Electron Attachment Step in Electron Capture Dissociation (ECD) and Electron Transfer Dissociation (ETD)

Abstract

We have made use of classical dynamics trajectory simultions and ab initio electronic structure calculations to estimate the cross sections with which electrons are attached (in electron capture dissociation (ECD)) or transferred (in electron transfer dissociation (ETD)) to a model system that contained both an S-S bond that is cleaved and a -NH 3 + positively charged site. We used a Landau-Zener-Stueckelberg curve-crossing approximation to estimate the ETD rates for electron transfer from a CH 3 -anion to the -NH 3 + Rydberg orbital or the S-S σ* orbital. We draw conclusions about ECD from our ETD results and from known experimental electron-attachment cross sections for cations and σ-bonds. We predict the cross section for ETD at the positive site of our model compound to be an order of magnitude larger than that for transfer to the Coulomb-stabilized S-S bond site. We also predict that, in ECD, the cross section for electron capture at the positive site will be up to 3 orders of magnitude larger than that for capture at the S-S bond site. These results seem to suggest that attachment to such positive sites should dominate in producing S-S bond cleavage in our compound. However, we also note that cleavage induced by capture at the positive site will be diminished by an amount that is related to the distance from the positive site to the S-S bond. This dimunition can render cleavage through Coulomb-assisted S-S σ* attachment competitive for our model compound. Implications for ECD and ETD of peptides and proteins in which SS or N-C R bonds are cleaved are also discussed, and we explain that such events are most likely susceptible to Coulomb-assisted attachment, because the S-S σ* and CdO π* orbitals are the lowest-lying antibonding orbitals in most peptides and proteins.

Dipole-Guided Electron Capture Causes Abnormal Dissociations of Phosphorylated Pentapeptides

“…The other charging proton was sequestered by the arginine guanidinium group which was not reduced. Structure 2a +• represents another example of a local energy minimum corresponding to a chargestabilized amide π* state as suggested previously [49][50][51]. The other cation-radical (2b +• ) was a standard guanidinium radical [52] resulting from one-electron reduction in the arginine residue.…”

Electron transfer dissociation of amide nitrogen methylated polypeptide cations

“…Specifically, this entails measurement of the relative contributions of proton transfer versus electron-transfer (PT versus ET), the partitioning between electron-transfer without dissociation (ET,noD) and with dissociation (ETD) within the overall ET channel, the partitioning between the products of backbone cleavages (e.g., the c-and z-type ions) and products from side-chain cleavages within the ETD channel, and the partitioning between the various backbone cleavages channels (i.e., specific c-and z-type ions). It is desirable to compare product partitioning at these various levels because the major effect of a particular variable in an ETD experiment, such as the nature of the reagent anion [29] or the nature of the charge bearing sites on the peptide [22], may be more manifest in one of the levels of partitioning than another. The partitioning of products is related on a percentage basis with the following relationships:…”

“…This intermediate can then readily undergo dissociation at the N-C ␣ bond to give rise to c-and z-type ions. An alternative to this mechanism has been proposed independently by Turecek and Simons [21][22][23]. In this picture, it is posited that electron capture occurs either directly in amide * orbitals that are stabilized by the electrostatic field present in the multiply charged ion, or an electron can transfer to such an orbital from a low-lying Rydberg state.…”

Effects of Peptide Backbone Amide-to-Ester Bond Substitution on the Cleavage Frequency in Electron Capture Dissociation and Collision-Activated Dissociation

“…Since the cross-section for direct capture of free hydrogen atoms by gas-phase peptides is quite low [25] , it appears that mainly hydrogen atoms from neutralized protonated groups solvated to carbonyls can be effectively transferred to carbonyl oxygens [23]. An alternative mechanism was put forward independently by Turecek and Simons and coworkers [26][27][28]. This mechanism suggests electron capture into the amide OCN π* orbital.…”