Valence parity renders z•-type ions chemically distinct

Here we report that the odd electron z(center dot)-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(center dot)-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(center dot)-type ion. By experiment we demonstrate that nearly half of all observed c- and z(center dot)-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.