Internal energy deposition in electron capture dissociation measured using hydrated divalent metal ions as nanocalorimeters

Electronic and geometric structure of cationic and neutral chromium and molybdenum ammonia complexes

High level quantum chemical approaches are used to study the geometric and electronic structures of M(NH₃)_n and M(NH₃)_n ⁺ (M = Cr, Mo for n = 1-6). These complexes possess a dual shell electronic structure of the inner metal (3d or 4d) orbitals and the outer diffuse orbitals surrounding the periphery of the complex. Electronic excitations reveal these two shells to be virtually independent of the other. Molybdenum and chromium ammonia complexes are found to differ significantly in geometry with the former adopting an octahedral geometry and the latter a Jahn-Teller distorted octahedral structure where only the axial distortion is stable. The hexa-coordinated complexes and the tetra-coordinated complexes with two ammonia molecules in the second solvation shell are found to be energetically competitive. Electronic excitation energies and computed IR spectra are provided to allow the two isomers to be experimentally distinguished. This work is a component of an ongoing effort to study the periodic trends of transition metal solvated electron precursors.

Rapid Determination of Activation Energies for Gas-Phase Protein Unfolding and Dissociation in a Q-IM-ToF Mass Spectrometer

Ion mobility-mass spectrometry has emerged as a powerful tool for interrogating a wide variety of chemical systems. Collision-induced unfolding (CIU), typically performed in time-of-flight instruments, has been utilized to obtain valuable qualitative insight into protein structure and illuminate subtle differences between related species. CIU experiments can be performed relatively quickly, but unfolding energy information obtained from them has not yet been interpreted quantitatively. While several methods can determine quantitative dissociation energetics for small molecules, clusters, and peptides, these methods have rarely been applied to proteins, and never to study unfolding. Here, we present a method to rapidly determine activation energies for protein unfolding and dissociation, built on a model for energy deposition during collisional activation. The method is validated by comparing activation energies for dissociation of three complexes with those obtained using blackbody infrared radiative dissociation (BIRD); values from the two methods are in agreement. Several protein monomers were unfolded using CIU, including multiple charge states of both cations and anions, and activation energies determined. ΔH^⧧ and ΔS^⧧ values are found to be correlated, leading to ΔG^⧧ values that lie within a narrow range (∼70-80 kJ/mol) and vary more with charge state than with protein identity. ΔG^⧧ is anticorrelated with charge density, highlighting the key role of Coulombic repulsion in gas-phase unfolding. Measured ΔG^⧧ values are similar to those computed for proton transfer within small peptides, suggesting that proton transfer is the rate-limiting step in gas-phase unfolding and providing evidence of a link between the Mobile Proton model and CIU.

Development of two-dimensional gas chromatography (GC×GC) coupled with Orbitrap-technology-based mass spectrometry: Interest in the identification of biofuel composition

Comprehensive gas chromatography (GC) has emerged in recent years as the technique of choice for the analysis of volatile and semivolatile compounds in complex matrices. Coupling it with high-resolution mass spectrometry (MS) makes a powerful tool for identification and quantification of organic compounds. The results obtained in this study showed a significant improvement by using GC×GC-EI-MS in comparison with GC-EI-MS; the separation of chromatogram peaks was highly improved, which facilitated detection and identification. However, the limitation of Orbitrap mass analyzer compared with time-of-flight analyzer is the data acquisition rate; the frequency average was about 25 Hz at a mass resolving power of 15.000, which is barely sufficient for the proper reconstruction of the narrowest chromatographic peaks. On the other hand, the different spectra obtained in this study showed an average mass accuracy of about 1 ppm. Within this average mass accuracy, some reasonable elemental compositions can be proposed and combined with characteristic fragment ions, and the molecules can be identified with precision. At a mass resolving power of 7.500, the scan rate reaches 43 Hz and the GC×GC-MS peaks can be represented by more than 10 data points, which should be sufficient for quantification. The GC×GC-MS was also applied to analyze a cellulose bio-oil sample. Following this, a highly resolved chromatogram was obtained, allowing EI mass spectra containing molecular and fragment ions of many distinct molecules present in the sample to be identified.

Mass spectrometry of aerosol particle analogues in molecular beam experiments

Nanometer-size particles such as ultrafine aerosol particles, ice nanoparticles, water nanodroplets, etc, play an important, however, not yet fully understood role in the atmospheric chemistry and physics. These species are often composed of water with admixture of other atmospherically relevant molecules. To mimic and investigate such particles in laboratory experiments, mixed water clusters with atmospherically relevant molecules can be generated in molecular beams and studied by various mass spectrometric methods. The present review demonstrates that such experiments can provide unprecedented details of reaction mechanisms, and detailed insight into the photon-, electron-, and ion-induced processes relevant to the atmospheric chemistry. After a brief outline of the molecular beam preparation, cluster properties, and ionization methods, we focus on the mixed clusters with various atmospheric molecules, such as hydrated sulfuric acid and nitric acid clusters, N_x O_y and halogen-containing molecules with water. A special attention is paid to their reactivity and solvent effects of water molecules on the observed processes.

