Comparison of collision- versus electron-induced dissociation of sodium chloride cluster cations

Uncovering the Pathway of Serine Octamer Magic Number Cluster Formation during Electrospray Ionization: Experiments and Simulations

Electrospray ionization (ESI) of serine (Ser) solution generates Ser8H+ as an abundant magic number cluster. ESI clustering of most other solutes yields nonspecific stoichiometries. It is unclear why Ser8H+ dominates in the case of Ser, and how Ser8H+ forms during ESI. Even the location of Ser8H+ formation is contentious (in solution, in ESI droplets, or elsewhere). Here we unravel key aspects of the l-Ser8H+ formation pathway. Harsh ion sampling conditions promote the collision-induced dissociation (CID) of regular ESI analytes. Unexpectedly, Ser8H+ was seemingly resistant against CID during ion sampling, despite its extremely low tandem mass spectrometry (MS/MS) stability. This unusual behavior reveals that Ser8H+ forms during ion sampling. We propose the following pathway: (1) Nonspecific Ser clusters are released when ESI droplets evaporate to dryness. These initial clusters cover a wide size range, from a few Ser to hundreds or thousands of monomers. (2) The clusters undergo dissociation during ion sampling, mostly via successive loss of neutral monomers. For any source activation voltage, there is a subpopulation of clusters for which this CID cascade tends to terminate at the octamer level, culminating in Ser8H+-dominated product distributions. Mobile proton molecular dynamics simulations were used to model the entire pathway. Ser8H+ structures formed in these simulations were consistent with ion mobility experiments. The most compact structures resembled the model of [Scutelnic, V. J. Am. Chem. Soc. 2018, 140, 7554-7560], with numerous intermolecular salt bridges and H-bonds. Our findings illustrate how the interplay of association and dissociation reactions across phase boundaries can culminate in magic number clusters.

Probing Metal Ion Adduction in the ESI Charged Residue Mechanism via Gas-Phase Ion/Ion Chemistry

The final stages of the charged residue mechanism/model (CRM) for ion generation via electrospray ionization (ESI) involves the binding of excess charge onto analyte species. Ions of both polarities can bind to the analyte with an excess of ions of the same polarity as the droplet. For large biomolecule/biocomplex ions, which are commonly the species of interest in native mass spectrometry (MS), the binding of acids and salts onto the analyte can lead to extensive broadening of ion signals due to adduction. Therefore, heating step(s) to facilitate desolvation and salt adduct removal are commonplace. In this work, we describe an approach to study the final stages of CRM using gas-phase ion/ion reactions to generate analyte ion/salt clusters of well-defined composition, followed by gas-phase collision-induced dissociation (CID). While there are many variables that can be studied systematically via this approach, the work described herein is focused on salt clusters of the form [Na10X11]-, where X = acetate (Ac-), chloride (Cl-), or nitrate (NO3-), in reaction with a common charge state of ubiquitin as well as several model peptides. Experiments in which equimolar quantities of each salt (i.e., NaAc, NaCl, and NaNO3) are subjected to ESI with ubiquitin (Ubi) and gas-phase ion/ion reaction studies involving [Na10X11]- and [Ubi + 6H]6+ show similar trends, in terms of the extent of sodium ion incorporation into the protein ions. Ion/ion reaction studies using model peptides show that the acetate-containing salt transfers significantly more Na+ ions into the peptide ions. Exchange of Na+ for H+ is shown to occur at the C-terminus and at up to all of the amide linkages using [Na10X11]-, whereas only the C-terminus engages in Na+/H+ exchange with [Na10Cl11]- and [Na10(NO3)11]-. In the latter cases, an additional Na+ is taken up as the excess positive charge, presumably due to solvation of the charge by multiple sites (e.g., carbonyl oxygens and basic sites).

