Ion-Cyclotron Double Resonance

102

The relative abundances of fragment ions in electron capture dissociation (ECD) are often greatly affected by the secondary and tertiary structures of the precursor ion, and have been used to derive the gas-phase conformations of the protein ions. In this study, it is found that resonance ejection of the charge reduced molecular ion during ECD resulted in significant changes in many fragment ion populations. The ratio of the ion peak intensities in the double resonance (DR)-ECD to that in the normal ECD experiment can be used to calculate the lifetime of the radical intermediates from which these fragments are derived. These lifetimes are often in the ms range, a time sufficiently long to facilitate multiple free radical rearrangements. These ratios correlate with the intramolecular noncovalent interactions in the precursor ion, and can be used to deduce information about the gas-phase conformation of peptide ions. DR-ECD experiments can also provide valuable information on ECD mechanisms, such as the importance of secondary electron capture and the origin of c·/z ions. (J Am Soc Mass

mentioning

confidence: 99%

Use of a double resonance electron capture dissociation experiment to probe fragment intermediate lifetimes

Lin

Cournoyer

O’Connor

2006

102

“…In a double resonance experiment [77,78], a frequency corresponding to the cyclotron resonance frequency of an ion suspected to be an intermediate in a consecutive dissociation process is continuously irradiated over the course of a dissociation experiment. The disappearance of lower mass ions confirms that the irradiated ion is an intermediary in the formation of those ions.…”

Section: Double Resonance Experimentsmentioning

confidence: 99%

Nonergodicity in electron capture dissociation investigated using hydrated ion nanocalorimetry

Leib

Donald

Bush

et al. 2007

118

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...

“…DR was initially developed to identify chemically coupled processes in equilibrium experiments conducted in ion cyclotron resonance mass spectrometers [49] and was later adapted for Fourier-transform mass spectrometry [50] and other kinds of mass spectrometry [51]. DR experiments are done by applying an on-resonance ejection waveform simultaneous with the dissociation event, with the aim of ejecting primary fragment ions before they undergo subsequent dissociation.…”

mentioning

confidence: 99%

Gas-phase dissociation pathways of multiply charged peptide clusters

Jurchen

García

Williams

2003

Numerous studies of cluster formation and dissociation have been conducted to determine properties of matter in the transition from the condensed phase to the gas phase using materials as diverse as atomic nuclei, noble gasses, metal clusters, and amino acids. Here, electrospray ionization is used to extend the study of cluster dissociation to peptides including leucine enkephalin with 7-19 monomer units and 2-5 protons, and somatostatin with 5 monomer units and 4 protons under conditions where its intramolecular disulfide bond is either oxidized or reduced. Evaporation of neutral monomers and charge separation by cluster fission are the competing dissociation pathways of both peptides. The dominant fission product for all leucine enkephalin clusters studied is a proton-bound dimer, presumably due to the high gas-phase stability of this species. The branching ratio of the fission and evaporation processes for leucine enkephalin clusters appears to be determined by the value of z 2 /n for the cluster where z is the charge and n the number of monomer units in the cluster. Clusters with low and high values of z 2 /n dissociate primarily by evaporation and cluster fission respectively, with a sharp transition between dissociation primarily by evaporation and primarily by fission measured at a z 2 /n value of ~0.5. The dependence of the dissociation pathway of a cluster on z 2 /n is similar to the dissociation of atomic nuclei and multiply charged metal clusters indicating that leucine enkephalin peptide clusters exist in a state that is more disordered, and possibly fluid, rather than highly structured in the dissociative transition state. The branching ratio, but not the dissociation pathway of [somatostatin 5 + 4H] 4+ is altered by the reduction of its internal disulfide bond indicating that monomer conformational flexibility plays a role in peptide cluster dissociation.Bulk properties of matter and the properties of isolated gas-phase atoms and molecules can have radically different characteristics in terms of structure, work function, ion solvation, etc. (for a review see reference [1]). One motivation behind cluster research is that the transitional characteristics between bulk and molecular properties can be discovered by studying intermediate states of matter. Investigators have explored a wide variety of materials, including charged droplets of various liquids [2,3], clusters of noble metals [4][5][6][7][8], metal-ligand clusters [1,[9][10][11], highly charged clusters of noble gasses [12,13], small molecules [14,15], and ultracold clusters of transition metals [16]. Recently, investigators have formed noncovalently bound clusters of biologically important molecules, including clusters of amino acids and small peptides [17][18][19][20][21][22][23][24][25][26][27][28], and have studied their dissociation processes [18,20,[26][27][28]. Multiply charged "clusters" of proteins consisting of multimeric, noncovalently associated protein complexes with a high degree of solution-phase structural specificity ...