Electron detachment dissociation

ETD lends the capabilities of ECD to linear ion trap mass spectrometers. The individual steps involved in the operation of an LTQ instrument in ETD mode (Fig. 9.39) are injection of multiply protonated peptides as delivered by an ESI source application of a DC offset to store these ions in the front section of the LIT followed by injection of reagent anions from the Cl source into the center of the LIT. Then all but the peptide precursor ions and the electron-donor reagent ions are ejecteeL Next the DC potential well is switched off and a secondary RF voltage is applied to the end lens plates of the LIT causing positive and negative ion populations to mix and react. The reaction period is ended by axial ejection of reagent anions while positive product ions are retained in the center section of the LIT. Finally, mass-selective radial ejection as usual yields the ETD spectrum [160]. The attractive ETD technique has also been implemented on LITs with axial ejection [144,166] and on LIT-orbitrap hybrids [167-169]. 

Unfortunately, ETD has limited applicability to doubly protonated peptide precursor ions, [M+2I1], while triply protonated ions fragment to reveal the full sequence due to the higher exothermicity of the electron capture (Fig. 9.40) [162]. 

Both ECD and ETD cause direct backbone cleavages in multiply positive peptide ions to deliver c- and z-type fragments which are highly informative for mass spectral sequencing. Especially when occurring more than once on a molecule, post-translational modifications like phosphorylation or sulfonation, reduce its tendency to form multiply protonated ions. Such analytes are best studied as negative ions, which are of course not amenable to ECD [170]. If the primary electrons 

Alternatively, electron ionization can occur anywhere along a polypeptide chain to locally form a positive radical ion that also may be regarded as an electron hole. This will attract an excess electron from one of the anionic sites of the ion and end up in mutual neutralization. The released energy effects electronic excitation, that, in turn, causes backbone cleavage [171]. This technique is termed electron detachment dissociation (EDD) [170-173]. 

EDD is not only useful for acidic peptides but also for oligonucleotides [174] and other analytes [152]. EDD has also been adapted to QITs [172]. Nonetheless, EDD is clearly less relevant in bioanalytical work than ECD or ETD. 

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A Dalton is defined as l/12tli of the mass of a C atom. It differs from an atomic mass unit (amu), which is defined as l/16th of the mass of a atom. Electron capture dissociation Electron detachment dissociation Electron-induced dissociation... 

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While most peptide dissociation is carried out in the positive ion mode, the negative ion mode is often better suited for acidic peptides, particularity those carrying acidic modifications (e.g., phosphorylations). There are a number of equivalent ion activation methods for peptide anions, involving ion-electron and ion-ion reactions, such as electron detachment dissociation (EDD) [49], negative electron transfer dissociation (NETD) [50, 51], and negative electron capture dissociation (nECD) [52]. 

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