Magic cluster sizes of cationic and anionic sodium chloride clusters explained by statistical modeling of the complete phase space

As one of the main components of sea salt aerosols, sodium chloride is involved in numerous atmospheric processes. Gas-phase clusters are ideal models to study fundamental physical and chemical properties of sodium chloride, which are significantly affected by the cluster size. Of particular interest are magic cluster sizes, which exhibit high intensities in mass spectra. In order to understand the origin of these magic cluster sizes, quantum chemical calculations at the CCSD(T)//DFT level are performed, yielding structures and binding energies of neutral (NaCl)x, anionic (NaCl)xCl- and cationic (NaCl)xNa+ clusters up to x = 8. Our calculations show that the clusters can easily isomerize, enabling dissociation into the lowest-energy isomers of the fragments. Energetics can explain the special stability of (NaCl)4Cl-, but (NaCl)4Na+ actually offers low-lying dissociation channels, despite being a magic cluster size. Collision-induced dissociation experiments reveal that the loss of neutral clusters (NaCl)x, x = 2, 4, is in most cases more favorable than the loss of NaCl or the atomic ion, i.e. sodium chloride clusters actually fragment via the cleavage of the entire cluster, not by evaporating small cluster building blocks. This is rationalized by the calculated high stability of even-numbered neutral clusters (NaCl)x, especially x = 2, 4. Analysis of the density of states and rate constants calculated with a modified Rice-Ramsperger-Kassel-Marcus (RRKM) equation called AWATAR - considering all energetically accessible isomers of reactants and fragments - shows that entropic effects are responsible for the magic cluster character of (NaCl)4Na+. In particular, low-lying vibrational modes provide a high density of states of the near-planar cluster. Together with the small contribution of an atomic ion to the sum of states in a loose transition state for dissociation, this leads to a very small unimolecular rate constant for dissociation into (NaCl)4 and Na+, which is the lowest energy fragmentation pathway. Thus, entropic effects may override energetics for certain magic cluster sizes.

Collision-Induced Dissociation of Electrosprayed NaCl Clusters: Using Molecular Dynamics Simulations to Visualize Reaction Cascades in the Gas Phase

Infusion of NaCl solutions into an electrospray ionization (ESI) source produces [Na_(n+1)Cl _n ]⁺ and other gaseous clusters. The n = 4, 13, 22 magic number species have cuboid ground state structures and exhibit elevated abundance in ESI mass spectra. Relatively few details are known regarding the mechanisms whereby these clusters undergo collision-induced dissociation (CID). The current study examines to what extent molecular dynamics (MD) simulations can be used to garner insights into the sequence of events taking place during CID. Experiments on singly charged clusters reveal that the loss of small neutrals is the dominant fragmentation pathway. MD simulations indicate that the clusters undergo extensive structural fluctuations prior to decomposition. Consistent with the experimentally observed behavior, most of the simulated dissociation events culminate in ejection of small neutrals ([NaCl] _i , with i = 1, 2, 3). The MD data reveal that the prevalence of these dissociation channels is linked to the presence of short-lived intermediates where a relatively compact core structure carries a small [NaCl] _i protrusion. The latter can separate from the parent cluster via cleavage of a single Na-Cl contact. Fragmentation events of this type are kinetically favored over other dissociation channels that would require the quasi-simultaneous rupture of multiple electrostatic contacts. The CID behavior of NaCl cluster ions bears interesting analogies to that of collisionally activated protein complexes. Overall, it appears that MD simulations represent a valuable tool for deciphering the dissociation of noncovalently bound systems in the gas phase. Graphical Abstract ᅟ.

Structural characterization of actinomycin D using multiple ion isolation and electron induced dissociation

Non-ribosomal peptides are bio synthesized using a range of enzymes that allow much more structural variability compared with "normal" peptides. Deviations from the standard amino acid structures are common features of this diverse class of natural products, making sequencing a challenging process. FTICR mass spectrometry, specifically the complementary tandem mass spectrometry techniques collision activated dissociation (CAD) and electron induced dissociation (EID), have been used to reveal structural information on the non-ribosomal peptide actinomycin D. EID was also combined with a multiple ion isolation method in order to provide an accurate (sub-ppm) internal calibration for the product ions. EID has been found to produce more detailed, complementary data than CAD for actinomycin D, with additional information being provided through fragmentation of the sodium and lithium adducts. Furthermore, the use of isolation in the FTICR cell was found to increase product ion intensities relative to the precursor ion, enabling significantly more peaks to be detected than when using EID alone. The combination of multiple ion isolation with EID, therefore, enables an accurate internal calibration of the fragment ions to be made (average mass uncertainty of <0.3 ppm), as well as increasing the degree of fragmentation of the compound, resulting in detailed structural information.

