Negative electron transfer dissociation

Wolff JJ, Leach 111 FE, Laremore TN, Kaplan DA, Easterling ML, Linhardt RJ, Amster JJ. Negative electron transfer dissociation of glycosaminoglycans. Anal Chem. 2010 82 3460-6. 

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]. 

A variation of EDD, termed negative electron transfer dissociation (NETD) employs fluoranthene, CieHio" , or Xe" ions instead of energetic electrons to induce radical cationic sites by charge transfer rather than by electron ionization [175]. NETD yields selective backbone cleavage at the Ca-C bonds, similar to EDD, but leaves the phosphorylation sites intact, thereby enabling the localization of posttranslational modifications (PTMs). 

Huzarska, M. Ugalde, I. Kaplan, D.A. Hartmer, R. Easterling, M.L. Polfer, N.C. Negative Electron Transfer Dissociation of Deprotonated Phosj opeptide Anions Chrace of Radical Cation Reagent and Competition Between Electron and Proton Transfer. Anal. Chem. 2010, 82,2873-2878. 

A functionally similar dissociation method, electron transfer dissociation (ETD) was more recently reported (32), specifically for use on QIT instruments, although it is also beginning to be adapted to other mass analyzers. ETD is accomplished by electron transfer to the analyte from a negatively charged species that is produced in a chemical ionization source and directed into the region where the analyte ions are trapped. For peptides and proteins, it produces spectra that... 

He, M. McLuckey, S.A. Increasing the negative charge of a macroanion in the gas phase via sequential charge inversion reactions. Ana/. Chem. 2004, 76,4189-4192. Gunawardena, H.P Emory, J.F. McLuckey, S.A. Phosphopeptide anion characterization via sequential charge inversion and electron-transfer dissociation. Anal. Chem. 2006, 78, 3788-3793. 

TOF mode and reflector TOF mode for the observation of labile modifications [45] (differences between linear and reflectron TOFs in the detection of metastable ions are detailed in Chapter 2). It may also be useful to try negative- and positive-ion modes, as certain species ionize more efficiently in the negative-ion mode [46]. An alternative to the MALDl-MS/MS analysis of modified peptides is electron-transfer dissociation (ETD)-based ESl-MS/MS, where the modification is retained during peptide ion fragmentation, making the assignment of the modification site more straightforward. 

Example The EC mass spectrum of benzo[a]pyrene, C20H12, shows the negative molecular ion exclusively at m/z 252 (Fig. 7.14). This spectrum is representative for EC spectra of polycyclic aromatic hydrocarbons (PAHs) [87,88]. One particular PAH, fluoranthene, has recently received much attention as its ion serves as electron donating reactant ion in electron transfer dissociation (ETD, Chap. 9.15). 

Excellent ion/molecule and ion/ion reactions capability. The LQIT readily allows for ion/molecule reactions and electron transfer dissociation (ETD) experiments because of the advantages discussed in i, ii, and iv above, and because of the unique ability of XQITs to trap both positive and negative ions simultaneously. 

The forward and reverse rate constants are thus equal at zero standard free energy. However, this will be difficult to check in practice, for both reactions are very slow, since a bond-breaking/bond-forming process endowed with a quite large internal reorganization is involved. The result is that dissociative electron transfer reactions are usually carried out with electron donors that have a standard potential largely negative to the dissociative standard potential. The reoxidation of the R, X- system is thus possible only with electron acceptors, D +, that are different from the D,+ produced in the reduction process (they are more powerful oxidants). There is no reason then that the oxidation mechanism be the reverse of the... 

One final example worth mentioning is the reductive alkylation/arylation with lithium and alkyl/aryl halides in liquid ammonia. This is a two-step process in which negatively charged nanotubes are formed via electron transfer from the metal. This step is relatively easy and fast due to the CNTs electron sink properties, and it enables exfoliation of the tubes through electrostatic repulsion in the second stage, the alkyl/aryl halides react with the charged tubes to form a radical anion which can dissociate into the alkyl radical and the halide anion, with the former species undergoing addition to the CNT sidewalls [42]. 

Of course the Co CNHj) breaks down rapidly in acid into Co + and 5NHJ. Precursor complex formation, intramolecular electron transfer, or successor complex dissociation may severally be rate limiting. The associated reaction profiles are shown in Fig. 5.1. A variety of rate laws can arise from different rate-determining steps. A second-order rate law is common, but the second-order rate constant is probably composite. For example, (Fig. 5.1 (b)) if the observed redox rate constant is less than the substitution rate constant, as it is for many reactions of Cr +, Eu +, Cu+, Fe + and other ions, and if little precursor complex is formed, then = k k2kz ). In addition, the breakdown of the successor complex would have to be rapid k > k 2). This situation may even give rise to negative (= A//° +... 

Through exothermic dissociation of a neutral excited state in molecule by electron transfer to an adjacent molecule. This process leads to the generation of geminately bound electron-hole pairs as precursors of free positive and negative charges in an organic solar cell. 

Desorption can proceed via several mechanisms. For solids with a negative electron alSnity such as Ar [49,149-151] and N2 [153], the extended electron cloud around a metastable center will interact repulsively with the surrounding medium and metastables formed at the film-vacuum interface will be expelled into vacuum (the so-called cavity expulsion mechanism [161]). Also permitted in solids with positive electron affinities (e.g., CO) is the transfer of energy intramolecular vibration to the molecule-surface bond with the resulting desorption of a molecule in lower vibrational level [153,155,158-160]. Desorption of metastables via the excitation of dissociative molecular (or excimer) electronic states is also possible [49,149-151,154,156,157]. A concise review of the topic can be found in Ref. 162. 

The increase in catalytic activity with the rise of metal content can be explained by the mutual charging of Cu nanoparticles by tunnel electron transfer between particles of different size. Presumably, negative charged particles formed in this case, among positively charged ones, facilitate initiation of the chain reaction (I) via dissociation of CC14. 

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