Charge transfer reactions between gas-phase hydrated electrons, molecular oxygen and carbon dioxide at temperatures of 80-300 K

The recombination reactions of gas-phase hydrated electrons (H2O)n˙(-) with CO2 and O2, as well as the charge exchange reaction of CO2˙(-)(H2O)n with O2, were studied by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry in the temperature range T = 80-300 K. Comparison of the rate constants with collision models shows that CO2 reacts with 50% collision efficiency, while O2 reacts considerably slower. Nanocalorimetry yields internally consistent results for the three reactions. Converted to room temperature condensed phase, this yields hydration enthalpies of CO2˙(-) and O2˙(-), ΔHhyd(CO2˙(-)) = -334 ± 44 kJ mol(-1) and ΔHhyd(O2˙(-)) = -404 ± 28 kJ mol(-1). Quantum chemical calculations show that the charge exchange reaction proceeds via a CO4˙(-) intermediate, which is consistent with a fully ergodic reaction and also with the small efficiency. Ab initio molecular dynamics simulations corroborate this picture and indicate that the CO4˙(-) intermediate has a lifetime significantly above the ps regime.

Thermochemistry of the Reaction of SF6 with Gas-Phase Hydrated Electrons: A Benchmark for Nanocalorimetry

The reaction of sulfur hexafluoride with gas-phase hydrated electrons (H2O)n(-), n ≈ 60-130, is investigated at temperatures T = 140-300 K by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. SF6 reacts with a temperature-independent rate of 3.0 ± 1.0 × 10(-10) cm(3) s(-1) via exclusive formation of the hydrated F(-) anion and the SF5(•) radical, which evaporates from the cluster. Nanocalorimetry yields a reaction enthalpy of ΔHR,298K = 234 ± 24 kJ mol(-1). Combined with literature thermochemical data from bulk aqueous solution, these result in an F5S-F bond dissociation enthalpy of ΔH298K = 455 ± 24 kJ mol(-1), in excellent agreement with all high-level quantum chemical calculations in the literature. A combination with gas-phase literature thermochemistry also yields an experimental value for the electron affinity of SF5(•), EA(SF5(•)) = 4.27 ± 0.25 eV.

Reductions of oxygen, carbon dioxide, and acetonitrile by the magnesium(II)/magnesium(I) couple in aqueous media: theoretical insights from a nano-sized water droplet

Reductions of O2, CO2, and CH3CN by the half-reaction of the Mg(II)/Mg(I) couple (Mg(2+) + e(-) → Mg(+•)) confined in a nanosized water droplet ([Mg(H2O)16](•+)) have been examined theoretically by means of density functional theory based molecular dynamics methods. The present works have revealed many intriguing aspects of the reaction dynamics of the water clusters within several picoseconds or even in subpicoseconds. The reduction of O2 requires an overall doublet spin state of the system. The reductions of CO2 and CH3CN are facilitated by their bending vibrations and the electron-transfer processes complete within 0.5 ps. For all reactions studied, the radical anions, i.e., O2(•-), CO2(•-), and CH3CN(•-), are initially formed on the cluster surface. O2(•-) and CO2(•-) can integrate into the clusters due to their high hydrophilicity. They are either solvated in the second solvation shell of Mg(2+) as a solvent-separated ion pair (ssip) or directly coordinated to Mg(2+) as a contact-ion pair (cip) having the (1)η-[MgO2](•+) and (1)η-[MgOCO](•+) coordination modes. The (1)η-[MgO2](•+) core is more crowded than the (1)η-[MgOCO](•+) core. The reaction enthalpies of the formation of ssip and cip of [Mg(CO2)(H2O)16](•+) are -36 ± 4 kJ mol(-1) and -30 ± 9 kJ mol(-1), respectively, which were estimated based on the average temperature changes during the ion-molecule reaction between CO2 and [Mg(H2O)16](•+). The values for the formation of ssip and cip of [Mg(O2)(H2O)16](•+) are estimated to be -112 ± 18 kJ mol(-1) and -128 ± 28 kJ mol(-1), respectively. CH3CN(•-) undergoes protonation spontaneously to form the hydrophobic [CH3CN, H](•). Both CH3CN and [CH3CN, H](•) cannot efficiently penetrate into the clusters with activation barriers of 22 kJ mol(-1) and ∼40 kJ mol(-1), respectively. These results provide fundamental insights into the solvation dynamics of the Mg(2+)/Mg(•+) couple on the molecular level.

Photoelectron imaging and theoretical study on nascent hydrogen bond network in microsolvated clusters of Au- (CH3OH)n (n = 1-5)

We first demonstrate the photoelectron spectroscopic evidence of the transition of two competitive solvation patterns in the Au(-)(CH3OH)n (n = 1-5) clusters. Quantum chemical calculations have been carried out to characterize the geometric structures, energy properties and hydrogen-bonded patterns, and to aid the spectral assignment. It has been found that the nonconventional hydrogen bonds dominate the small clusters (n = 1 and 2), whereas the conventional hydrogen bonds play more and more important role from n = 2 to n = 5. This finding provides concrete hydrogen bond network evolution of Au(-) surrounded by methanol molecules.

The effective temperature of ions stored in a linear quadrupole ion trap mass spectrometer

The extent of internal energy deposition into ions upon storage, radial ejection, and detection using a linear quadrupole ion trap mass spectrometer is investigated as a function of ion size (m/z 59 to 810) using seven ion-molecule thermometer reactions that have well characterized reaction entropies and enthalpies. The average effective temperatures of the reactants and products of the ion-molecule reactions, which were obtained from ion-molecule equilibrium measurements, range from 295 to 350 K and do not depend significantly on the number of trapped ions, m/z value, ion trap q z value, reaction enthalpy/entropy, or the number of vibrational degrees of freedom for the seven reactions investigated. The average of the effective temperature values obtained for all seven thermometer reactions is 318 ± 23 K, which indicates that linear quadrupole ion trap mass spectrometers can be used to study the structure(s) and reactivity of ions at near ambient temperature.

