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

Abstract

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(H Z O)lSf+ or [Ca(HzO)d z +, 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(HzO)d z + at longer irradiation times. In contrast, this effect is negligible for [Ca(H Z O)3Zf+, 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. [3][4][5][6][7], and even tertiary structure can be obtained [2,8,9]. Since the introduction of this electron capture dissociation (ECD) method by Zubarev and colleagues [1], who combined thermally generated electrons with trapped ions in a Fourier-transform ion cyclotron resonance (FT/ICR) mass spectrometer, others have demonstrated that similar fragmentation pathways can be obtained when the electron is captured from an atom [10,11] or from molecular anions (12,13]. These methods provide a new route to obtain structural information from intact proteins and large peptides, making applications such as "top-down" proteomics [7] feasible.Ion-electron recombination or electron capture (EC) is exothermic by a value corresponding to the recombination energy (RE) [14,15] into internal modes of the precursor ion and into translational, rotational, and vibrational modes of the dissociation products. The extent of internal energy deposition upon ion activation can be measured using "chemical thermometers," which are ions that have fragmentation pathways with known activation energies and entropies. For example, Cooks and colleagues used Fe(CO)s+. to compare the energy deposition of collision-induced dissociation and surface-induced dissociation [16]. The appearance energies for fragments of Fe(CO)s +. are known and their formation occurs with similar entropies, so that the internal energy deposition is directly reflected by the fragment ion abundances. A measure of the internal energy deposition into molecular ions of n-butylbenzene molecules can be obtained from the relative abundance of fragment ions at m/z 91 and 92 [17][18][19]. Th...

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

“…Precursor ions were isolated and after 50 ms, electrons were introduced into the ion cell from a heated cathode by pulsing the cathode housing to Ϫ1.5 V for 120 ms for the transition-metal containing nanodrops and to Ϫ1.4 V for 40 ms for the triply charged clusters. A detailed description of the experimental setup used here has been previously reported [23,28]. Between 500 -1000 spectra were coadded to improve the signal-to-noise ratios in these experiments.…”

“…Ions that dissociate via sequential fragmentation and that have known threshold dissociation energies and similar dissociation entropies have also been used [18 -20]. From the abundance of the product ions, information about both the average energy and the width of the energy deposition can be inferred.An alternative approach to measuring internal energy deposition uses nanometer-sized hydrated ions as "nanocalorimeters," to precisely measure the amount of energy that is deposited into these ions upon activation [21][22][23][24][25][26][27][28][29][30]. When these ions are activated, they sequentially lose water molecules.…”

Sequential water molecule binding enthalpies for aqueous nanodrops containing a mono-, di- or trivalent ion and between 20 and 500 water molecules

“…, clusters of water molecules containing a single ion. 9–27 In this way, any effects of impurities or counter ions are eliminated. By investigating size selected clusters, it is possible to monitor how thermodynamic quantities, such as water molecule binding enthalpies, evolve with cluster size.…”

“…These data are especially important to accurately model ion-induced water nucleation 6,7 in the atmosphere and for ion nanocalorimetry. 9–13 In the latter method, hydrated ions are irradiated, for example, with slow electrons that can lead to a one-electron reduction of the ion. 9 The recombination energy (RE), which corresponds to the energy released due to the ion-electron recombination, leads to the evaporation of water molecules from the hydrated ion.…”

“…9–13 In the latter method, hydrated ions are irradiated, for example, with slow electrons that can lead to a one-electron reduction of the ion. 9 The recombination energy (RE), which corresponds to the energy released due to the ion-electron recombination, leads to the evaporation of water molecules from the hydrated ion. The RE can be obtained from the number of water molecules that are lost by modeling of this thermochemical process for water evaporation, which is based on the sequential water molecule binding energies as well as energy that partitions into translational, rotational and vibrational modes of the departing water molecules.…”