Novel Peptide Ion Chemistry Associated with Gold (I) Cationization: Preferential Cleavage at Lysine Residues.

•Gold (I) cationization of lysine containing peptides results in a novel peptide oxidation reaction upon collision-induced dissociation.•A hydride transfer to gold rapidly followed by ammonia loss gives rise to an abundant product 215Da lower in mass than the precursor ion.•The 215Da loss (HAuNH3) generates a cyclic iminium ion on the peptide backbone.•Subsequent collisional activation of this product ion tends leads to preferential cleavage at the iminium ion site to give b-type and y-H-NH3 ions.•The net effect is that gold (I) cationization of lysine-containing peptide ions leads to preferential cleavage at lysine residues.