Reactions of CH3SH and CH3SSCH3 with gas-phase hydrated radical anions (H2O)n(•-), CO2(•-)(H2O)n, and O2(•-)(H2O)n

The chemistry of (H(2)O)(n)(•-), CO(2)(•-)(H(2)O)(n), and O(2)(•-)(H(2)O)(n) with small sulfur-containing molecules was studied in the gas phase by Fourier transform ion cyclotron resonance mass spectrometry. With hydrated electrons and hydrated carbon dioxide radical anions, two reactions with relevance for biological radiation damage were observed, cleavage of the disulfide bond of CH(3)SSCH(3) and activation of the thiol group of CH(3)SH. No reactions were observed with CH(3)SCH(3). The hydrated superoxide radical anion, usually viewed as major source of oxidative stress, did not react with any of the compounds. Nanocalorimetry and quantum chemical calculations give a consistent picture of the reaction mechanism. The results indicate that the conversion of e(-) and CO(2)(•-) to O(2)(•-) deactivates highly reactive species and may actually reduce oxidative stress. For reactions of (H(2)O)(n)(•-) with CH(3)SH as well as CO(2)(•-)(H(2)O)(n) with CH(3)SSCH(3), the reaction products in the gas phase are different from those reported in the literature from pulse radiolysis studies. This observation is rationalized with the reduced cage effect in reactions of gas-phase clusters.

How hot are your ions in TWAVE ion mobility spectrometry?

Effective temperatures of ions during traveling wave ion mobility spectrometry (TWIMS) analysis were measured using singly protonated leucine enkephalin dimer as a chemical thermometer by monitoring dissociation of the dimer into monomer, as well as the subsequent dissociation of monomer into a-, b-, and y-ions, as a function of instrumental parameters. At fixed helium cell and TWIMS cell gas flow rates, the extent of dissociation does not vary significantly with either the wave velocity or wave height, except at low (<500 m/s) wave velocities that are not commonly used. Increasing the flow rate of nitrogen gas into the TWIMS cell and decreasing the flow rate of helium gas into the helium cell resulted in greater dissociation. However, the mobility distributions of the fragment ions formed by dissociation of the dimer upon injection into the TWIMS cell are nearly indistinguishable from those of fragment ions formed in the collision cell prior to TWIMS analysis for all TWIMS experiments. These results indicate that heating and dissociation occur when ions are injected into the TWIMS cell, and that the effective temperature subsequently decreases to a point at which no further dissociation is observed during the TWIMS analysis. An upper limit to the effective ion temperature of 449 K during TWIMS analysis is obtained at a helium flow rate of 180 mL/min, TWIMS flow rate of 80 mL/min, and traveling wave height of 40 V, which is well below previously reported values. Effects of ion heating in TWIMS on gas-phase protein conformation are presented.

Ions in size-selected aqueous nanodrops: sequential water molecule binding energies and effects of water on ion fluorescence

The effects of water on ion fluorescence were investigated, and average sequential water molecule binding energies to hydrated ions, M(z)(H(2)O)(n), at large cluster size were measured using ion nanocalorimetry. Upon 248-nm excitation, nanodrops with ~25 or more water molecules that contain either rhodamine 590(+), rhodamine 640(+), or Ce(3+) emit a photon with average energies of approximately 548, 590, and 348 nm, respectively. These values are very close to the emission maxima of the corresponding ions in solution, indicating that the photophysical properties of these ions in the nanodrops approach those of the fully hydrated ions at relatively small cluster size. As occurs in solution, these ions in nanodrops with 8 or more water molecules fluoresce with a quantum yield of ~1. Ce(3+) containing nanodrops that also contain OH(-) fluoresce, whereas those with NO(3)(-) do not. This indirect fluorescence detection method has the advantages of high sensitivity, and both the size of the nanodrops as well as their constituents can be carefully controlled. For ions that do not fluoresce in solution, such as protonated tryptophan, full internal conversion of the absorbed 248-nm photon occurs, and the average sequential water molecule binding energies to the hydrated ions can be accurately obtained at large cluster sizes. The average sequential water molecule binding energies for TrpH(+)(H(2)O)(n) and a doubly protonated tripeptide, [KYK + 2H](2+)(H(2)O)(n), approach asymptotic values of ~9.3 (n ≥ 11) and ~10.0 kcal/mol (n ≥ 25), respectively, consistent with a liquidlike structure of water in these nanodrops.

Competition between Birch reduction and fluorine abstraction in reactions of hydrated electrons (H2O)n(-) with the isomers of di- and trifluorobenzene

The reactions of the isomers of di- and trifluorobenzene with hydrated electrons (H(2)O)(n)(-), n = 19-70, have been studied by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. While Birch reduction, i.e. H atom transfer to the aromatic ring, was observed for all studied isomers, a strong dependence on the substitution pattern was observed for fluorine abstraction. Nanocalorimetry combined with G3 calculations are used to analyze the thermochemistry of the reactions. Fluorine abstraction is at least 100 kJ mol(-1) more exothermic than Birch reduction, yet the latter is the dominant reaction pathway for all three difluorobenzene isomers. Fluorine abstraction and Birch reduction face activation barriers of comparable magnitude. The relative barrier height is sensitive to the substitution pattern. Birch reduction occurs selectively with 1,3- and 1,4-difluorobenzene in a nanoscale aqueous environment.

