Side‐chain radical losses from radical cations allows distinction of leucine and isoleucine residues in the isomeric peptides Gly‐XXX‐Arg

Sequencing of peptides via low‐energy collision‐induced dissociation of protonated peptides typically yields b n and y n sequence ions. The isomeric residues leucine and isoleucine rarely can be distinguished in these experiments since they give b n and y n sequence ions of the same m/z . Siu's pioneering work on electrospray ionization of copper complexes of peptides (Chu IK, Rodriquez CF, Lau TC, Hopkinson AC, Siu KWM. J. Phys. Chem. B 2000; 104: 3393) provides a way of forming radical cations of peptides in the gas phase. This method was used to generate M +· ions of the two isomeric peptides Gly‐Leu‐Arg and Gly‐Ile‐Arg in order to compare their fragmentation reactions. Both radical cations fragment to give even electron y 2 and y 1 sequence ions as well as side‐chain radical losses of CH 3 and CH 3 CH 2 for isoleucine and (CH 3 ) 2 CH for leucine. In contrast the [M + H] + and [M + 2H] 2+ ions do not allow distinction between the isomeric leucine and isoleucine peptides. Copyright © 2002 John Wiley & Sons, Ltd.