Matrix‐dependent a/x pair and overdegraded w/y/z ions generated by radical‐directed dissociation of peptide radical cations [M]+ in matrix‐assisted laser desorption/ionization in‐source decay

Abstract

AbstractFragmentation of peptide radical cations [M].+ has been examined using matrix‐assisted laser desorption/ionization (MALDI) in‐source decay (ISD) with hydrogen‐abstracting nitro‐substituted matrices. The ISD spectra of peptides containing an arginine (Arg) residue at carboxyl (C)‐termini showed preferential [w]+ ions when 4‐nitro‐1‐naphthol (4,1‐NNL) matrix was used, whereas the use of 3,5‐dinitrosalicylic acid (3,5‐DNSA) resulted in preferential [x]+ ions. Minor or some [d]+, [x]+, [y]+, and [z]+ ions were also observed. For peptides containing Arg residue at amino (N)‐termini, the ISD spectra showed preferential [a]+ ions independent of matrix used. The observed [a]+, [w]+, [x]+, [y]+, and [z]+ ions can be rationally explained by radical‐directed dissociation (RDD) of the peptide radical cations [M].+, although [d]+ ions may be formed via Norrish Type I cleavage and/or by RDD of [M].+ ions. The formation of overdegraded [d]+, [w]+, [y]+, and [z]+ ions is discussed from the standpoint of the internal energy of radical cations [M].+ and radical fragment ions [a + H].+ and [x + H].+ deposited via collisional interactions with excited matrix molecules in the MALDI plume. The radical site of the peptide cations [M].+ was presumed to be backbone amide nitrogen, from MALDI‐ISD data with three different deuterated amino acids.