Molecular radical cations of oligopeptides

An unprecedented method of producing molecular radical cations of oligopeptides in the gas phase has been discovered. Electrospraying a methanolic mixture of a Cu(II)-amine complex, e.g., Cu^II(dien)(NO₃)₂ (where dien = diethylenetriamine), and an oligopeptide (M) yields the [Cu_II(dien)M]·²⁺ ion, whose collision-induced dissociation (CID) produces [Cu^I(dien)]⁺ and M^·+, the molecular cation of the oligopeptide. Abundant M·⁺ is apparent when the oligopeptide contains both a tyrosyl and a basic residue-arginyl, lysyl, or histidyl. These structural requirements are similar to those in the metalloradical enzyme process in photosystem II. Tandem mass spectrometry of M·⁺ produces fragment ions that are both common to and also different from [M + H]⁺. The fragmentation chemistry of M·⁺ and of its products appear to be radical driven. © 2000 American Chemical Society.