An improved cluster pair correlation method for obtaining the absolute proton hydration energy and enthalpy evaluated with an expanded data set

An improved cluster pair correlation method that is based on the method originally introduced by Tuttle et al. ( Tuttle et al. J. Phys. Chem. A 2002 , 106 , 925 - 932 ) was developed and evaluated using a significantly larger data set than used previously. With this larger data set, values for the absolute proton hydration free energy of -259.3 and -265.0 kcal/mol were obtained using the original and improved method, respectively. The former value is ∼4.5 kcal/mol less negative than previously reported values obtained with the same method but with smaller data sets. The dependence of this value on data set size indicates that the uncertainty in the original method may be greater than previously realized. The improved method has the advantages of higher precision, and the effects of cluster size on the proton hydration free energy and enthalpy values can be more readily evaluated. Data for ions with extreme pK(a)s, many of which were included in previous estimates of the proton hydration free energy, were found to be unreliable and were eliminated from the extended data set. There is only a subtle effect of cluster size on the Gibbs free energy values, and within the limits of the approximation inherent in the cluster pair correlation method, the "best" value for the standard absolute proton hydration free energy obtained with this new method and larger data set is -263.4 kcal/mol (average for clusters with 4-6 water molecules). The absolute proton hydration enthalpy values decrease from -273.1 to -275.3 kcal/mol with increasing cluster size (one to six water molecules, respectively). This trend, along with an anomalously high value for the absolute proton hydration entropy, indicates that the enthalpy obtained with this method may not have converged for these relatively small clusters.

Repeatability and reproducibility of product ion abundances in electron capture dissociation mass spectrometry of peptides

Site-specific reproducibility and repeatability of electron capture dissociation (ECD) in Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) are of fundamental importance for product ion abundance (PIA)-based peptide and protein structure analysis. However, despite the growing interest in ECD PIA-based applications, these parameters have not yet been investigated in a consistent manner. Here, we first provide a detailed description of the experimental parameters for ECD-based tandem mass spectrometry performed on a hybrid linear ion trap (LTQ) FT-ICR MS. In the following, we describe the evaluation and comparison of ECD and infrared multiphoton dissociation (IRMPD) PIA methodologies upon variation of a number of experimental parameters, for example, cathode potential (electron energy), laser power, electron and photon irradiation periods and pre- irradiation delays, as well as precursor ion number. Ranges of experimental parameters that yielded an average PIA variation below 5% and 15% were determined for ECD and IRMPD, respectively. We report cleavage site-dependent ECD PIA variation below 20% and correlation coefficients between fragmentation patterns superior to 0.95 for experiments performed on three FT-ICR MS instruments. Overall, the encouraging results obtained for ECD PIA reproducibility and repeatability support the use of ECD PIA as a complementary source of information to m/z data in radical-induced dissociation applied for peptide and protein structure analysis.

Average sequential water molecule binding enthalpies of M(H2O)(19-124)2+ (M = Co, Fe, Mn, and Cu) measured with ultraviolet photodissociation at 193 and 248 nm

The average sequential water molecule binding enthalpies to large water clusters (between 19 and 124 water molecules) containing divalent ions were obtained by measuring the average number of water molecules lost upon absorption of an UV photon (193 or 248 nm) and using a statistical model to account for the energy released into translations, rotations, and vibrations of the products. These values agree well with the trend established by more conventional methods for obtaining sequential binding enthalpies to much smaller hydrated divalent ions. The average binding enthalpies decrease to a value of ~10.4 kcal/mol for n > ~40 and are insensitive to the ion identity at large cluster size. This value is close to that of the bulk heat of vaporization of water (10.6 kcal/mol) and indicates that the structure of water in these clusters may more closely resemble that of bulk liquid water than ice, owing either to a freezing point depression or rapid evaporative cooling and kinetic trapping of the initial liquid droplet. A discrete implementation of the Thomson equation using parameters for liquid water at 0 °C generally fits the trend in these data but provides values that are ~0.5 kcal/mol too low.

"Weighing" photon energies with mass spectrometry: effects of water on ion fluorescence

We report a new, highly sensitive method for indirectly measuring fluorescence from ions with a discrete number of water molecules attached. Absorption of a 248 nm photon by hydrated protonated proflavine, PH(+)(H(2)O)(n) (n = 13-50), results in two resolved product ion distributions that correspond to full internal conversion of the photon energy (loss of approximately 11 water molecules) and to partial internal conversion of the photon energy and emission of a lower energy photon (loss of approximately 6 water molecules). In addition to fluorescence, a long-lived triplet state with a half-life of approximately 0.5 s (for n = 50) is formed. The energy of the emitted photon can be obtained from the number of water molecules lost from the precursor to form each distribution. The photon energies generally red shift from approximately 450 to 580 nm with increasing cluster size (the onset of the PH(+)(aq) fluorescence spectrum is 600 nm and the maximum is 518 nm) consistent with preferential stabilization of the first excited singlet state versus the ground state. The fluorescence quantum yield of PH(+)(H(2)O)(n) for n > or = 30 is 0.36 +/- 0.02, the same as that in bulk solution, and increases dramatically with decreasing cluster sizes, due to less efficient conversion of electronic-to-vibrational energy. The high sensitivity of this method should make it possible to perform Forster resonance energy transfer experiments with gas-phase biomolecules in a microsolvated environment to investigate how a controlled number of water molecules facilitates dynamical motions in proteins or other molecules of interest.

