Characterization of the product ions from the collision-induced dissociation of argentinated peptides.

Tandem mass spectrometry performed on a pool of 18 oligopeptides shows that the product ion spectra of argentinated peptides, the [bn + OH + Ag]+ ions and the [yn - H + Ag]+ ions bearing identical sequences are virtually identical. These observations suggest strongly that these ions have identical structures in the gas phase. The structures of argentinated glycine, glycylglycine, and glycylglycylglycine were calculated using density functional theory (DFT) at the B3LYP/DZVP level of theory; they were independently confirmed using HF/LANL2DZ. For argentinated glycylglycylglycine, the most stable structure is one in which Ag+ is tetracoordinate and attached to the amino nitrogen and the three carbonyl oxygen atoms. Mechanisms are proposed for the fragmentation of this structure to the [b2 + OH + Ag]+ and the [Y2 - H + Ag]+ ions that are consistent with all experimental observations and known calculated structures and energetics. The structures of the [b2 - H + Ag]+ and the [a2 - H + Ag]+ ions of glycylglycylglycine were also calculated using DFT. These results confirm earlier suggestions that the [b2 - H + Ag]+ ion is an argentinated oxazolone and the [a2 - H + Ag]+ an argentinated immonium ion.