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)(NO 3 ) 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.