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

The internal energy deposited in both on- and off-resonance collisional activation in Fourier transform ion cyclotron resonance mass spectrometry is measured with ion nanocalorimetry and is used to obtain information about the dissociation energy and entropy of a protonated peptide. Activation of Na(+)(H(2)O)(30) results in sequential loss of water molecules, and the internal energy of the activated ion can be obtained from the abundances of the product ions. Information about internal energy deposition in on-resonance collisional activation of protonated peptides is inferred from dissociation data obtained under identical conditions for hydrated ions that have similar m/z and degrees-of-freedom. From experimental internal energy deposition curves and Rice-Ramsperger-Kassel-Marcus (RRKM) theory, dissociation data as a function of collision energy for protonated leucine enkephalin, which has a comparable m/z and degrees-of-freedom as Na(+)(H(2)O)(30), are modeled. The threshold dissociation energies and entropies are correlated for data acquired at a single time point, resulting in a relatively wide range of threshold dissociation energies (1.1 to 1.7 eV) that can fit these data. However, this range of values could be significantly reduced by fitting data acquired at different dissociation times. By measuring the internal energy of an activated ion, the number of fitting parameters necessary to obtain information about the dissociation parameters by modeling these data is reduced and could result in improved accuracy for such methods.

Measuring the extent and width of internal energy deposition in ion activation using nanocalorimetry

The recombination energies resulting from electron capture by a positive ion can be accurately measured using hydrated ion nanocalorimetry in which the internal energy deposition is obtained from the number of water molecules lost from the reduced cluster. The width of the product ion distribution in these experiments is predominantly attributable to the distribution of energy that partitions into the translational and rotational modes of the water molecules that are lost. These results are consistent with a singular value for the recombination energy. For large clusters, the width of the energy distribution is consistent with rapid energy partitioning into internal vibrational modes. For some smaller clusters with high recombination energies, the measured product ion distribution is narrower than that calculated with a statistical model. These results indicate that initial water molecule loss occurs on the time scale of, or faster than energy randomization. This could be due to inherently slow internal conversion or it could be due to a multi-step process, such as initial ion-electron pair formation followed by reduction of the ion in the cluster. These results provide additional evidence for the accuracy with which condensed phase thermochemical values can be deduced from gaseous nanocalorimetry experiments.

Gas-phase chemistry of multiply charged bioions in analytical mass spectrometry

Ion chemistry has long played an important role in molecular mass spectrometry (MS), as it is central to the use of MS as a structural characterization tool. With the advent of ionization methods capable of producing gaseous ions from large biomolecules, the chemistry of gaseous bioions has become a highly active area of research. Gas-phase biomolecule-ion reactions are usually driven by interactions with neutral molecules, photons, electrons, ions, or surfaces. Ion dissociation or transformation into different ion types can be achieved. The types of reaction products observed depend on the characteristics of the ions, the transformation methods, and the time frame of observation. This review focuses on the gas-phase chemistries of ions derived from the electrospray ionization of peptides, proteins, and oligonucleotides, with particular emphasis on their utility in bioanalysis. Various ion-transformation strategies, which further facilitate structural interrogation by converting ions from one type to another, are also summarized.

Electron-induced dissociation of doubly protonated betaine clusters: controlling fragmentation chemistry through electron energy

The [M21+2H]2+ cluster of the zwitterion betaine, M = (CH3)3NCH2CO2, formed via electrospray ionisation (ESI), has been allowed to interact with electrons with energies ranging from >0 to 50 eV in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. The types of gas-phase electron-induced dissociation (EID) reactions observed are dependent on the energy of the electrons. In the low-energy region up to 10 eV, electrons are mainly captured, forming the charge-reduced species, {[M21+2H]+*}*, in an excited state, which stabilises via the ejection of an H atom and one or more neutral betaines. In the higher energy region, above 12 eV, a Coulomb explosion of the multiply charged clusters is observed in highly asymmetric fission with singly charged fragments carrying away more than 70% of the parent mass. Neutral betaine evaporation is also observed in this energy region. In addition, a series of singly charged fragments appears which arise from C-X bond cleavage reactions, including decarboxylation and CH3 group transfer. These latter reactions may arise from access of electronic excited states of the precursor ions.

Directly relating reduction energies of gaseous Eu(H2O)n(3+), n = 55-140, to aqueous solution: the absolute SHE potential and real proton solvation energy

In solution, half-cell potentials are measured relative to other half-cells resulting in a ladder of thermodynamic values that is anchored to the standard hydrogen electrode (SHE), which is assigned an arbitrary value of exactly 0 V. A new method for measuring the absolute SHE potential is introduced in which reduction energies of Eu(H(2)O)(n)(3+), from n = 55 to 140, are extrapolated as a function of the geometric dependence of the cluster reduction energy to infinite size. These measurements make it possible to directly relate absolute reduction energies of these gaseous nanodrops containing Eu(3+) to the absolute reduction enthalpy of this ion in bulk solution. From this value, an absolute SHE potential of +4.11 V and a real proton solvation energy of -269.0 kcal/mol are obtained. The infrared photodissociation spectrum of Eu(H(2)O)(119-124)(3+) indicates that the structure of the surface of the nanodrops is similar to that at the bulk air-water interface and that the hydrogen bonding of interior water molecules is similar to that in aqueous solution. These results suggest that the environment of Eu(3+) in these nanodrops and the surface potential of the nandrops are comparable to those of the condensed phase. This method for obtaining absolute potentials of redox couples has the advantage that no explicit solvation model is required, which eliminates uncertainties associated with these models, making this method potentially more accurate than previous methods.

