination of O2+

We report data on a study into the vibrational dependence in the dissociative recombination of O+2 (X2Πg). The experiment has been performed with a merged-beam apparatus in the heavy-ion storage ring, CRYRING. We present the total rate coefficients as function of collision energy for five different vibrational populations of the ion beam and the partial rate coefficients and branching fractions near 0 eV collision energy for the vibrational levels v = 0, 1 and 2. Furthermore, we report on the control over and characterization of vibrational populations. The total rate coefficient is found to be weakly dependent on the vibrational excitation. The rate decreases upon increasing vibrational excitation, though only within an order of magnitude. The partial rate coefficients are strongly dependent on the vibrational level; with the v = 0 oxygen ions recombining the fastest. The partial branching fractions are also found to be strongly dependent on the vibrational level, specifically the O(1S) quantum yield which increases strongly upon increasing vibrational level.

Vibrationally resolved rate coefficients and branching fractions in the dissociative recombination of O2+

We have studied the dissociative recombination of the first three vibrational levels of O(2) (+) in its electronic ground X (2)Pi(g) state. Absolute rate coefficients, cross sections, quantum yields and branching fractions have been determined in a merged-beam experiment in the heavy-ion storage ring, CRYRING, employing fragment imaging for the reaction dynamics. We present the absolute total rate coefficients as function of collision energies up to 0.4 eV for five different vibrational populations of the ion beam, as well as the partial (vibrationally resolved) rate coefficients and the branching fractions near 0 eV collision energy for the vibrational levels v=0, 1, and 2. The vibrational populations used were produced in a modified electron impact ion source, which has been calibrated using Cs-O(2)(+) dissociative charge transfer reactions. The measurements indicate that at low collision energies, the total rate coefficient is weakly dependent on the vibrational excitation. The calculated thermal rate coefficient at 300 K decreases upon vibrational excitation. The partial rate coefficients as well as the partial branching fractions are found to be strongly dependent on the vibrational level. The partial rate coefficient is the fastest for v=0 and goes down by a factor of two or more for v=1 and 2. The O((1)S) quantum yield, linked to the green airglow, increases strongly upon increasing vibrational level. The effects of the dissociative recombination reactions and super elastic collisions on the vibrational populations are discussed.

Vibrational mode and collision energy effects on reaction of H2CO+ with C2D4

We report the effects of collision energy (Ecol) and five different H2CO+ vibrational modes on the reaction of H2CO+ with C2D4 over the center-of-mass E(col) range from 0.1 to 2.1 eV. Properties of various complexes and transition states were also examined computationally. Seven product channels are observed. Charge transfer (CT) has the largest cross section over the entire energy range, substantially exceeding the hard sphere cross section at high energies. Competing with CT are six channels involving transfer of one or more hydrogen atoms or protons and one involving formation of propanal, followed by hydrogen elimination. Despite the existence of multiple deep wells on the potential surface, all reactions go by direct mechanisms, except at the lowest collision energies, where short-lived complexes appear to be important. Statistical complex decay appears adequate to account for the product branching at low collision energies, however, even at the lowest energies, the vibrational effects are counter to statistical expectations. The pattern of Ecol and vibrational mode effects provide insight into factors that control reaction and interchannel competition.

A QCT study of the cross-section, energy and angular distributions of the OH + D 2 → HOD + D reaction at ET = 0.28 eV on the YZCL2 surface

Reaction cross-section, product energy and angular distributions for the OH + D 2 → HOD + D reaction have been calculated using the quasiclassical trajectory method with the recently developed YZCL2 potential energy surface at 0.28 eV collision energy. Although the calculated cross-section, σ r = 0.077 ± 0.002 Å 2, underestimates the experimental value, σ r = 0.22 ± 0.05 Å 2, it compares better with experiment than earlier surfaces. Calculated HOD vibrational distributions, product translational and angular distributions are also compared with experiments and with estimates of earlier surfaces. Overall, the YZCL2 surface provides a somewhat better match with experiments than earlier surfaces, but integration of trajectories is about one thousand times slower.

Vibrational Mode Effects as a Probe of Inter-channel Coupling in the Reactions of Formaldehyde Cation with Ammonia and Water

We report the effects of collision energy (E c o l ) and five different H 2 CO + vibrational modes on reaction of H 2 CO + with ND 3 and D 2 O over the center-of-mass E c n l range from 0.1 to 2.1 eV. Properties of various complexes and transition states were also examined computationally. For water, the only reaction is proton transfer (PT), going by a direct mechanism over the entire E c o l range, with a cross section near the collision limit. H 2 CO + vibrational excitation has no effect on reaction with water. Three product channels are observed in reaction with ammonia. Both proton transfer (PT) and charge transfer (CT) have large cross sections over the entire energy range. Hydrogen abstraction by H 2 CO + from ammonia (HA) accounts for <2% of the total product signal but is a mechanistically interesting channel. Both PT and HA go by direct mechanisms over most of the E c o l range, but complex mediation may be important at the lowest energies. All three channels are mode-specifically affected by H 2 CO + vibrational excitation. Vibration controls total reactivity but has essentially no effect on product branching. Charge transfer during reactant approach appears to have a large effect on subsequent PT and HA reactions.

The H + N2O → OH + N2 Reaction Dynamics on an Interpolated QCISD Potential Energy Surface. A Quasiclassical Trajectory Study

A global ab initio interpolated potential-energy surface (PES) for the H + N2O → OH + N2 reaction has been constructed using the GROW package of Collins and co-workers. The ab initio calculations have been performed using the quadratic configuration interaction with single and double excitations (QCISD) and QCISD with quasipertubative treatment of triple excitations QCISD(T) methods. By use of this PES, a detailed quasiclassical trajectory study of integral and differential cross sections, product rovibrational populations and internal energy distributions are presented. It is found that the main mechanism for the title reaction is via an indirect process in which the H atom attaches to the N end of N2O and then undergoes a 1,3-hydrogen shift, leading to products. However, the direct abstraction process in which the H atom attacks the O end generating OH + N2 products accounts for ∼30% of the global reactivity at 1.5 eV collision energy. Both mechanisms contribute distinctly to the various reaction properties, and it is demonstrated that the trajectories associated with the direct mechanism have shorter collision times than those proceeding via the indirect mechanism. The theoretical integral cross sections as a function of collision energy are in good agreement with the experimental values at translational energies below 1.20 eV. At higher collision energies, the theoretical calculations on the present PES predict values of the cross sections smaller than those determined experimentally. The OH vibrational branching ratio OH(v‘ = 1)/OH(v‘ = 0) given by the theoretical calculations is considerably larger than that obtained experimentally by Brouard and co-workers. However, the calculated OH(v‘ = 0,1) rotational populations at 1.48 eV reproduce to a large extent the measurements. Also, a good accordance is found with the experimental OH state resolved differential cross sections at 1.48 eV. The calculated kinetic energy release distributions and triple scattering angle-recoil velocity differential cross sections for state-resolved OH(v‘,N‘) products show that a substantial fraction of the total energy goes into rotational excitation of the N2 coproduct due to the direct mechanism and that there exists a strong pair correlation in the angular and velocity distributions of the product molecules formed in coincidence.