Influence of a 4‐aminomethylbenzoic acid residue on competitive fragmentation pathways during collision‐induced dissociation of metal‐cationized peptides

Formation of [b n  + 17 + cat] + is a prominent collision‐induced dissociation (CID) pathway for Li + ‐ and Na + ‐cationized peptides. Dissociation of protonated and Ag + ‐cationized peptides instead favors formation of the rival b   n + /[b n −1 + cat] + species. In this study the influence of a 4‐aminomethylbenzoic acid (4AMBz) residue on the relative intensities of [b 3 −1 + cat] + and [b 3  + 17 + cat] + fragment ions was investigated using several model tetrapeptides including those with the general formula A(4AMBz)AX and A(4AMBz)GX (where X = G, A, V). For Li + ‐ and Na + ‐cationized versions of the peptides there was a significant increase in the intensity of [b 3 −1 + cat] + for the peptides that contain the 4AMBz residue, and in some cases the complete elimination of the [b 3  + 17 + cat] + pathway. The influence of the 4AMBz residue may be attributed to the fact that [b 3 −1 + cat] + would be a highly conjugated species containing an aromatic ring substituent. Comparison of CID profiles generated from Na + ‐cationized AAGV and A(4AMBz)GV suggests an apparent decrease in the critical energy for generation of [b 3 −1 + Na] + relative to that of [b 3  + 17 + Na] + when the aromatic amino acid occupies a position such that it leads to the formation of the highly conjugated oxazolinone, thus leading to an increase in formation rate for the former compared to the latter. Copyright © 2007 John Wiley & Sons, Ltd.