The reduction of water clusters H+(H2O)n to (OH-)(H2O)m by double electron transfer from Cs atoms

(H(+))(H(2)O)(n) ions (n = 1-72) at 50 keV energies were brought to collide with caesium atoms. The analysis of the products formed for clusters having n > 4 shows that this leads to the formation of a population of (OH(-))(H(2)O)(m) ions with a variable number m. On average, more than half of the water molecules are lost from the cluster in the process. A model can explain the experimental observations where two successive collisions occur within a time period of less than 100 ns. One-electron transfer from caesium to water leading to the loss of one hydrogen atom occurs at each stage. While the first stage is by itself exothermic, the second stage requires additional energy from collisional energy transfer.

Directly relating gas-phase cluster measurements to solution-phase hydrolysis, the absolute standard hydrogen electrode potential, and the absolute proton solvation energy

Solution-phase, half-cell potentials are measured relative to other half-cell potentials, resulting in a thermochemical ladder that is anchored to the standard hydrogen electrode (SHE), which is assigned an arbitrary value of 0 V. A new method for measuring the absolute SHE potential is demonstrated in which gaseous nanodrops containing divalent alkaline-earth or transition-metal ions are reduced by thermally generated electrons. Energies for the reactions 1) M(H(2)O)(24)(2+)(g) + e(-)(g)-->M(H(2)O)(24)(+)(g) and 2) M(H(2)O)(24)(2+)(g) + e(-)(g)-->MOH(H(2)O)(23)(+)(g) + H(g) and the hydrogen atom affinities of MOH(H(2)O)(23)(+)(g) are obtained from the number of water molecules lost through each pathway. From these measurements on clusters containing nine different metal ions and known thermochemical values that include solution hydrolysis energies, an average absolute SHE potential of +4.29 V vs. e(-)(g) (standard deviation of 0.02 V) and a real proton solvation free energy of -265 kcal mol(-1) are obtained. With this method, the absolute SHE potential can be obtained from a one-electron reduction of nanodrops containing divalent ions that are not observed to undergo one-electron reduction in aqueous solution.

Electron-capture dissociation and collision-induced dissociation of lanthanide metal-ligand complexes and lanthanide metal-ligand complexes bound to phosphopeptides

Collision-induced dissociation (CID) and electron-capture dissociation (ECD) of lanthanide metal(III)-ligand complexes and lanthanide metal(III)-ligand-phosphopeptide complexes have been investigated using a Fourier transform-ion cyclotron resonance mass spectrometer (FT-ICR MS). Ternary adducts were formed for [LnL(3+)+solvent anion(s)(n-)]((3-n)+) [Ln=europium, terbium and ytterbium, L=heptadentate ligand, solvent anion=acetate or trifluoromethane-sulphonate (triflate)]. Results show that ECD provides much more diagnostically useful information than CID. ECD was found to systematically cleave N-C bonds in the "arms" of the ligand, similar to the N-C(alpha) cleavage of peptides, generating two fragmentation sites per arm of the ligand. The most intense and informative fragment ions were obtained from the terbium complex and it is believed that this is a consequence of terbium's greater reduction potential: the greater the reduction potential, the greater the ligand fragmentation; the lower the reduction potential, the more likely the molecule is to relinquish the solvent anion. The choice of solvent is also shown to be important. In general, the complexes studied fragment easily to lose CH(3)CO(2)H; however, particularly for ECD, the complexes retain the triflate anion causing the ligand to fragment instead, thus providing much more useful information. The triflate anion can be displaced by a phosphopeptide to create a lanthanide metal-ligand-phosphopeptide adduct. ECD is able to sequence the phosphopeptide, locating the site of phosphorylation bound to [LnL](3+) and also confirm the identity of the ligand. Small differences in the fragmentation of the lanthanide metal-ligand-phosphopeptide adducts follow the trend Eu <Tb <Yb suggesting that charge density may now be a more significant factor than metal ion reduction potential. ECD analysis of the triflate salts of the terbium complexes is most informative in developing a method to optimise structural information that can be gained from this group of molecules by mass spectrometry.

Nanocalorimetry in mass spectrometry: a route to understanding ion and electron solvation

A gaseous nanocalorimetry approach is used to investigate effects of hydration and ion identity on the energy resulting from ion-electron recombination. Capture of a thermally generated electron by a hydrated multivalent ion results in either loss of a H atom accompanied by water loss or exclusively loss of water. The energy resulting from electron capture by the precursor is obtained from the extent of water loss. Results for large-size-selected clusters of Co(NH(3))(6)(H(2)O)(n3)(+) and Cu(H(2)O)(n2)(+) indicate that the ion in the cluster is reduced on electron capture. The trend in the data for Co(NH(3))(6)(H(2)O)(n3)(+) over the largest sizes (n >/= 50) can be fit to that predicted by the Born solvation model. This agreement indicates that the decrease in water loss for these larger clusters is predominantly due to ion solvation that can be accounted for by using a model with bulk properties. In contrast, results for Ca(H(2)O)(n2)(+) indicate that an ion-electron pair is formed when clusters with more than approximately 20 water molecules are reduced. For clusters with n = approximately 20-47, these results suggest that the electron is located near the surface, but a structural transition to a more highly solvated electron is indicated for n = 47-62 by the constant recombination energy. These results suggest that an estimate of the adiabatic electron affinity of water could be obtained from measurements of even larger clusters in which an electron is fully solvated.

