Electron capture, femtosecond electron transfer and theory: A study of noncovalent crown ether 1,n-diammonium alkane complexes
Abstract
Complexes of doubly protonated 1,n-diaminoalkanes with one or two molecules of 18-crown-6-ether undergo consecutive and competitive dissociations upon electron capture from a free thermal electron and femtosecond collisional electron transfer from Na and Cs atoms. The electron capture dissociation (ECD) and electron capture-induced dissociation (ECID) mass spectra show very different products and product ion intensities. In ECD, the reduced precursor ions dissociate primarily by loss of an ammonium hydrogen and the crown ether ligand. In ECID, ions from many more dissociation channels are observed and depend on whether collisions occur with Na or Cs atoms. ECID induces highly endothermic C
C bond cleavages along the diaminoalkane chain, which are not observed with ECD. Adduction of one or two crown ethers to diaminoalkanes results in different electron capture cross-sections that follow different trends for ECD and ECID. Electron structure calculations at the B3-PMP2/6-311++G(2d,p) level of theory were used to determine structures of ions and ion radicals and the energetics for protonation, electron transfer, and ion dissociations for most species studied experimentally. The calculations revealed that the crown ether ligand substantially affected the recombination energy of the diaminoalkane ion and the electronic states accessed by electron attachment.
Introduction
Ammonium radicals are transient species that formally have nine valence electrons on nitrogen and thus violate the venerable octet rule [1]. Simple ammonium radicals such as NH4
and a number of alkyl and aryl ammonium derivatives of the RNH3, R2NH2, R3NH and R4N-type have been studied extensively by spectroscopy [2], mass spectrometry [3], and ab initio theory [4], and their fundamental electronic properties and unimolecular dissociations are mostly well understood. Recently, the properties of ammonium radicals have become of increased interest owing to their role in dissociations of biomolecular ions, primarily peptides and proteins, upon reduction by capture of a free electron [5] or transfer from atoms [6] or anions [7] in the gas phase. The interaction of an electron with a multiply protonated protein can result in extensive backbone cleavage from which primary [5], and even tertiary structure information may be obtained [5b–d]. Unlike other ion activation techniques, electron capture can result in retention of labile post-translational modifications [5e–i], which substantially increases the information that may be gained from tandem mass spectrometry.
Chemical recognition in mass spectrometry has shown potential for analytical applications in protein structure elucidation. For example, charged ammonium groups in peptide lysine residues have been found to form gas-phase complexes with crown ethers (CE) [8], [9] from which solution-phase structural information may be inferred. For example, 18-C-6-E adduction in solution to various proteins has been used to qualitatively probe the surface accessibility of proteins in solution from the number of CE adducted and the charge state envelopes [9]. Activation of CE adducted peptides by thermal techniques results in loss of the CE prior to covalent bond cleavage [8], [10]. Recently, gas-phase complexes of 18-C-6-E with small doubly charged peptide ions containing one or two lysine residues were studied by collisional electron transfer and the CE had a large effect on the competitive formation of sequence fragments [6a]. Furthermore, collisional electron transfer to a CE adducted dipeptide resulted in backbone fragments, some of which retained the noncovalent CE ligand [6a]. These results raised some fundamental questions about the nature of electronic states that are formed by attaching an electron to CE·ammonium complexes. The present paper reports a systematic study of electron-ion recombination involving 1,n-diaminoalkane ions and their complexes with one and two molecules of 18-crown-6-ether. Electron-ion recombination is realized either as electron capture by doubly protonated ions trapped in an ion-cyclotron resonance cell, or as femtosecond electron transfer to doubly protonated ions from alkali metal atoms performed at high ion velocity in a beam experiment. The experimental data are complemented by ab initio and density functional calculations for selected species and interesting comparisons between the two techniques are made.
Section snippets
Materials
1,n-Diaminoalkanes and 18-crown-6-ether were purchased from Sigma–Aldrich (Milwaukee, WI) and used as received. 1,n-Diaminoalkane·CE complexes were prepared in situ by mixing the components in 1:1 or 1:2 molar ratios in methanol or aqueous methanol solutions. Gas-phase ion complexes are represented such that a doubly protonated 1,n-diaminoalkane with m number of 18-crown-6-ethers (m = 1 or 2) is n-mCE2+. Species which have undergone deuterium labeling such that they contain x number of D atoms
Ion formation
Electron capture and transfer were studied for a series of cations either free or in complexes with one or two molecules of CE. Diaminoalkanes with chain lengths from n = 4 to 12 were investigated. Doubly protonated ions were produced by electrospray ionization of free amines or in mixtures with CE. The ions are denoted as 42+–122+ for free diammonium, 4-1CE2+–12-1CE2+ for complexes with one molecule of CE, and analogously, 4-2CE2+–12-2CE2+ for the complexes with two CE molecules.
The efficiency
Conclusions
This comparative study of ECD and ECID of model ammonium dications points to different energies and time scales in dissociations triggered by electron capture and fast collisional electron transfer. The main dissociation due to loss of an ammonium hydrogen atom occurs in both ECD and ECID. However, the spectra substantially differ in the consecutive dissociations by loss of the CE ligand. In addition, the ECID spectra show unique minor dissociations that can be assigned to the formation of
Acknowledgements
FT thanks the National Science Foundation for support through grants CHE-0349595 and CHE-0750048 for experiments and CHE-0342956 for computations, and the University of Aarhus, Denmark for a Visiting Professor fellowship in June–August 2007. ERW thanks the National Science Foundation (CHE-041593 and CHE-0718790) and the University of Aarhus, Denmark, for a visiting professor fellowship in 2005, and AISH gratefully acknowledges the funding from the European Project ITS-LEIF (RII 3/02 6016).
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