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

Abstract

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. (J Am Soc Mass Spectrom 2010, 21, 615-625) © 2010 American Society for Mass Spectrometry C apture of a low-energy electron by a multiply charged peptide or protein can result in extensive and relatively non-selective fragmentation of the amide backbone, from which information about the amino acid sequence [1-3], post-translational modification sites [3][4][5], and higher-order structure [2,6,7] can be obtained. Similar fragmentation can be obtained by transferring an electron to the peptide or protein cation from a molecular anion [8] or from atoms [9,10]. These methods are being applied to both "bottom up" and "top-down" approaches to protein analysis.An important parameter in understanding how activated ions fragment is the extent of internal energy that is deposited in the activation step. For electron capture dissociation (ECD) [1-6], the recombination energy (RE) resulting from capture of an electron from a multiply charged peptide or protein has been estimated to be between 4 and 7 eV [1,11,12], based in part on known or estimated thermochemical data. Dissociation may occur from excited electronic states or from ground states [13,14], so that the internal energy deposited into an ion may not be a singular value.Some ions have been used as chemical thermometers to measure internal energy deposition [15][16][17][18]. For example, dissociation of the molecular ion of n-butylbenzene results in formation of product ions at m/z 91 and 92, the ratio of which provides information about the average internal energy of the activated precursor [15,16]. Ions that dissociate via sequential fragmentation and that have known threshold dissociation energies and similar dissociation entropies have also been used [18 -...

Average Sequential Water Molecule Binding Enthalpies of M(H2O)19−1242+ (M = Co, Fe, Mn, and Cu) Measured with Ultraviolet Photodissociation at 193 and 248 nm

“…These results are consistent with electron capture experiments with hydrated divalent and trivalent metal ions which indicate that for smaller clusters with large recombination energies, the rate of ion dissociation, at least for some of the initial water molecules lost, can be competitive with the rate of conversion from electronic-to-vibrational energy, that is, nonergodic dissociation, whereas for larger clusters, the rate of energy conversion occurs much faster than the rate of water molecule loss. 52,55 The sum of the dehydration rate constants from n to n -12 increases nearly monotonically as a function of cluster size from n ) 26 to 35, slightly plateaus from n ) 35 to 37, and continues to increase with increasing size for n > 37. The deviation in the monotonically increasing values is similar to the data in Figure 3 for Co, in which the average water molecules lost increases sharply and nearly monotonically from 11.2 to 12.6 for n ) 29-33, decreases slightly to 12.3 at n ) 37, and monotonically increases again with increasing size.…”

“…13,14,46,[49][50][51][52][53][54][55][56][57][58] The latter method, in which recombination energies (REs) of extensively hydrated ions are obtained from the number of water molecules lost upon electron capture, has been used to obtain values for the absolute potential of the standard hydrogen electrode (SHE) and absolute proton hydration energy. [49][50][51] In these experiments, a large number of water molecules can be lost from the reduced precursor.…”

Benzylammonium Thermometer Ions: Internal Energies of Ions Formed by Low Temperature Plasma and Atmospheric Pressure Chemical Ionization

“…Thermometer ions are useful for investigating the extent of energy deposited into ions upon ion formation and transfer [6][7][8][9], storage [10], and during ion activation [11]. Thermometer ions, such as the widely used 4-substituted benzylpyridiniums, have sufficiently low bond dissociation enthalpies (BDEs) that they fragment predictably and primarily by a single dissociation pathway [6][7][8].…”

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

“…The internal energy deposition can be estimated from the relative abundances of the product ions in the dissociation mass spectrum of Na 30 as a function of the energy necessary to form each product ion, which is the sum of the sequential threshold dissociation energies and the energy that partitions into the products in the form of translational, rotational, and vibrational energy [44,48]. Hydration enthalpies for small clusters of sodium have been measured [55], but values for the much larger clusters used here have not.…”

“…When a significant amount of energy is deposited into an ion on a time-scale shorter than dissociation, such as typically occurs when large hydrated ions capture an electron [48], the temperature of the activated ion can be high. Some of this deposited energy is partitioned into translational and rotational modes of the products and the rest of this energy goes into breaking the interactions between the water molecules and the cluster.…”