C_(alpha)-C Backbone Fragmentation Dominates in Electron Detachment Dissociation of Gas-Phase Polypeptide Polyanions

摘要

Fragmentation of peptide polyanions by electron detachment dissociation (EDD) has been induced by electron irradiation of deprotonated polypeptides [M-nH]~(n-) with >10eV electrons.EDD has been found to lead preferentially to alpha~(centre dot) and x fragment ions (C_(alpha)-C backbone cleavage) arising from the dissociation of oxidized radical anions [M-nH]~((n-1)-centre dot).We demonstrate that C_(alpha)-C cleavages,which are otherwise rarely observed in tandem mass spectrometry,can account for most of the backbone fragmentation,with even-electron x fragments dominating over radical alpha~(centre dot) ions.Ab initio calculations at the B3LYP level of theory with the 6-311 + G(2p,2d)//6-31 + G(d,p) basis set suggested a unidirectional mechanism for EDD (cleavage always N-terminal to the radical site),with a',x formation being favored over a,x~(centre dot) fragmentation by 74.2kJ mol~(-1).Thus,backbone C_(alpha)-C bonds N-terminal to proline residues should be immune to EDD,in agreement with the observations.EDD may find application in mass spectrometry for such tasks as peptide sequencing and localization of labile post-translational modifications,for example,those introduced by sulfation and phosphorylation.EDD can now be performed not only in Fourier transform mass spectrometry,but also in far more widely used quadrupole (Paul) ion traps.