Specific cleavage at peptide backbone C α –C and CO–N bonds during matrix‐assisted laser desorption/ionization in‐source decay mass spectrometry with 5‐nitrosalicylic acid as the matrix

The use of 5‐nitrosalicylic acid (5‐NSA) as a matrix for in‐source decay (ISD) of peptides during matrix‐assisted laser desorption/ionization (MALDI) is described herein. Mechanistically, the decay process is initiated by a hydrogen abstraction from a peptide backbone amide nitrogen by 5‐NSA. Hydrogen abstraction results in formation of an oxidized peptide containing a radical amide nitrogen. Subsequently, the C α –C bond N‐terminal to the peptide bond is cleaved to form an a ·/ x fragment pair. The C α –C bonds C‐terminal to Gly residues were less susceptible to cleavage than were those of other residues. C α –C bonds N‐terminal to Pro and Sar residues were not cleaved by the aforementioned mechanism; instead, after hydrogen abstraction from a Pro or Sar C α –H bond, the peptide bond N‐terminal to the Pro was cleaved yielding b ‐ and y ‐series ions. We also show that fragments produced by MALDI 5‐NSA‐induced ISD were formed independently of the ionization process. Copyright © 2011 John Wiley & Sons, Ltd.