Investigation of energy deposited by femtosecond electron transfer in collisions using hydrated ion nanocalorimetry

Ion nanocalorimetry is used to investigate the internal energy deposited into M (2+)(H 2O) n , M = Mg ( n = 3-11) and Ca ( n = 3-33), upon 100 keV collisions with a Cs or Ne atom target gas. Dissociation occurs by loss of water molecules from the precursor (charge retention) or by capture of an electron to form a reduced precursor (charge reduction) that can dissociate either by loss of a H atom accompanied by water molecule loss or by exclusively loss of water molecules. Formation of bare CaOH (+) and Ca (+) by these two respective dissociation pathways occurs for clusters with n up to 33 and 17, respectively. From the threshold dissociation energies for the loss of water molecules from the reduced clusters, obtained from binding energies calculated using a discrete implementation of the Thomson liquid drop model and from quantum chemistry, estimates of the internal energy deposition can be obtained. These values can be used to establish a lower limit to the maximum and average energy deposition. Not taking into account effects of a kinetic shift, over 16 eV can be deposited into Ca (2+)(H 2O) 33, the minimum energy necessary to form bare CaOH (+) from the reduced precursor. The electron capture efficiency is at least a factor of 40 greater for collisions of Ca (2+)(H 2O) 9 with Cs than with Ne, reflecting the lower ionization energy of Cs (3.9 eV) compared to Ne (21.6 eV). The branching ratio of the two electron capture dissociation pathways differs significantly for these two target gases, but the distributions of water molecules lost from the reduced precursors are similar. These results suggest that the ionization energy of the target gas has a large effect on the electron capture efficiency, but relatively little effect on the internal energy deposited into the ion. However, the different branching ratios suggest that different electronic excited states may be accessed in the reduced precursor upon collisions with these two different target gases.

Electron capture by a hydrated gaseous peptide: effects of water on fragmentation and molecular survival

The effects of water on electron capture dissociation products, molecular survival, and recombination energy are investigated for diprotonated Lys-Tyr-Lys solvated by between zero and 25 water molecules. For peptide ions with between 12 and 25 water molecules attached, electron capture results in a narrow distribution of product ions corresponding to primarily the loss of 10-12 water molecules from the reduced precursor. From these data, the recombination energy (RE) is determined to be equal to the energy that is lost by evaporating on average 10.7 water molecules, or 4.3 eV. Because water stabilizes ions, this value is a lower limit to the RE of the unsolvated ion, but it indicates that the majority of the available RE is deposited into internal modes of the peptide ion. Plotting the fragment ion abundances for ions formed from precursors with fewer than 11 water molecules as a function of hydration extent results in an energy resolved breakdown curve from which the appearance energies of the b 2 (+), y 2 (+), z 2 (+*), c 2 (+), and (KYK + H) (+) fragment ions formed from this peptide ion can be obtained; these values are 78, 88, 42, 11, and 9 kcal/mol, respectively. The propensity for H atom loss and ammonia loss from the precursor changes dramatically with the extent of hydration, and this change in reactivity can be directly attributed to a "caging" effect by the water molecules. These are the first experimental measurements of the RE and appearance energies of fragment ions due to electron capture dissociation of a multiply charged peptide. This novel ion nanocalorimetry technique can be applied more generally to other exothermic reactions that are not readily accessible to investigation by more conventional thermochemical methods.

Infrared spectroscopy of Cr+(H2O) and Cr2+(H2O): the role of charge in cation hydration

Singly and doubly charged chromium-water ion-molecule complexes are produced by laser vaporization in a pulsed-nozzle cluster source. These species are detected and mass-selected in a specially designed time-of-flight mass spectrometer. Vibrational spectroscopy is measured for these complexes in the O-H stretching region using infrared photodissociation spectroscopy and the method of rare gas atom predissociation. Infrared excitation is not able to break the ion-water bonds in these systems, but it leads to elimination of argon, providing an efficient mechanism for detecting the spectrum. The O-H stretches for both singly and doubly charged complexes are shifted to frequencies lower than those for the free water molecule, and the intensity of the symmetric stretch band is strongly enhanced relative to the asymmetric stretch. Partially resolved rotational structure for both complexes shows that the H-O-H bond angle is greater than it is in the free water molecule. These polarization-induced effects are enhanced in the doubly charged ion relative to its singly charged analog.

Effects of electron kinetic energy and ion-electron inelastic collisions in electron capture dissociation measured using ion nanocalorimetry

Ion nanocalorimetry is used to measure the effects of electron kinetic energy in electron capture dissociation (ECD). With ion nanocalorimetry, the internal energy deposited into a hydrated cluster upon activation can be determined from the number of water molecules that evaporate. Varying the heated cathode potential from -1.3 to -2.0 V during ECD has no effect on the average number of water molecules lost from the reduced clusters of either [Ca(H2O)15]2+ or [Ca(H2O)32]2+, even when these data are extrapolated to a cathode potential of zero volts. These results indicate that the initial electron kinetic energy does not go into internal energy in these ions upon ECD. No effects of ion heating from inelastic ion-electron collisions are observed for electron irradiation times up to 200 ms, although some heating occurs for [Ca(H2O)17]2+ at longer irradiation times. In contrast, this effect is negligible for [Ca(H2O)32]2+, a cluster size typically used in nanocalorimetry experiments, indicating that energy transfer from inelastic ion-electron collisions is negligible compared with effects of radiative absorption and emission for these larger clusters. These results have significance toward establishing the accuracy with which electrochemical redox potentials, measured on an absolute basis in the gas phase using ion nanocalorimetry, can be related to relative potentials measured in solution.

