Valence parity renders z(*)-type ions chemically distinct

J Am Chem Soc. 2008 May 21;130(20):6388-94. doi: 10.1021/ja7099985. Epub 2008 Apr 29.


Here we report that the odd electron z (*) -type ions formed by the electron-based peptide dissociation methods (electron capture or transfer, ECD or ETD) have distinctive chemical compositions from other common product ion types. Specifically, b-, c-, and y-type ions have an odd number of atoms with an odd valence (e.g., N and H), while z (*)-type ions contain an even number of atoms with an odd valence. This tenet, referred to as the valence parity rule, mandates that no c-type ion shall have the same chemical composition, and by extension mass, as a z (*) -type ion. By experiment we demonstrate that nearly half of all observed c- and z (*) -type product ions resulting from 226 ETD product ion spectra can be assigned to a single, correct, chemical composition and ion type by simple inspection of the m/ z peaks. The assignments provide (1) a platform to directly determine amino acid composition, (2) an input for database search algorithms, or (3) a basis for de novo sequence analysis.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Amino Acids / chemistry
  • Fourier Analysis
  • Ions / chemistry
  • Peptide Mapping
  • Peptides / analysis
  • Peptides / chemistry*
  • Tandem Mass Spectrometry / methods*
  • Yeasts / chemistry


  • Amino Acids
  • Ions
  • Peptides