Nonergodicity in electron capture dissociation investigated using hydrated ion nanocalorimetry

Abstract

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 2ϩ , Pathway I occurs exclusively for n Ն 30 whereas Pathway II occurs exclusively for n Յ 22 with a sharp transition in the branching ratio for these two processes that occurs for n Ϸ 24. The number of water molecules lost by both pathways increases with increasing cluster size reaching a broad maximum between n ϭ 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 ϳ4.4 and 5.2 eV, respectively, for Ca(. This value is approximately the same as the calculated ionization energies of M(H 2 O) n ϩ , M ϭ 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 2ϩ , n ϭ 4 -6, the average internal energy deposition is estimated to be 4.2-4.6 eV. The maximum possible energy deposited into the n ϭ 5 cluster is Ͻ7.1 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. . The effectiveness of the bottom-up method for complex samples can be enhanced by using multidimensional separations. Clemmer and coworkers elegantly demonstrated that combining on-line liquid chromatography (LC) with ion mobility spectrometry and MS can greatly improve separations without increasing analysis times over LC/MS alone [2][3][4]. In contrast, the "top-down" approach to protein characterization has the advantage that de novo sequencing, including the identification and structural localization of labile posttranslational modifications, can be done directly on protein mixtures without proteolysis [5,6]. This top-down approach has greatly benefited from the development of electron capture dissociation (ECD), a method pioneered by McLafferty and coworkers [6][7][8][9]. In a typical ECD experim...

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.…”

“…Instead, more accurate RE values can be obtained from the sum of the sequential water molecule binding energies (E 0 ) and the sum of the average energy lost to the translational, rotational, and vibrational modes of the products for each sequential water molecule that is lost. Under the conditions of these experiments (Ͻ10 Ϫ8 Torr), collisional and radiative cooling should be minimal [21][22][23]. Clusters with fewer than 55 water molecules dissociate faster than the timescale of the experiment so kinetic shift effects are negligible [25].…”

“…In general, both smaller and larger clusters lose fewer water molecules than intermediate size clusters. For example, EC of Ca(H 2 O) n 2ϩ results in the loss of 2.8, 10.2, and 9.0 water molecules for n ϭ 4, 32, and 47, respectively [23]. At smaller cluster sizes, fewer water molecules are lost despite high RE values because the binding energies of water molecules closest to the metal ions are higher and because more energy partitions into translational and rotational modes of the products due to the high initial effective temperatures of the reduced clusters.…”

“…In addition to the extent of water molecule loss depending on cluster size, the competition between loss of exclusively water molecules (Pathway I) and the loss of a H atom and water molecules (Pathway II) upon EC by M(H 2 O) n zϩ also depends on n. Pathway II is favored at small cluster sizes, whereas Pathway I is dominant at larger sizes, with a relatively rapid transition occurring between both pathways at n ϳ 24 and ϳ44, for M ϭ Ca 2ϩ [23], and M ϭ La 3ϩ , respectively. Mn is the only divalent metal ion that dissociates by both pathways with 32 water molecules (Figure 1a), although both pathways are observed for all the transition-metal ions with 24 water molecules [27].…”

“…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.…”