1D-O2-1820 PDF
Where Does Electron Go in Metal-Peptide Complex? An Approach to Address the General Mechanism of Electron Transfer Mass Spectrometry
ECD and ETD have been widely used for peptide/protein sequencing. Among the most accepted mechanisms, ECD/ETD involves recombination of electron and proton in a multiply-protonated molecule. The mechanism should indicate that decreasing the number of excess proton in precursors impairs the reduction efficiency and reduces the fragment ion yield. However, we have recently found that ETD mass spectra of metal-peptide complexes are more informative than those obtained from the protonated molecules with lower charge states and that the number of excess protons does not correlate with fragment ion yield.
The “Utah–Washington” model has been alternatively proposed. This model is based on direct electron attachment to π* amide orbital of the backbone amide group. This model would be applicable to the ETD process of metal-peptide complex.
We focused on the ETD of Zn2+-trihistidine complex, which did not contain any remote proton. Electron transfer to the complex induced N–Cα bond cleavage. Ab initio calculations showed that the charge-reduced zinc-peptide radical can exist as low-energy zwitterionic amide π* states, which underwent N–Cα bond dissociation. The formation pathway for the ETD products was calculated using Gaussian09. In conclusion, the ETD mass spectrum of the complex could be explained by “Utah-Washington” model.