Structural characterization of polyketides using high mass accuracy tandem mass spectrometry

The tandem mass spectrometry techniques electron-induced dissociation (EID) and collision-activated dissociation (CAD) have been compared as tools for providing detailed structural information of polyketides. Polyketides are an important class of natural products that account for a significant proportion of the drugs currently in clinical use. Three polyketide natural products, namely erythromycin A, lasalocid A, and iso-lasalocid A, were subjected to both CAD and EID, and their fragment ions were assigned with sub-part-per-million accuracy. The number of fragment ions detected through EID was much greater than for CAD, leading to a greater amount of structural information obtained for each polyketide, albeit with a decreased signal-to-noise ratio. The effect of different bound cations on the fragment pattern of the isomers lasalocid A and iso-lasalocid A was studied, with CAD and EID performed on the [M + H](+), [M + Na](+), [M + Li](+), and [M + NH(4)](+) precursor ions. The lithiated species were found to produce the greatest degree of fragmentation and enabled detailed structural information on the isomers to be obtained. Multistage mass spectrometry (MS(3)) experiments, combining CAD and EID, could also be performed on the lithiated species, generating new fragment information which enables the two isomers to be distinguished. Combining CAD and EID for the structural characterization of polyketides will therefore be a useful tool for identifying and characterizing unknown polyketides and their biosynthetic intermediates.

Electron induced dissociation: a mass spectrometry technique for the structural analysis of trinuclear oxo-centred carboxylate-bridged iron complexes

We report electron induced dissociation (EID) Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry of the singly-charged cations [Fe(3)O(CH(3)COO)(6)](+) and [Fe(3)O(HCOO)(6)+H(2)O](+). Trinuclear oxo-centered carboxylate-bridged iron complexes of this type are of interest due to their electronic and magnetic properties, and because of their role as synthetic precursors of single molecule magnets. EID of these complexes is particularly efficient and provides detailed information about the triangular core, and the nature and number of ligands. EID behavior is in marked contrast to the collision induced dissociation (CID) of these species. Whereas EID allows virtually complete structural characterization, the structural information provided by CID is very limited. The results suggest that EID is particularly suitable for the structural analysis of singly-charged polynuclear metal complexes.

Comparison of collision- versus electron-induced dissociation of Pt(II) ternary complexes of histidine- and methionine-containing peptides

Incubation of the histidine-containing peptides (GH, HG, GGH, GHG, HGG) and methionine-containing peptides (GM, MG, GGM, GMG, MGG) with the platinum complexes [Pt(terpy)Cl](+) (A) and [Pt(dien)Cl](+) (B) followed by electrospray ionisation (ESI) led to a number of singly and doubly charged ternary platinum peptide complexes, including [Pt(L)M](2+) and [Pt(L)M-H](+) (where L = the ligand terpy or dien; M is a peptide). Each of the [Pt(L)M](2+) complexes was subjected to electron capture dissociation (ECD), collision-induced dissociation (CID) and electron-induced dissociation (EID), while each of the [Pt(L)M-H](+) complexes was subjected to CID and EID. Results from ECD suggest that the free electron is captured by the metal ion thus weakening the bonds to its ligands. In the case of the ligand terpy, which binds more strongly than dien, this weakening leads to the loss of the peptide. The minor products in the ECD spectra of [Pt(terpy)M](2+) complexes do show fragmentation along the peptide backbone, but the ions observed are of the a-, b-, and y-type. For the complexes with methionine-containing peptides, a marker ion, [Pt(L)SCH(3)](+), was found which is indicative of binding of Pt to the methionine side chain. For the histidine-containing peptides, an ion containing platinum, the auxiliary ligand, and the histidine imine was observed in many instances, thus indicating the binding of the histidine side chain to the metal, but other modes of Pt coordination (N-terminus) were also found to be competitive. These findings are consistent with a recent finding (Sze et al. J. Biol. Inorg. Chem. 2009; 14: 163) that Pt occupies the methionine-rich copper(I)-binding site rather than histidine-rich copper(II)-binding site in the CopC protein.