Absolute standard hydrogen electrode potential measured by reduction of aqueous nanodrops in the gas phase

In solution, half-cell potentials are measured relative to those of other half cells, thereby establishing a ladder of thermochemical values that are referenced to the standard hydrogen electrode (SHE), which is arbitrarily assigned a value of exactly 0 V. Although there has been considerable interest in, and efforts toward, establishing an absolute electrochemical half-cell potential in solution, there is no general consensus regarding the best approach to obtain this value. Here, ion-electron recombination energies resulting from electron capture by gas-phase nanodrops containing individual [M(NH3)6]3+, M = Ru, Co, Os, Cr, and Ir, and Cu2+ ions are obtained from the number of water molecules that are lost from the reduced precursors. These experimental data combined with nanodrop solvation energies estimated from Born theory and solution-phase entropies estimated from limited experimental data provide absolute reduction energies for these redox couples in bulk aqueous solution. A key advantage of this approach is that solvent effects well past two solvent shells, that are difficult to model accurately, are included in these experimental measurements. By evaluating these data relative to known solution-phase reduction potentials, an absolute value for the SHE of 4.2 +/- 0.4 V versus a free electron is obtained. Although not achieved here, the uncertainty of this method could potentially be reduced to below 0.1 V, making this an attractive method for establishing an absolute electrochemical scale that bridges solution and gas-phase redox chemistry.

Abundant b-type ions produced in electron capture dissociation of peptides without basic amino acid residues

We have investigated electron capture dissociation (ECD) of doubly protonated peptides with few or no basic amino acid residues (BAARs). For peptides containing one His, abundant b-type ions were only found when His was located adjacent to the N-terminus. Interestingly, b-type ions, particularly b(5)(+), were found to be the dominant product ions in ECD of peptides without BAARs. Fragmentation patterns of luteinizing hormone releasing hormone (LHRH) and vasopressin (VP), containing one Arg and one His, respectively, were compared to those of Q(8)-LHRH and oxytocin (OT) in which the BAAR is replaced with a non-BAAR. More b-type ions were found for Q(8)-LHRH and OT than for LHRH and VP. We also performed ECD of melittin and found no b-type ions from ECD of the 4+ charge state; however, many low abundance b-type ions were produced in ECD of the 5+ charge state. Possible mechanisms for the formation of b-type ions are discussed and we propose that such ions are formed as a consequence of protons being located at backbone amide nitrogens.

Hydration of the Calcium Dication: Direct Evidence for Second Shell Formation from Infrared Spectroscopy

Infrared laser action spectroscopy in a Fourier-transform ion cyclotron resonance mass spectrometer is used in conjunction with ab initio calculations to investigate doubly charged, hydrated clusters of calcium formed by electrospray ionization. Six water molecules coordinate directly to the calcium dication, whereas the seventh water molecule is incorporated into a second solvation shell. Spectral features indicate the presence of multiple structures of Ca(H2O)(7)2+ in which outer-shell water molecules accept either one (single acceptor) or two (double acceptor) hydrogen bonds from inner-shell water molecules. Double-acceptor water molecules are predominantly observed in the second solvent shells of clusters containing eight or nine water molecules. Increased hydration results in spectroscopic signatures consistent with additional second-shell water molecules, particularly the appearance of inner-shell water molecules that donate two hydrogen bonds (double donor) to the second solvent shell. This is the first reported use of infrared spectroscopy to investigate shell structure of a hydrated multiply charged cation in the gas phase and illustrates the effectiveness of this method to probe the structures of hydrated ions.

Nonergodicity in electron capture dissociation investigated using hydrated ion nanocalorimetry

Hydrated divalent magnesium and calcium clusters are used as nanocalorimeters to measure the internal energy deposited into size-selected clusters upon capture of a thermally generated electron. The infrared radiation emitted from the cell and vacuum chamber surfaces as well as from the heated cathode results in some activation of these clusters, but this activation is minimal. No measurable excitation due to inelastic collisions occurs with the low-energy electrons used under these conditions. Two different dissociation pathways are observed for the divalent clusters that capture an electron: loss of water molecules (Pathway I) and loss of an H atom and water molecules (Pathway II). For Ca(H(2)O)(n)(2+), Pathway I occurs exclusively for n >or= 30 whereas Pathway II occurs exclusively for n <or= 22 with a sharp transition in the branching ratio for these two processes that occurs for n approximately 24. The number of water molecules lost by both pathways increases with increasing cluster size reaching a broad maximum between n = 23 and 32, and then decreases for larger clusters. From the number of water molecules that are lost from the reduced cluster, the average and maximum possible internal energy is determined to be approximately 4.4 and 5.2 eV, respectively, for Ca(H(2)O)(30)(2+). This value is approximately the same as the calculated ionization energies of M(H(2)O)(n)(+), M = Mg and Ca, for large n indicating that the vast majority of the recombination energy is partitioned into internal modes of the ion and that the dissociation of these ions is statistical. For smaller clusters, estimates of the dissociation energies for the loss of H and of water molecules are obtained from theory. For Mg(H(2)O)(n)(2+), n = 4-6, the average internal energy deposition is estimated to be 4.2-4.6 eV. The maximum possible energy deposited into the n = 5 cluster is <7.1 eV, which is significantly less than the calculated recombination energy for this cluster. There does not appear to be a significant trend in the internal energy deposition with cluster size whereas the recombination energy is calculated to increase significantly for clusters with fewer than 10 water molecules. These, and other results, indicate that the dissociation of these smaller clusters is nonergodic.