Characterization of an exception to the 'even-electron rule' upon low-energy collision induced decomposition in negative ion electrospray tandem mass spectrometry

Yang Cai; Zhenzhen Mo; Nalaka S Rannulu; Bing Guan; Srinivasan Kannupal; Bruce C Gibb; Richard B Cole

doi:10.1002/jms.1706

Characterization of an exception to the 'even-electron rule' upon low-energy collision induced decomposition in negative ion electrospray tandem mass spectrometry

J Mass Spectrom. 2010 Mar;45(3):235-40. doi: 10.1002/jms.1706.

Authors

Yang Cai¹, Zhenzhen Mo, Nalaka S Rannulu, Bing Guan, Srinivasan Kannupal, Bruce C Gibb, Richard B Cole

Affiliation

¹ Department of Chemistry, University of New Orleans, New Orleans, LA 70148, USA. ycai@chnola-research.org

PMID: 20014161
DOI: 10.1002/jms.1706

Abstract

Electrospray-generated precursor ions usually follow the 'even-electron rule' and yield 'closed shell' fragment ions. We characterize an exception to the 'even-electron rule.' In negative ion electrospray mass spectrometry (ES-MS), 2-(ethoxymethoxy)-3-hydroxyphenol (2-hydroxyl protected pyrogallol) easily formed a deprotonated molecular ion (M-H)(-) at m/z 183. Upon low-energy collision induced decomposition (CID), the m/z 183 precursor yielded a radical ion at m/z 124 as the base peak. The radical anion at m/z 124 was still the major fragment at all tested collision energies between 0 and 50 eV (E(lab)). Supported by computational studies, the appearance of the radical anion at m/z 124 as the major product ion can be attributed to the combination of a low reverse activation barrier and resonance stabilization of the product ions. Furthermore, our data lead to the proposal of a novel alternative radical formation pathway in the protection group removal of pyrogallol.