Theoretical Study on the Electron Capture Dissociation Correlated with Proton Transfer Processes.

Abstract

A novel approach was proposed for elucidating the reaction mechanism of the electron capture dissociation (ECD) recently introduced powerful technique to characterize peptides and proteins. Against a mechanism presented so far that amide bonds catch the hydrogen atom released from the protonated peptide through electron capture, we present another mechanism focusing the proton rearrangement among the sites of the nitrogen and the oxygen atoms. This proton transfer produces various types of protonated peptides that should be responsible for the cleavage after electron capture. We carried out MO calculations on the ECD processes of glycylserine as a model peptide to elucidate how the process of proton transfer correlates with backbone cleavages. Based on the calculated energy and the molecular geometry for various stages of species concerned with the ECD process, mechanisms for proton transfer and electron capture through the Franck-Condon process were discussed. Presented here are the correlation of protonated sites to types of product ions unique in ECD and the importance of unoccupied MO's to understand the bond cleavage reactions connected with the Franck-Condon process.